A COMPARATIVE PRIMATE ANATOMY - University of Cape Town

A COMPARATIVE PRIMATE ANATOMY 

Dissection Manual 

Edited by: 

Rebecca Rogers Ackermann 

Version 1.0 

© Copyright 2003

FOREWARD 

This dissection manual was prepared in conjunction with a course in Comparative Primate Anatomy taught 

jointly by Professors J Cheverud, G Conroy, and J Phillips-Conroy, at Washington University in St. Louis in the 

mid 1990’s. The dissections in this course focused on the skeleton and musculature of the limbs, specifically in 

a comparative context. As most dissection guides are Homo-centric, Homo sapiens is used as a reference 

species against which we compared the anatomy of the non-human primates. It also includes brief sections on 

behavior and ecology, as well as summary comparative anatomy sections. The choice of species was solely a 

function of availability, and included three New World monkeys (Saguinus oedipus, Saimiri sciureus, Ateles 

sp.), one African prosimian (Galago crassicaudatus), two southeast Asian macaques (Macaca fascicularis, M. 

mulatta), one African baboon (Papio hamadryas), and one great ape (Pongo pygmaeus). This guide is by no 

means a comprehensive guide, and is instead intended as a working document. As such, any comments or 

additional contributions to this guide are welcome, and will be included in updated versions. 

Importantly, this dissection guide was a collective effort, and a number of graduate and undergraduate students 

at Washington University contributed text and graphics to this work. These contributors include: Rebecca 

Rogers Ackermann, Jeff Baliff, Susan Foxman, Jennifer Helbig, Jonathan Lesser, Mark Mosbacher, Sam 

Senturia, Debbie Sklar, Patricia Sothman. Without this group cooperation (a good primate trait!) and the 

support of the faculty at Washington University, this manual would not have happened. 

REBECCA ROGERS ACKERMANN 

MARCH 2003

Comparative Primate Anatomy Behavior & Ecology 

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COTTON-TOP TAMARIN 

Saguinus oedipus 

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© Copyright 2003 

SECTION 1: 

BEHAVIOR AND ECOLOGY 

The cotton-top tamarin inhabits the forested areas of northern Columbia. These diurnal primates feed largely on 

fruits, insects, and spiders. Occasionally the tamarin will eat buds, soft young leaves, and vertebrates. While 

knowledge of their social structure is sketchy, they seem to be basically monogamous, although recent research 

suggests that there may be a “divorce” element to this monogamy, or that perhaps the structure is actually 

polyandrous. They reside in concentrations of 30-180 per square kilometer, with a home range of about 10 ha. 

These home ranges overlap, but contact is agnostic. Additionally, they tend to remain in extended family 

groups (1-19 individuals), and often form larger groups (20-40 individuals). These groups contain a dominant 

mated pair, their new young, and a group of subordinate young. Each troop has a defended territory. 

Tamarins are approximately 9 inches long and weigh about 450g (1 lb). They are easily identifiable by their flat 

faces and distinctive markings; the cotton-top tamarin has brown, black, and white body hair, with a plume of 

white hair on the top of its head. There is no sexual dimorphism. They reproduce seasonally (Jan-June). 

Typical gestation length is 140-145 days, with twinning as the norm. The father assists at birth and both parents 

participate in the care of the infants, which ride on their backs for the first 6-7 weeks of life. Tamarins spend 

much of their time grooming. 

Tamarins are arboreal quadrupeds who spend a great deal of time walking on branches. However, they also 

spend much of their time climbing trunks, and even occasionally leaping/hopping. Adaptations for this 

locomotor behavior are apparent in their anatomy. 

Kavanagh, M. (1983) A Complete Guide to Monkeys, Apes, and Other Primates. New York: The Viking Press. 

Napier, J.R. and P.H. Napier (1985) The Natural History of the Primates. Cambridge, Mass.: The MIT Press. 

Nowak, R.M. (1991) Walker’s Mammals of the World. Baltimore and London: The John Hopkins University 

Press, 5th ed, Vol. 1. 

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THICK-TAILED BUSHBABY 

Galago crassicaudatus 

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The thick-tailed bushbaby is the largest of all the galagos. The average weight of males (1,510g n=8) is slightly 

higher than females (1,258g n=9) (Petter and Petter-Rousseaux, 1979). This species is nocturnal. 

This species has been observed throughout eastern and southern Africa. Highest population densities are found 

in humid subtropical evergreen forests in which fruiting trees are plentiful. Populations are also found in dense 

riparian vegetation, subtropical orchards, and open woodland (Charles-Dominique and Bearder, 1979). 

The thick-tailed bushbaby, while anatomically suited to leaping (Fleagle, 1988), actually exhibits a tendency 

towards quadrupedalism. In fact, the musculature and skeleton of this species gives no clues to indicate that 

this animal locomotes via arboreal running and walking. Yet locomotor studies reveal that quadrupeal running 

and walking occurs arboreally, along the top of horizontal branches, as well as terrestrially. Saltation is rare. 

Jumping between trees is minimized by either climbing between connecting terminal branches or descending to 

the ground. No patterns have been observed in preference of using supports at a particular height. This species 

is flexible to individual forest structure despite its preference for horizontal supports (Charles-Dominique and 

Bearder, 1979). 

Gum, fruit, nectar, seeds, and insects comprise the annual diet of the thick-tailed bushbaby, yet this diet varies 

seasonally. Gums are eaten throughout the year, but become important during the dry season when fruit and 

nectar are scarce. At one study area Charles-Dominique and Bearder (1979) reported the following annual 

percentages for food intake: gum 62%; fruit 21%; flower secrections 8%; seeds 4%; insects and other items 5%. 

Lorisiformes do not live in social groups typical of diurnal primates. Instead, contact is limited to shared 

sleeping sites, brief encounters while foraging, and mother-infant relationships. At sleeping sites, animals have 

been observed huddling, allogrooming, and playing. Communication away from sleeping sites is maintained 

through a variety of mechanisms. Olfactory communication (e.g., urine washing) is used by both sexes to 

convey information without direct contact. Vocal communication is also frequent, and the loud cry of this 

species is the inspiration for the common name of “bushbaby.” 

Charles-Dominique, P. and S. K. Bearder (1979) Field studies of lorisid behavior: Methodological aspects. In 

The Study of Prosimian Behavior, ed. G.A. Doyle and R.D. Martin, pp. 567-630. New York: Academic Press. 

Fleagle, J.G. (1988) Primate Adaptation and Evolution. New York: Academic Press. 

Petter, J.-J., and A.Petter-Rousseaux (1979) Classification of the prosimians. In The Study of Prosimian 

Behavior, ed. G.A. Doyle and R.D. Martin, pp. 1-44. New York: Academic Press. 

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SQUIRREL MONKEY 

Saimiri sciureus 

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The genus Saimiri is attributed to the subfamily Cebinae within the family Cebidae and the infraorder 

Platyrrhini. There is some debate as to the number of species, but most researchers recognize two Saimiri 


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species: Saimiri sciureus and Saimiri oerstedi. Saimiri is the smallest of the cebines with a body mass that 

ranges from 0.6 to 1.1 kg. They are sexually dimorphic with the males usually weighing more than the females. 

The head and body length is 260-360 mm. The tail of the squirrel monkey is very long (350-425 mm in adults) 

and prehensile only in infants. 

Geographically, Saimiri ranges from Costa Rica and Panama in the north, to central Brazil and Bolivia in the 

south, and from central and northeastern Brazil in the east, to just east of the Andes in Peru in the west. While 

these platyrrhines are found throughout the rain forests of Central and South America, they prefer riverine and 

secondary forests to primary forests. The squirrel monkey can be found primarily in the lower levels of the 

forest. Terborgh (1983) reported a home range of more than 250 hectares for Saimiri sciureus. Saimiri has been 

reported to have a day range as large as 4 km. 

Squirrel monkeys are often seen in polyspecific groups with Cebus monkeys. Terborgh (1983) reported that the 

mean troop size of Saimiri studied at Coch Cashu in Peru was 35 individuals ranging from 30-40. Thorington 

(1968) studied a troop of 18 individuals of Saimiri in Colombia which increased to 22 during a 10-week period 

due to births. During the day the group divided into smaller groups of pregnant females, females with young, 

and adult males, but during the night they stayed together in the large group. The population densities of this 

genus ranges from 20-30 individuals per km2 in Colombia to 151-528 individuals per km2 in Peru. 

Fleagle and Mittermeier (1980) define Saimiri as a frugivore that spends much time foraging for insects while 

travelling between fruit trees. Janson and Boinski (1992) noted that squirrel monkeys eat vertebrate prey and 

but seldom feed on animals that are greater than 50g. Janson and Boinski also noted that caterpillars make up 

about half of the faunal component of Saimiri’s diet. Terborgh (1983) reported that Saimiri sciureus’s diet 

consisted of 18% fruit and 82% prey. 

Overall Saimiri is classified as an arboreal quadruped that often leaps. Fleagle and Mittermieier (1980) reported 

that when Saimiri sciureus travelled between feeding locations it engaged in slightly more bouts of arboreal 

quadrupedalism (55%) than bouts of leaping (42%). However, when feeding, the difference between these two 

modes of locomotion was much greater (quadrupedalism - 87% and leaping - 11%). 

Eisenberg, JF (1989) Mammals of the Neotropics: The Northern Neotropics, Volume 1. University of Chicago 

Press: Chicago. 

Fleagle, JG (1988) Primate Adaptation and Evolution. Academic Press: San Diego. 

Fleagle, JG and RA Mittermeier (1980) Locomotor behavior, body size, and comparative ecology of seven 

Surinam monkeys. American Journal of Physical Anthropology 52:301-314. 

Janson, CH and S Boinski (1992) Morphological and behavioral adaptations for foraging in generalist 

primates: The case of the cebines. American Journal of Physical Anthropology 88:483-498. 

Nowak, RM (1991) Walker’s Mammals of the World: Fifth Edition. Johns Hopkins University Press: 

Baltimore. 

Terborgh, J (1983) Five New World Primates: A Study in Comparative Ecology. Princeton University Press: 

Princeton. 

Thorington, RW (1968) Observations of squirrel monkeys in a Colombian forest. In RA Rosenblum and RW 

Cooper (Eds.): The Squirrel Monkey. Academic Press: New York, pp. 69-85. 


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LONG-TAILED, OR CRAB-EATING MACAQUE 

Macaca fascicularis 

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The long-tailed, or crab-eating macaque is found throughout southeast Asia from Burma to the Philippines and 

further southward through Indochina, Malaysia, and on all the islands of Indonesia except for Celebes 

(Sulawesi). M. fascicularis is considered to be an ecologically opportunistic species which is able to thrive in 

habitats ranging from primary to secondary forests, and from riverine to coastal forests. The long-tailed 

macaque is also a very successful edge species which has learned to inhabit, and exploit plantations, parks, and 

gardens. In fact, it has been noted that population densities of the long-tailed macaque are sometimes higher in 

disturbed habitats than in undisturbed habitats. 

Group size in M. fascicularis ranges from 10 to 50 individuals, although groups containing nearly 100 

individuals have been reported. Home range size varies between 40 and 100 ha, and day ranges are reportedly 

less than one km. Groups are matrilineal, and males transfer between groups frequently. 

M. fascicularis is a sexually dimorphic species. Males weigh 5 kg, and females weigh 3 kg on average. Social 

grooming accounts for the majority of social interactions, and grooming is primarily solicited and provided by 

the females. Grooming is directed toward close relatives, and close relatives provide support during agonistic 

encounters. Playful behavior occurs between infants, and between juveniles. Adult males are the least socially 

active group members. 

Like other macaques, M. fascicularis possesses a generalized anatomy adapted for quadrupedal locomotion. 

However, the long-tailed macaque is a highly arboreal species which spends the bulk of its feeding and 

traveling time in the mid-canopy of the forests it inhabits. Though the majority of the long-tailed macaques’ 

locomotion is in the form of quadrupedal walking and running, some leaping has been observed, and hind-limb 

hanging is used during feeding. The long-tailed, or crab-eating macaque is a frugivore. It supplements its diet 

with leaves, with a considerable proportion of invertebrates (crabs and termites), and with small vertebrates. 

Chivers, D.J. (1973) An introduction to the socio-ecology of Malayan forest primates. In Comparative Ecology 

and Behavior of Primates. Michael, P.P. and Crook, J.H. (eds.) 

Crockett, C.M. and Wilson, W.L. (1980) The ecological seperation of M. nemenstrina and M. fascicularis in 

Sumatra. In: Lindberg, D.G. (ed.)The Macaques. Van Nostrand Reinhold, New York. 

Fleagle, J.G. (1988) Primate Adaptation and Evolution. Academic Press. New York. 

Goosen, C. (1991) Social Grooming in Primates. In: The Order Primates. Stephens, M.E. (ed.) Kendall/Hunt. 

Iowa. 

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RHESUS MACAQUE 

Macaca mulatta 

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The rhesus macaque, Macaca mulatta (subfamily Cercopithecinae), ranges from Afghanistan eastward through 

South and Southeast Asia. Data obtained from a study on rhesus monkeys in Nepal noted the high adaptive 


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flexibility of the rhesus, considered to be the greatest of any nonhuman primate species. The macaques occupy 

habitats including temple grounds and parkland, heavily populated towns, lowland monsoon forest, and upper 

montane forest. Various studies have found that rhesus can feed on natural forest vegetation as well as take 

advantage of human food sources provided by agriculture, enabling them to coexist with man as well as on their 

own. Moreover, behavioral data obtained from temple ground-living populations found 20% of rhesus 

aggression involved other species (i.e. dogs, humans); of this total, 80% was directed from other species toward 

the monkeys, 20% from the monkeys toward other species, with population density remaining stable during the 

study period. Teas et. al (1980) conclude that this indicates the notable ability of rhesus monkeys to deal with 

adverse conditions of crowding and competition, particularly in areas of human occupation. Troop size ranges 

from 20-51 individuals, averaging 32 individuals. Troop composition is generally 8% male, 35% female, 30% 

juveniles, 15% yearlings, and 13% infants. These figures vary with season, year, and particular ecological 

conditions, however. The population density of a lowland monsoon region in Nepal was 0.29 rhesus groups per 

square kilometer. Home range varies from 2.5 to 24 hectares, depending on habitat, with overlap of 10-20% in 

some areas, up to 80% in more crowded areas. Despite such extensive home range overlap, each group 

maintains a core area of exclusive use. 

Teas, J., Richie, T., Taylor, H., and Southwick, C. (1980) Population patterns and behavioral ecology of rhesus 

monkeys (Macaca mulatta) in Nepal. In Lindburg, D. (ed.), The Macaques: Studies of Ecology, Behavior, and 

Evolution, Van Nostrand Reinhold, pp. 247-262. 

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HAMADRYAS BABOON 

Papio hamadryas 

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Hamadryas baboons (Papio hamadryas) are Old World monkeys belonging to the subfamily Cercopithecinae. 

P. hamadryas is one of the 5 species within Papio (in addition to P. anubis, P. papio, P. cynocephalus, P. 

ursinus). However, the specific status of the other 4 species is debated. P. hamadryas and P. anubis have 

formed an apparently stable hybrid zone where their ranges meet in Ethiopia, but the two are still considered 

distinct species. 

In P. hamadryas, head and body length is 610-762 mm and tail length is 362-610 mm. Adult body weights 

range between 12,000 grams and 21,000 grams, with considerable size sexual dimorphism; females are almost 

half the size of males. Pelage in the young animals is brown, and this becomes ash gray with age. Older males 

possess a heavy mane around the neck and shoulders. Hymadryas baboons, who are terrestrial quadrupeds, 

have similar upper and lower limb lengths; intermembral indices center around 95. 

P. hamadryas is geographically distributed throughout upper Egypt, northeastern Sudan, eastern Ethiopia, 

northern Somalia, and the southwestern Arabian Peninsula. They inhabit open woodland, savannahs, grassland, 

and rocky hill country. The diurnal hymadryas baboons forage and travel primarily by terrestrial, quadrupedial 

walking and running. However, the animals often climb trees or rocky cliffs for sleeping and resting. They 

generally occur at low densities, with recorded densities at 1.8-3.4 animals per square kilometer. Day ranges 

are considerable (6-19 km), and home ranges may range up to 28 square kilometers. Home ranges of 

neighboring bands may overlap by 50%. 

Hamadryas baboons eat a variety of vegetable and fruit matter, but seem to concentrate on whatever is easily 

available at a given time. They consume ripe fruits, roots, tubers, grass seeds, and leaves. In addition, they are 

opportunistic faunivores, taking small mammals, invertebrate and insects. 

The basic social unit is the one-male group (OMG), which consists of one or two males, several females, and 

their offspring. Usually, two or three OMG are associated in autonomous foraging groups called bands (30-85 


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animals). Up to three or four bands may congregate at one rock or sleeping cliff to form a troop. The males of 

the OMG defend the same set of females from the advances of other males, and actually herd the females by 

tracking any stray females and biting them in the neck and back to keep the group together. Both males and 

females may depart from their natal unit, at about 2 years in males and 3.5 years in females. Breeding occurs 

throughout the year, although there are birth peaks in Ethiopia in May-June and November-December. The 

average estrous cycle is 30 days, gestation is 170-173 days, and single births are the norm. The age of sexual 

maturity is 7 years in males and 5 years in females. 

Fleagle, JG. (1988) Primate adaptation and evolution. San Diego: Academic Press Inc. 

Nowak, RM. (1991) Walker’s Mammals of the World. Volume 1. Baltimore: Johns Hopkins University Press. 

Stammbach, E. (1987) Desert, forest, and montane baboons: multilevel societies. In BB Smuts, et. al., Primate 

Societies. Chicago: The University of Chicago Press. 

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SPIDER MONKEY 

Ateles 

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Spider monkeys (Ateles) are diurnal Cebids from the subfamily Atelinae whose distribution ranges from the 

southern Amazon in Brazil to the Neotropical regions of Mexico. This is a vast area consisting of rainforests of 

various types in which many other primate species thrive, including Howler monkeys (Alloutta), Squirrel 

monkeys (Saimiri), and Owl monkeys (Aotus). 

The genus Ateles consists of four species, A. geoffroyi, A. fusciceps, A. belzebuth, and A. paniscus, all of which 

are allopatric. All species of spider monkeys are frugivorous, complementing their diet with insects, leaves, and 

other items. 

Spider monkeys are aboreal and have an extremely dexterous prehensile tail which allows them to hang from 

trees and use their hands for the collection of fruit. Their range is primarily in the upper to mid canopy in 

mature forests, yet they can locomote on the ground if necessary. Locomotion in Ateles is extremely variable, 

ranging from semi-brachiation to brief moments of bipedal walking. However, the majority of locomotion is a 

composite of arboreal quadrupedalism and semi-brachiation (suspensory locomotion). The semi-brachiation 

used by Ateles differs from that of the true brachiators, genus Hylobates primarily in the use of the tail. The tail 

is used virtually all of time during the various modes of locomotion and is employed during postural behavior as 

well. Spider monkeys often feed while suspended using various combinations of the upper and hind limbs and 

the tail, an important feature that affects the musculoskeletal system greatly. 

Spider monkeys can be differentiated from other Platyrrhines on the basis of their social organization. 

Members of the genus Ateles form large polygamous groups characterized by a fission-fusion social 

organization lacking strict dominance heirarchies. This social organization is very similar to that of 

Chimpanzees (genus Pan). In Ateles, there are two major levels of organization: the group and the subgroup. 

Groups consist of up to 35 individuals who scatter throughout the day yet occasionally regroup at night. 

Subgroups are fluctuating units in which various individuals forage together. Membership in the subgroup is 

not constant and changes on a daily basis or even more frequently. However, despite the apparent randomnesss, 

male spider monkeys often form stable subgroups which are cohesive, reflecting a tendency towards male 

affiliation. Females migrate in and out of subgroups individually or with their young. 


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Cant, J.G.H. Ecology, Locomotion, and Social Organization of Spider Monkeys (Ateles geoffroyi). Ph.D. 

Thesis, University of California, Davis, 1977. 

Eisenberg, J.F. Communication Mechanisms and Social Integration in the Black Spider Monkey, Ateles 

fusciceps and Related Species. Smithsonian Contr. Zoo, 113 (1989). 

Kellogg, R. and E.A.Goldman. Review of Spider Monkeys. Proceedings U.S. National Museum, 96: 1-45, 1944. 

Symington, M.M. Food Competition and Foraging Party Size in the Black Spider Monkey. Behavior, 105 

(1988), 117-34. 

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ORANGUTAN 

Pongo pygmaeus 

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The orangutan’s main habitat are the rain forests of insular South East Asia, specifically the islands of Borneo 

and Sumatra. There are two recognized subspecies of the orangutan which correspond to the particular islands; 

Pongo pygmaeus pygmaeus from Borneo and Pongo pygmaeus abelii found on Sumatra. The orangutan is the 

largest primate which is almost completely arboreal (males weigh about 70 kg and females approximately 37 

kg). Because of its large size, the orangutan does not brachiate while moving in their arboreal habitat. The 

main form of arboreal locomotion for the orangutan is quadrumanus climbing, used to achieve maximal weight 

distribution across as many supports as possible. While terrestrial, the orangutan utilizes a particular form of 

quadrupedism known as “fist-walking” where they use the sides of their hands and feet as the main bodily 

support. Orangutans exhibit that modification of quadrupedism because of their elongated phalanges. 

The diet of the orangutan consists mainly of fruit: mangoes, figs and durian being the most common elements; 

however, they do supplement their diet with insects such as ants, termites and honeybees. Also some predation 

on young gibbons has been observed. During the rainy season, many of the fruits are not as abundant and the 

orangutans eat more bark, leaves and pith to make up for the lack of fruit. 

Unlike most primates, the orangutans are relatively unsocial animals. The most common group observed are 

females with their offspring. Solitary adult males have home ranges which overlap the home ranges of several 

adult females. The adult males are highly territorial and actively maintain their domains by displays and loud 

calls. The females do not display swellings during estrus and are receptive throughout their cycles; however, 

some swellings are seen during pregnancy. The gestation period is approximately 264 days, weaning occurs 

around 3 years and sexual maturity around 6-7 years. Orangutans have life spans of approximately 50 years. 

Galdikas BM (1988) Orangutan diet, range and activity at Tanjung Puting, Central Borneo. Int. J. Primat. 9: 

1-35. 

Hamburg DA and ER McCown (eds.) (1979) The Great Apes. Reading, MA: The Benjamin/Cummings 

Publishing Company. 

Napier JR and PH Napier (1994) The Natural History of the Primate Cambridge, MA: MIT Press. 

Richard AF (1985) Primates in Nature. New York: W.H. Freeman and Company. 

Tuttle RH (1986) Apes of the World: Their Social Behavior, Communication, Mentality, and Ecology. Park 

Ridge, NJ: Noyes Publications. 


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Comparative Primate Anatomy Back, Shoulder & Forelimb 

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INTRODUCTION 

Homo sapiens 

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SECTION 2: 

BACK, SHOULDER AND FORELIMB 

The major muscles responsible for the movement of the arm and pectoral girdle in humans can be divided into 

five groups based on the movements they produce; muscles that elevate the arm, lower the arm, abduct and flex 

the humerus, adduct and extend the humerus, and rotatie and stabilize the shoulder. Some muscles are 

responsible for movements in more than one of these categories. 

The muscles that elevate the arm are the trapezius, serratus anterior, supraspinatus, deltoid and pectoralis major. 

When the arm is lowered, the levator scapulae, rhomboids, latissimus dorsi, and pectoralis minor muscles are 

engaged. Abduction and flexion of the humerus are accomplished by the deltoid, supraspinatus, and pectoralis 

major. Adduction and extension of the humerus is carried out by latissimus dorsi, pectoralis major, and the 

deltoid muscles. The shoulder is rotated and stabilized by the infraspinatus, teres minor, subscapularis, teres 

major, and coracobrachialis. (Each muscle description includes medial/proximal attachments, lateral/distal 

attachments, and the main actions when engaged.) 

Trapezius 

--medial attachment - medial third of superior nuchal line, external occipital protuberance, ligamentum nuchae 

spinous processes of C7 to T12 vertebrae 

--lateral attachment - lateral third of clavicle, acromion, spine of scapula 

--main actions - superior fibers elevate the arm, middle fibers retract, inferior fibers depress the scapula; the 

superior and inferior fibers act together in superior rotation of the scapula. 

Serratus Anterior 

--proximal attachment - external surfaces of the lateral parts of the first through eighth ribs 

--distal attachment - anterior surface of the medial border of the scapula 

--main actions - protracts scapula and hold it against thoracic wall, rotates scapula 

Supraspinatus (rotator cuff muscle) 

--medial attachment - supraspinous fossa of scapula 

--lateral attachment - superior facet on greater tubercle of humerus 

--main actions - helps deltoid to abduct arm and acts with rotator cuff muscles 

Deltoid 

--proximal attachment - lateral third of clavicle, acromion, and spine of scapula 

--distal attachment - deltoid tuberosity of humerus 

--main actions - anterior part flexes and medially rotates arm, middle part abducts arm, posterior part extends 

and laterally rotates arm 

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Pectoralis Major 

--proximal attachment - clavicular head of the muscle attaches on the anterior surface of the medial half of the 

clavicle; sternocostal head attaches on the anterior surface of the sternum, superior six costal cartilages and the 

aponeurosis of the external oblique muscle 

--distal attachment - lateral lip of the intertubercular groove of the humerus 

--main actions - adducts and medially rotates humerus, draws scapula anteriorly and inferiorly, the clavicular 

head acting alone flexes humerus, and sternoclaviuclar head extends humerus 

Levator Scapulae 

--medial attachment - posterior tubercles of the transverse processes of C1 to C4 vertebrae 

--lateral attachment - superior part of the medial border of the scapula 

--main actions - elevates scapula and tilts its glenoid cavity inferiorly by rotating scapula 

Rhomboid Major and Minor 

--medial attachment - minor attaches at the ligamentum nuchae and spinous processes of C7 and T1 vertebrae 

--lateral attachment - medial border of scapula from level of spine to inferior angle 

--main actions - retracts scapula and rotates it to depress the glenoid cavity, and fixes scapula to thoracic wall. 

Latissimus dorsi 

--medial attachment - spinous process of the inferior six thoracic vertebrae, thoracolumbar fascia, iliac crest, 

and inferior 3 or 4 ribs. 

--lateral attachment - floor of intertubercular groove of humerus 

--main actions - extends, adducts, and medially rotates humerus (raises body toward arms during climbing) 

Pectoralis Minor 

--proximal attachment - third through fifth ribs near costal cartilages 

--distal attachment - medial border and superior surface of coracoid process of scapula 

--main actions - stabilizes scapula by drawing it inferiorly and anteriorly against thoracic wall 

Infraspinatus (rotator cuff muscle) 

--medial attachment - infraspinous fossa of scapula 

--lateral attachment - middle facet on greater tubercle of humerus 

--main actions - laterally rotates arm, helps to hold humeral head in glenoid cavity of scapula 

Teres Minor (rotator cuff muscle) 

--medial attachment - superior part of lateral border of scapula 

--lateral attachment - inferior facet on greater tubercle of humerus 

--main actions - laterally rotates arm, helps to hold humeral head in glenoid cavity of scapula 

Subscapularis (rotator cuff muscle) 

--medial attachment - subscapular fossa 

--lateral attachment - lesser tubercle of humerus 

--main actions - medially rotates arm and adducts it , helps to hold humeral head in glenoid cavity 

Teres Major 

--medial attachment - dorsal surface of inferior angle of scapula 

--lateral attachment - medial lip of intertubercular groove of humerus 

--main actions - adducts and medially rotates arm 

Coracobrachialis 

--proximal attachment - tip of coracoid process of scapula 

--distal attachment - middle third of medial surface of humerus 

--main actions - helps to flex and adduct arm 

The muscles that move the forearm in humans can be divided into those groups that flex the elbow, extend the 

elbow, pronate the forearm, and supinate the forearm. The biceps brachii, brachialis, and brachioradialis 


9


muscles produce flexion at the elbow, while the triceps brachii and anconeus muscles extend the elbow. 

Pronation of the forearm is carried out by the pronator teres and pronator quadratus. Supination of the forearm 

is produced by the biceps brachii and supinator muscles. 

Biceps Brachii 

--proximal attachment - short head attaches on the tip of the coracoid process of the scapula, long head 

originates at the supraglenoid tubercle of the scapula 

--distal attachment - radial tuberosity and fascia of arm via the bicipital aponeurosis 

--main actions - supinates forearm and when supine flexes forearm 

Brachialis 

--proximal attachment - distal half of anterior surface of humerus 

--distal attachment - coronoid process and tuberosity of ulna 

--main actions - flexes forearm in all positions 

Brachioradialis 

--proximal attachment - proximal two-thirds of the supracondylar ridge of the humerus 

--distal attachment - lateral surface of distal end of radius 

--main actions - flexes forearm 

Triceps Brachii 

--proximal attachment - long head begins at infraglenoid tubercle of glenoid, lateral head is at the posterior 

surface of humerus superior to radial groove, medial head originates on the posterior surface of the humerus 

inferior to the radial groove 

--distal attachment - proximal end of olecranon of ulna, fascia of forearm 

--main actions - extends forearm (chief extensor of forearm), long head steadies head of abducted humerus 

Anconeus 

--proximal attachment - lateral epicondyle of humerus 

--distal attachment - lateral surface of olecranon and superior part of posterior surface of ulna 

--main actions - assists triceps in extending forearm, stabilizes elbow joint, abducts ulna during pronation 

Pronator Teres 

--proximal attachment - medial epicondyle of humerus, coronoid process of ulna 

--distal attachment - middle of lateral surface of radius 

--main actions - pronates forearm and flexes it 

Pronator Quadratus 

--proximal attachment - distal fourth anterior surface of ulna 

--distal attachment - distal fourth anterior surface of radius 

--main actions - pronates forearm, deep fibers bind ulna and radius together 

Supinator 

--proximal attachment - lateral epicondyle of humerus, radial collateral and anular ligaments, supinator fossa, 

crest of ulna 

--distal attachment - lateral, posterior, and anterior surfaces of proximal third of radius 

--main actions - supinates forearm 

The muscles that move the hand in humans are divided into the long flexors and the long extensors of the hand 

and digits. The flexor group includes flexor carpi radialis, flexor carpi ulnaris, palmaris longus, flexor 

digitorum profundus, and flexor pollicus longus. The extensors are extensor carpi radialis longus, extensor 

carpi radialis brevis, extensor carpi ulnaris, extensor digitorum, extensor digiti minimi, extensor indicus, 

extensor pollicus longus, extensor pollicus brevis, and abductor pollicus longus. 

Flexor Carpi Radialis 

--proximal attachment - medial epicondyle of humerus 


10


--distal attachment - base of second metacarpal bone 

--main actions - flexes hand and abducts it 

Flexor Carpi Ulnaris 

--proximal attachment - humeral head originates at medial epicondyle of humerus, ulnar head originates at 

olecranon and posterior border of ulna 

--distal attachment - pisiform bone, hook of hamate bone, fifth metacarpal bone 

--main actions - flexes hand and adducts it 

Palmaris Longus 

--proximal attachment - medial epicondyle of humerus 

--distal attachment - distal half of flexor retinaculum, palmar aponeurosis 

--main actions - flexes hand and tightens palmar aponeurosis 

Flexor Digitorum Superficialis 

--proximal attachment - humeroulnar head originates at medial epicondyle of humerus, ulnar collateral 

ligament, and coronoid process of the ulna, radial head originates on superior half of anterior border of radius 

--distal attachment - bodies of the middle phalanges of medial four digits 

--main actions - flexes middle phalanges of medial four digits (stronger action - flexes proximal phalanges and 

hand) 

Flexor Digitorum Profundus 

--proximal attachment - proximal three-fourths of the medial and anterior surfaces of the ulna, interosseous 

membrane 

--distal attachment - bases of distal phalanges of medial four digits 

--main actions - flexes distal phalanges of medial four digits 

Flexor Pollicus Longus 

--proximal attachment - anterior surface of radius, interosseous membrane 

--distal attachment - base of distal phalanx of thumb 

--main actions - flexes phalanges of thumb 

Extensor Carpi Radialis Longus 

--proximal attachment - lateral supracondylar ridge of humerus 

--distal attachment - base of second metacarpal bone 

--main actions - extend and abduct hand at wrist joint 

Extensor Carpi Radialis Brevis 


--distal attachment - base of third metacarpal 

--main actions - extend and abduct hand at wrist joint 

Extensor Carpi Ulnaris 

--proximal attachment - lateral epicondyle of humerus and posterior border of ulna 

--distal attachment - base of fifth metacarpal bone 

--main actions - extends and adducts hand at wrist 

Extensor Digitorum 


--distal attachment - extensor expansions of medial four digits 

--main actions - extends medial four digits at metacarpophalangeal joints, extends at wrist joints 

Extensor Digiti Minimi 


--distal attachment - extensor expansion of fifth digit 

--main actions - extends fifth digit at metacarpophalangeal and interphalangeal joints 


11


Extensor Indicus 

--proximal attachment - posterior surface of ulna, interosseous membrane 

--distal attachment - extensor expansion of second digit (index finger) 

--main actions - extends second digit and helps to extend hand 

Extensor Pollicus Longus 

--proximal attachment - posterior surface of middle third of ulna, interosseous membrane 

--distal attachment - base of distal phalanx of thumb 

--main actions - extends distal phalanx of thumb at metacarpophlangeal and interphalangeal joints 

Extensor Pollicus Brevis 

--proximal attachment - posterior surface of radius, interosseous membrane 

--distal attachment - base of proximal phalanx of thumb 

--main actions - extends proximal phalanx of thumb at carpometacarpal joint 

Abductor Pollicus Longus 

-- proximal attachment - posterior surfaces of ulna and radius, interosseous membrane 

--distal attachment - base of first metacarpal bone 

--main actions - abducts thumb and extends it at carpometacarpal joint 

The deep muscles of the back can be grouped one category as erectors. This region of the body is where one 

would expect to find strong contrasts between the human and other primates (especially those with tails). The 

muscle included here are the iliocostalis, longissimus, and spinalis. Together, these make up the erector spinae 

muscle group. The main actions of this muscle group are to act as the chief extensor of the vertebral column, 

bend vertebral column posteriorly, and to control movement during flexion of the vertebral column. 

Iliocostalis 

--origin - broad tendon attached inferiorly to the posterior part of the iliac crest, the posterior aspect of the 

sacrum, the sacroiliac ligaments, and the sacral and inferior lumbar spinous process 

--insertion - angles of the ribs 

-- main actions - acting unilaterally it laterally flexes the head or the vertebral column, bilaterally it extends the 

head and part or all of the vertebral column 

Longissimus 

--origin - same as iliocostalis 

--insertion - transverse processes of thoracic and cervical vertebrae, mastoid process 

--main actions - same as iliocostalis 

Spinalis 

--origin - same as iliocostalis and longissimus 

--insertion - extends from spinous processes in the superior lumbar and inferior thoracic regions to the spinous 

processes in the superior thoracic region 

--main actions - same as iliocostalis and longissimus 

Aiello, Leslie and Dean, Christopher (1990). An Introduction to Human Evolutionary Anatomy. Academic 

Press: London. 

Moore, Keith L. (1992). Clincally Oriented Anatomy. Williams and Wilkins: Baltimore. 


12


______________________________ 



______________________________ 

One of the main differences between the forelimb of a tamarin and that of a human involves the medial-lateral 

lengthening of the scapula which helps to properly position the humerus for quadrupedal locomotion. For the 

most part, the muscles and their insertions are the same in the two animals, however, the osteological changes 

necessary for the different locomotor forms may reposition the muscles relative to the joints, thereby altering 

their functions. These differences, in turn, cause relative changes in the proportions of the muscles. I will 

discuss both the absolute changes in muscles and the relative changes in their form due to function in a specific 

order: those muscles that move the arm and pectoral girdle, those muscles that move the forearm, and those 

muscles that move the wrist and hand. 

The tamarin has only three muscles that are not found in the arm and pectoral girdle of the human-atlantoscapularis 

anterior, atlantoscapularis posterior, and pectoralis abdominus. Atlantoscapularis 

anterior originates from the spinous process of the atlas vertebra and inserts on the acromion of the scapula. It 

functions as a upward rotator of the scapula. Atlantoscapularis posterior originates on the transverse process 

of the atlas and inserts on the anterior medial border of the scapula, and functions as a replacement for levator 

scapulae which raises the scapula. The pectoralis abdominus was not observable, as it was removed when the 

specimen was eviscerated. However, it should arise from the sheath of rectus abdominus, insert into the 

humerus, and flex the arm. 

Some of these muscles that act on the arm and shoulder in the tamarin have different origin and insertion sites 

and areas compared with humans, largely due to relative differences in the sizes of the muscles. The origin of 

trapezius does not extend as far up the spine in tamarins as in humans, with only a few fibers reaching the 

nuchal crest, and the muscle in general is less substantial. Latissimus dorsi is also less extensive, as there is 

no portion of latissimus dorsi which originates from the iliac crest. Teres major is robust compared with 

humans, as is serratus anterior, while the origin of the rhomboids is more extensive, traveling up into the 

cervical region. Also, the deltoid is not very large, but is distinctly divided into its three functional regions-anterior, 

middle, and posterior. 


Figure 1: Cotton-top tamarin back. (A) 

deltoid muscle, (B) atlantoscapularis anterior, 

(C) trapezius. 

13


As a whole, these differences in the muscle sizes and attachment areas result from functional differences. 

Tamarins, as arboreal quadrupeds, don't use their arms above their heads. Therefore, their arm-raising muscles- 

-deltoid, trapezius--are relatively underdeveloped. (While serratus anterior is used in arm-raising, its good 

development in the tamarin is probably due to the fact that it is also used to support the rib cage in quadrupeds 

and functions as a sling.) In contrast, the muscles which are used for quadrupedal propulsion--latissimus dorsi, 

teres major, and presumably pectoralis major (if my animal had one)--are relatively overdeveloped. 

The muscles that move the forearm are the same in tamarins and humans with one exception. The 

dorsoepitrochlearis originates from the tendon of latissimus dorsi and inserts into the olecranon process of the 

ulna. It functions as an extensor of the elbow. 

The arm flexors of the tamarin are highly developed (biceps brachii and brachioradialis), relative to humans. 

The extensors of the arm, in particular triceps brachii, are also highly developed, as well as being more 

extensive. As an arboreal quadruped, the elbow of the tamarin remains in a constant state of flexion, with both 

the flexors and extensors of the arm almost continuously active during motion. Therefore, the muscular 

morphology of the tamarin arm reflects its mode of locomotion. 

The muscles that move the hand and wrist in the tamarin are the same as in humans. There are, however, some 

relative differences in the size of these muscles. As a whole, the flexors of the hand are extremely well 

developed. This is probably a reflection of the fact that, as arboreal quadrupeds, they spend much of their time 

not only balancing on branches, but actively grasping these branches both when walking of them and when 

climbing. 

______________________________ 



______________________________ 


Figure 2: The tamarin arm. (A) triceps 

brachii, (B) extensor group. 

The well developed musculature of the forelimbs of G. crassicaudatus indicates that the forelimbs are used in 

locomotion. This could either indicate that this species moves quadrupedally or by leaping. (Crompton (1984) 

distinguishes between leaping, in which the animal includes use of its forelimbs, and hopping, in which the 

animal locomotes entirely with its hindlimbs. It is important to note that not all saltation requires use of the 

forelimbs). 

14


Superficial back and shoulder: 

The muscles of Galago crassicaudatus in the superficial back and shoulder are primarily similar to Homo. 

The omohyoid is not considered a back or shoulder muscle in Homo , yet in G. crassicaudatus it originates at 

the vertebral border of the scapula, near the scapular spine. The levator scapula muscle in this species can be 

demarcated into more distinct muscles than the single levator scapula in Homo. The levator scapula ventralis 

originates on the spine of the scapula near the acromion and inserts onto the body of C1. The levator scapula 

dorsalis originates from the vertebral border of the scapula, and inserts onto the transverse processes of C1 to 

C5 or C6. The deltoid is clearly divided into three functional groups (posterior, middle, and anterior) and is 

relatively small. 

Differences between G. crassicaudatus and Homo in muscle size relative to body size can be accounted for by 

functional differences in postural behavior: the thick-tailed bushbaby is an arboreal quadruped, and the human 

is a biped. Unlike Homo,, G. crassicaudatus utilizes its superficial back and shoulder muscles to maintain 

balance, locomote, and support its body weight. The serratus anterior is relatively robust, forming a weight 

bearing sling around the thorax. This muscle, in conjunction with the equally robust rhomboids (major, minor, 

and capitus) and the trapezius, stabilizes the scapula when the forelimbs are bearing weight. The trapezius is 

not as relatively well developed as other superficial back muscles because galagos do not use them to raise their 

arms above their heads -- one of this muscle’s functions in humans. The latissimus dorsi, teres major, and 

pectoralis major are used to transfer the weight of the animal on to the arm when standing still, to adduct the 

arm in order to balance on arboreal supports, and to propel the animal when locomoting. They are, 

understandable, relatively well developed. In addition to these muscles of the "sling," G. crassicaudatus has an 

expansive pectoralis abdominus muscles. This muscle originates along the aponeurosis of the rectus 

abdominus at the mid-line, and inserts onto the medial aspect of the deltoid tuberosity. 


Figure 3: Galago chest: (latissimus dorsi, (B) serratus anterior, (C) pectoralis major, (E) deltoid. 

15


Upper arm and shoulder: 

G. crassicaudatus has all of the upper arm and shoulder muscles that Homo has, and has one additional 

muscle: epitrochlearis. Epitrochlearis takes origin from the olecranon process of the ulna and distal tendon of 

the triceps and inserts onto the latissimus dorsi, near its insertion on the humerus. This muscle is an additional 

flexor at the elbow. All of the upper arm muscles are well developed as a result of the functional demands of 

arboreal quadrupedalism. Those muscles responsible for flexion and extension of the humerus at the shoulder 

joint propel the body during locomotion. The flexors and extensors of the ulna at the elbow are always being 

used to stabilize the continuously flexed elbow, as well as propel the body. The biceps are also used in 

supination. This motion is important in an arboreal setting where the supports may be skewed in relation to the 

long axis of the body. 

The forearm, wrist, and hand: 

The muscles of the forearm, wrist, and hand are also relatively well developed as a result of the functional 

demands of arboreal quadrupedalism. All of the muscles found in Homo are also present in G. crassicaudatus 

with the exception of extensor digiti minimi. In the galago, this muscle is absent. Its functional equivalent is 

found in the extensor digitorum communis. This muscle is like the extensor digitorum of Homo --inserting 

onto the first phalange of digits 2-4, yet extensor digitorum communis also attaches to digit 5. The exacting 

manual dexterity exhibited by Homo is not necessary to G. crassicaudatus This species can afford to move 

digits 2-5 more as a functional unit. 

Crompton, R.H. (1984). Foraging, habitat structure and locomotion in two species of Galago. In Adaptations 

for Foraging in Non-human Primates, ed. P.S. Rodman and J.G.H. Cant, pp. 73-111. New York: Columbia 

University Press. 

Stevens, J.L., Edgerton, V.R., Haines, D.E., and D. M. Meyer (1981). An Atlas and Source Book of the Lesser 

Bushbaby, Galago senegalensis. Boca Raton, Florida: CRC Press, Inc. 

______________________________ 


Saimiri 

______________________________ 

Back and shoulder: 

With few exceptions, the muscles of Saimiri originate and insert in the same fashion as in Homo. Saimiri has 

two muscles not possessed by Homo: the dorsoepitrochlearis and the atlantoscapularis. The 

dorsoepitrochlearis is actually part of the triceps group that runs from the olecranon fascia and the medial 

epicondyle of the humerus to the latissimus dorsi where it attaches. This muscle is important for the arboreally 

adapted quadrupedal squirrel monkey because it aids in extension of the arm at the shoulder and extension of 

the forearm at the elbow. The altlantoscapularis originates on the transverse process of the atlas and inserts on 

the medial part of the cranial border of the scapula. The main function of this muscle is to move the shoulder 

cranially. 

There are a few more differences between Saimiri and Homo in the back and shoulder. Saimiri is characterized 

by undifferentiated rhomboids (major and minor), absence of the serratus posterior inferior, and a more 

robust serratus anterior. The rhomboides major and minor are either undifferentiated or the rhomboides 

minor may be absent. The serratus posterior inferior is occasionally absent in humans. The relatively thick 

serratus anterior is easily understood when one considers the morphological adaptation of the squirrel 


16


monkey. Because this taxon is an arboreal quadruped, this muscle acts as a sling holding up the torso when the 

monkey is in a quadrupedal stance. In addition, it works with the trapezius and the rhomboids to fix the the 

vertebral border of the scapula. This muscle can also aid in respiration. 

The deltoid and the pectoralis group have important roles for Saimiri. In the squirrel monkey, the deltoid 

serves a major role in protraction, retraction and abduction of the arm. All of these actions are very important 

for the quadruped. The pectoralis group is relatively robust, serves as an adductor and medial rotator of the 

humerus, and helps to support the torso on the arm. The pectoralis minor acts as a depressor and inward 

rotator of the scapula. One major difference in the pectoral group is that Saimiri possesses a pectoralis 

abdominalis. This muscle is not present in Homo. The muscle originates on both the costal region and the 

xiphoid process and inserts on the upper humerus. The main function of the pectoralis abdominalis, which is 

very well developed in Saimiri, is to adduct the arm at the shoulder joint. 

Upper arm and shoulder: 


Figure 4: Saimiri arm. (A) 

brachioradialis, (B), deltoid, (C) 

unknown muscle. 

Saimiri differs from Homo in upper arm muscular morphology. Saimiri is characterized by a relatively more 

robust teres major and a well developed and large triceps group (particularly the long head of the triceps). 

The teres major and triceps morphology can best be explained by their function; they are both extensors of the 

arm at the shoulder joint. Arm extension plays an important role in the locomotion of the quadruped. 

The flexor group in the upper arm of Saimiri is essentially the same as in Homo. The main differences lie in the 

relative sizes of the muscles. The coracobrachialis is relatively smaller in Saimiri and functions as an adductor 

and flexor of the humerus at the shoulder joint. The biceps brachii is well developed in the squirrel monkey 

and has two important functions for this arboreal quadruped: it acts as a flexor of the humerus at the shoulder 

joint, and also as a powerful supinator of the forearm. This is extremely important in an arboreal environment. 

Another difference in the upper arm musculature is the well developed and robust brachialis. This muscle acts 

as a powerful forearm flexor in Saimiri and is important for arboreal quadrupedalism. The brachioradialis is 

well developed in the squirrel monkey. Functionally the brachioradialis serves to flex the forearm, an 

important function for the arboreal quaduped. There is one additional muscle that this specimen of Saimiri 

possesses. This muscle appears to originate from the deltoid muscle near its attachment on the humerus and 

runs distally down the arm. It has a slight connection at the anterior bend of the elbow and then continues 

distally down the arm riding just above the brachioradialis and has a common attachment with this muscle. 

17


The forearm, wrist and hand: 

Both the flexor and extensor groups of the forearm and hand are essentially the same in Saimiri and Homo. The 

only major difference is that both of these muscle groups are a little more developed and relatively more robust 

in Saimiri in comparison with Homo. 

Hartman, CG & WL Strauss (1961). The Anatomy of the Rhesus Monkey (Macaca mulatta). New York: Hafner 

Publishing. 

Larson, SG (1994). Functional morphology of the shoulder in primates. In DL Gebo (ed.): Postcranial 

Adaptations in Nonhuman Primates. DeKalb: NIU Press. pp. 45-69. 

______________________________ 

LONG-TAILED OR CRAB-EATING MACAQUE 


______________________________ 

Although the long-tailed macaque relies on quadrupedalism in both arboreal and terrestrial settings, the over all 

patterns of muscle origin and insertion resembles the musculature in humans. Resemble, however, does not 

mean identical, and as we shall see the differences that do exist in Macaca fascicularis help optimize the 

locomotor pattern of the species. Other muscles do exist i.e., panniculus carnosus (see below), that do not 

appear in humans and have little, if any, influence on the movement of the limbs. 

Panniculus: The panniculus muscles are subcutaneous muscles which move the skin. These muscles originate 

from the superficial fascia and insert onto the internal surface of the skin. Most of the panniculus muscles do 

not exert any force, or influence on the limbs of the primate body. The panniculus carnosus does assist in the 

movement of the humerus in the long-tailed macaque. The panniculus carnosus, characterized by a thin, fan 

shape exists in two portions. The caudal part originates from the superficial fascia overlying the anterior 

surface of the thigh and gluteal region, and from the lumbar fascia. The thoracic part originates from the dorsal 

and caudal portions of the skin, and ultimately interdigates with the fibers from the caudal part. While the 

caudal part inserts onto the skin covering the thorax, the thoracic part inserts by a tendon on to the humerus. 

The thoracic portion assists the pectoralis minor in lowering the arm. 

Superficial back and shoulder: 

Trapezius (cervical and thoracic portions): The trapezius orginates and inserts at the same locations as in 

humans. However, where as the clavicular insertion is usually missing in humans, it is always present in 

macaques. The cervical portion of the trapezius aids in elevation of the scapula while the thoracic portion 

adducts, depresses, and stabalizes the scapula. Functionally, the presence of the clavicular insertion may assist 

in, or increase the level of elevation, or caudal to cranial movement, of the scapula. In quadrupedal locomotion 

the scapula moves in a caudal to cranial direction with the spine in a, more or less, horizontal orientation to the 

ground. This differs from bipedal locomotion where the scapula is oriented in the sagittal plane. Elevation of 

the scapula for a bipedal animal occurs when the arm is raised, and not during locomotion. Elevation of the 

scapula does not occur during quadrupedal locomotion, rather the scapula moves in a caudal to cranial direction, 

and perhaps the clavicular insertion of the trapezius aids in this particular movement. 

Atlantoscapularis anterior/posterior: The levator scapulae is the functionally equivalent muscle in humans. 

In the long-tailed macaque the two portions of the atlantoscapularis lie directly underneath the cervical portion 

of the trapezius. Both the anterior and posterior portions of the atlantoscapularis orginate from the transverse 


18


process of the atlas. The anterior portion, inserting on the superior and lateral part of the scapular spine as well 

as on the acromion, assists in the elevation of the scapula. The posterior part, inserting on the vertebral border 

of the scapula superior to the scapular spine, assists in the medial rotation and elevation of the scapula. 

Rhomboid capitis/cervicis/dorsi: Humans posses rhomboid major and minor which are equivalent to 

rhomboid dorsi and rhomboid cervicis in macaques, respectively. The equivalent to rhomboid capitis 

occasionally occurs in humans, and is called rhomboid occipitalis when it does. The capitis part originates 

from the medial secton of the superior nuchal line. The cervicis originates from the ligamentum nuchae. The 

dorsi part originates from the spinous processes of the cervical and the first six or seven thoracic vertebrae. The 

three parts of the rhomboid insert along the vertebral border of the scapula with the capitis portion most 

cranial, and the dorsi portion most caudal. The rhomboids adduct and rotate the scapula. 

Latissimus dorsi: The latissimus dorsi does not extend to the iliac spine in Macaca fascicularis as it does in 

humans. The origin, insertion, and action of the latissimus dorsi are the same in humans and in Macaca 

fascicularis. Unlike humans, where the more medial and caudal portion of the trapezius overlays the latissimus 

doris, in the long-tailed macaque the more medial and cranial portion of the latissimus dorsi overlays the 

trapezius. It appears as if the latissimus dorsi forms a pocket around a portion of the trapezius, and the 

covered portion of the trapezius does not attach to the spinous processes of the vertebral column. 

Serratus anterior: The serratus anterior in the long-tailed macaque derives its origin from the transverse 

processes of the cervical vertebrae, and the inferior border of the first nine to ten ribs. In humans, the serratus 

anterior does not originate from the cervical vertebrae, and it attaches to the superior portion of the ribs. The 

insertion on the vertebral border of the scapula is identical in humans and the long-tail. As in humans, the 

serratus anterior aids in respiration, and abducts the scapula. Unlike humans, the serratus anterior in the 

long-tailed macaque forms a sling around the the ribs and helps in supporting the chest cavity. 

The remaining muscles of the back and shoulder (teres major, teres minor, supra and infraspinatus, and the 

subscapularis) in Macaca fascicularis do not differ from their orientation in humans. 

Secondary muscles of respiration: 

Serratus posterior superior originates from the spinous processes of the thoracic vertebra, and inserts on the 

first through fifth ribs. The muscle consists of three thin, ovoid, shaped bellies and aids in respiration. The 

serratus posterior inferior is approximately five times as large as the serratus posterior superior. The muscle 

consists of six ovoid bellies, each attached to the ribs by a long, thin tendon; each belly is connected to the next 

by a layer of fascia. The muscle fibers run superior laterally, and it also aids in respiration. 


Figure 5: M. 

fascicularis 

medial arm. (A) 

flexor carpi 

radialis, (B) 

palmaris longus, 

(C) biceps brachii 

19


Shoulder: 

The muscles responsible for the motions of the shoulder (the deltoid, pectoralis major, pectoralis minor, and 

pectoralis abdominalis) do not signifcantly differ from their orientation in humans. Most notably, the 

pectoralis major in Macaca fascicularis and Macaca mulatta lacks the clavicular origin which occurs in 

humans, as well as in other primates. The lack of the origin on the clavicle, and the stronger origin from the 

entire sternum is related to the quadrupedal locomotor pattern of the macaque. In quadrupedalism the limbs 

need to move in, and remain in a para-sagital orientation; thus the pectoralis major in macaques aids in holding 

the upper limbs in position in the para-sagital plane. A second, minor difference is noted in the deltoid. Unilke 

humans, and unlike some other primates i.e., Saimiri, the separation of the deltoid into three distinct parts in 

relation to its spinous, acromial, and clavicular insertions is very distinct in the long-tailed macaque. 

Upper arm : 

The upper arm in Macaca fascicularis greatly resembles the upper arm in humans. The brachialis and the 

coracobrachialis are reduced in comparison to humans. The pronators, pronator teres and pronator 

quadratus, and the supinator do not differ from their orientation in humans. The greatest differences lie in the 

triceps complex. The long and lateral heads of the triceps are proportionally much larger than in humans. The 

extent to which the triceps are enlarged may be directly related to the extension of the forearm, and the the 

propulsion of the animals during quadrupedal locomotion. The dorso-epitrochlearis, a part of the triceps 

complex, occurs in Macaca fascicularis and not in humans. This thin, sheet-like muscle which orginates from 

the tendonous insertion of the latissimus dorsi, wraps around the posterior and medial portion of the triceps 

group to insert, by an a tendonous sheet, on to the olecranon fascia. The dorsi-epitrochlearis assists in the 

extension of the forearm. The brachioradalis in the long-tailed macaque has a higher point of origin on the 

deltoid ridge of the humerus than in humans. The insertion and action are the same in the long-tail as they are 

in humans. However, the extent of flexion in long-tailed macaques is limited by the shape and orientation of 

the brachioradalis. When trying to extend the forearm, the brachioradalis becomes taut and for full extension 

would need to be cut. Thus, the forearm remains in partial flexion. 

Flexors of the hand: 

There are no great differences between the flexors of the hand in Macaca fascicularis and in humans. Both the 

flexor carpi radialis, and the palmaris longus show no differences from humans. The flexor carpi ulnaris in 

Macaca fascicularis is proportionally much larger than in humans. The flexor digitorum superficialis lacks 

the radial origin which humans possess. The muscles listed above are responsible for the flexion of the wrist, 

and the flexion of the proximal phalanges, not including the thumb. The flexor digitorum profundus 

eliminates the inidividual muscle of the flexor policis longus by incorporating it into the body of the profundus 

group. As the tendon of the flexor digitorum profundus separates, the tendons insert into the distal phalanges 

of all five of the digits, and when stimulated causes a closed fist to form. 

Extensors of the hand: 

The extensors in Macaca fuscicularis are very similar to those in humans. Extensor carpi radialis longus, 

extensor carpi radialis brevis, and extensor carpi ulnaris show no discernible differences from their 

orientation in humans. The communis group of extensors separates into two groups. The extensor digitorum 

communis originates, as do the other extensors, from the lateral epicondyle of the humerus, and inserts on the 

dorsal side of the first through fourth digits. The tendonous insertions differ from humans in the communis 

group. At first, they appear as four distinct tendons deriving from the communis muscle. The four tendons join 

together as they pass over the metacarpals. Then, as the tendonous sheet approaches the digits, it separates into 

four tendons and they insert on to the digits. A second belly of the communis group, the extensor digitorum 

quatri and quinti proprius, extends off of the larger group and leads to two separate tendons which insert on to 

the fourth and fifth digits. The other extensors; extensor digiti minimi, extensor indicis, extensor pollicus 

longus, extensor pollicus brevis are all present and do not exhibit any great differences from humans. 


20


Agur, M.R. (1991) Grant’s atlas of anatomy. Gardner, J.N. (ed.) Williams and Wilkins. Baltimore, Md. 

Aiello, L., and Dean, C. (1990) An introduction to human evolutionary anatomy. Acedemic Press. San Diego, 

Ca. 

Berringer, O.M., Browing, F.M., and Schroeder, C.R. (1968) An atlas and dissection manual of rhesus monkey 

anatomy. Animal Laboratory Aids. Tallahassee, Fl. 

Netter, F.H. (1989) Atlas of human anatomy. Colacino, S. (ed.) Ciba-Geiba Corporation. Summit, N.J. 

______________________________ 



______________________________ 

Muscles of the superficial back and shoulder: 

When considering the muscles of the superficial back and shoulder in the rhesus macaque, one must keep in 

mind the macaque’s quadrupedal locomotion. Most of the muscles, decribed below in detail, act upon the 

proximal humerus differently than in humans. Most importantly, they are more directly involved in retration 

and adduction of the arm, as a result of their altered insertion points when compared to Homo (see below). This 

is to be expected in quadrupedalism, as the arm must necessarily be kept close to the body and must be able to 

retract in the parasagittal plain with propulsion coming from the legs muscles. 

Unlike Homo, the rhesus macaque pectoralis major has no origin from the medial clavicle. Rather, the muscle 

is composed of the pars capsularis and pars sternalis. The pars capsularis arises from the sternoclavicular joint 

and manubrium, while the pars sternalis originates along the length of the sternum running deep to the pars 

capsularis. As in Homo, both insert upon the anterior border of the intertubercular sulcus of the humerus below 

the synovial joint capsule of the shoulder. 

Action: contrary to Homo, the pectoralis major has less of a role in flexion of the upper limb since it has no 

clavicular origin. Rather, it acts in adduction f the limb, especially from an elevated position. 

Whereas the pectoralis minor inserts upon the coracoid process of the scapula in humans, in rhesus macaques 

it inserts into an aponeurotic sheet that extends long the intertubercular sulcus of the humerus upward over the 

lesser tuberosity to the shoulder joint capsule. 

Action: although the insertion is slightly different from Homo, the muscle produces the same action of rotating 

the scapula and shoulder joint inward and downward (caudalward), working in conjunction with the rhomboids 

and trapezius. 

The pectoralis abdominalis, absent in Homo, originates from the sheath of the rectus abdominis caudalward 

to the xiphoid process and lateral to the midline and inserts upon the humerus by an aponeurosis just distal to 

the insertion of the pectoralis minor, proximal to the panniculus carnosus insertion. 

Action: contraction produces the adduction of the upper limb, aiding the pectoralis major and minor. Also 

functions as an arm flexor. 


21



Figure 6: Rhesus macaque 

superficial arm. Reproduced from 

Hartman & Straus (1933). 

The panniculus carnosus, absent in Homo runs superficial to the other trunk muscles, originating from the 

anterolateral surface of the thigh and gluteal region and from the abdominal wall. The fibers continue cranially 

along the side of the trunk to insert into the deep pectoral aponeurosis, with the pectoral abdominalis insertion 

just proximal. 

Action: mainly causes skin movement. Whereas Hill (1974) does not consider it effective in upper limb 

movement, Hartman and Strauss (1933) note that it aids the pectoralis major in depressing the arm from an 

elevated position. 

Of the superficial back muscles, the trapezius does not significantly differ from that of Homo. The latissimus 

dorsi differs from Homo in that there are no fibers which originate from the crest of the ilium, either fleshily or 

through an aponeurotic attachment. 

Whereas Homo has the rhomboid major and minor, the rhesus macaque has three components composing the 

rhomboideus: 

1) pars capitis, arising from the occipital bone and inserting upon the vertebral border of the scapula 

just above the scapular spine. 

2) pars cervicis, arising from the ligamentum nuchae and inserting caudal to the pars capitis. 

Equivalent to the rhomboid minor in Homo. 

3) pars dorsi, arising caudal to the pars cervicis from the upper thoracic spinous processes and 

inserting caudal to the same muscle on the vertebral border of the scapula, extending to the glenovertebral 

angle. Equivalent to the rhomboid major in Homo. 

Action: despite the slightly different musculature, the rhesus rhomboideus functions to adduct the scapula and 

play a role in scapular rotation, as is the case in Homo. 

The atlantoscapularis posterior resembles the levator scapulae of Homo in that it originates from the dorsal 

aspect of the transverse process of the atlas, inserting upon the vertebral border of the scapula cranial to the 

scapular spine. The rhesus differs markedly from Homo in the atlantoscapularis anterior, arising from the 

22


ventral border of the transverse process of the atlas and inserting upon the lateral half of the scapular spine and 

acromion. Homo lacks anything resembling the atlantoscapularis anterior. 

Action: the atlantoscapularis anterior and posterior both function to rotate the scapula cranially, the former 

muscle being more effective. 

Whereas both the deltoid and pectoralis major have a clavicular origin in Homo, the deltoid dominates the 

clavicle in Macaca mulatta, with the pectoralis major arising from the manubrium and sternum. Unlike Homo, 

the most dorsal aspect of the deltoid arises from an aponeurosis that extends from the scapular spine to the 

glenovertebral angle of the scapula; in Homo, this portion of the muscle arises from an aponeurotic attachment 

both to the spine and superior border of the scapula. Thus, the dorsal portion of the rhesus deltoid is positioned 

more laterocaudally than in Homo. 

Action: the deltoid function in the protraction, retraction, and abduction of the humerus. 

The serratus anterior, supraspinatus, infraspinatus, subscapularis, teres minor, teres major muscles are 

all similar to those of Homo. 

The coracobrachialis differs from Homo in the presence of two component muscles, the coracobrachialis 

profundus and coracobrachialis medius. Homo lacks the former. The coracobrachialis profundus arises partly 

from the coracoid process and from the deep surface of the common coracoid tendon to insert upon the surgical 

neck of the humerus. 

Action: the coracobrachialis profundus rotates the upper limb medially. 

Muscles of the upper arm: 

Overall, the musculature of the upper arm is similar in the macaque and Homo. The main differences arise due 

to the macaque’s quadrupedal locomotion; muscles aiding in active and/or maintained extension of the forearm 

are emphasized both in size and specific insertion sites. 

The brachioradialis originates closer to the insertion of the deltoid on the humerus than in Homo. As a 

powerful forearm flexor, the muscle probably aids the macaque in manipulation of objects with the forelimbs. 

Perhaps the greatest difference in arm musculature from Homo is in the macaque triceps brachii; this 

difference is manifest not in origin/insertion sites, but in sheer size. The macaque has extremely robust triceps 

due to the necessity of supporting its upper body solely on its arms when in the quadrupedal posture. As part of 

this group, and absent in humans, the dorso-epitrochlearis aids in this function. It originates from the tendon 

of the latissimus dorsi and runs along the medial aspect of the humerus to attach by an aponeurosis to the 

superficial olecranon fascia and the medial epicondyle of the humerus. The remainder of the upper arm 

muscles--biceps brachii, brachialis, anconeus lateralis, pronator teres, pronator quadratus, and supinator- 

-are all similar to that of Homo. 

Long flexors of the hand and digits: 

The superficial flexor muscles of the forearm are similar to those in Homo in function, origins, and insertions. 

They differ as far as size is concerned--the flexor carpi ulnaris and palmaris longus are proportionately larger 

and more developed in the rhesus macaque. The flexor carpi radialis is similar to that in Homo. One level 

deeper, the flexor digitorum sublimis differs from the human flexor digitorum superficialis in the absense of a 

radial origin. Moreover, there is a deep insertion of some fibers into the medial margin of the radial portion of 

the flexor digitorum profundus. This latter muscle differs from Homo in that it incorporates the flexor pollicis 

longus in one of its two bellies: 1) the radial portion arises from the medial apect of the radius, and from the 

interosseous membrane; it provides the tendons for digits I-III; 2) the ulnar portion originates from the lateral 

and volar spects of the ulna and also from the interosseous membrane; it provides the tendons for digits III-V. 

Long extensors of the hand and digits: 


23


The superficial extensors of the wrist--extensor carpi radialis longus, extensor carpi radialis brevis, and 

extensor carpi ulnaris--are similar to Homo. The extensor digitorum communis, equivalent to the human 

extensor digitorum, differs from Homo in that the four separate tendons are united as far distally as the 

metacarpal heads, after which they diverge and progress toward digits II-V. Moreover, there is not marked 

continuation of the tendons to the terminal phalanx of each digit, as the case is in Homo. The macaque 

extensor digiti quarti proprius and extensor digiti quinti proprius originate from the lateral epicondyle of 

the humerus and have a common tendon which divides in two parts at variable positions in the forearm. The 

tendon of the e.d. quarti proprius inserts upon the dorsum of the fourth finger; this muscle is absent in Homo. 

The tendon of the e.d. quinti proprius inserts upon the dorsum of the fifith finger; this muscle is equivalent to 

the human extensor digiti minimi. 

Deep to the above, the abductor pollic longus, and extensor pollicis longus are similar to that in Homo; the 

extensor pollicis brevis is absent in the macaque. The extensor digiti secundi proprius is an equivalent 

muscle to the human extensor indicus, inserting upon the dorsum of digit II; however, the macaque also has an 

extensor digiti tertii proprius which arises with the previous muscle from the lateral border of the distal ulna, 

and inserts via a tendon upon the dorsum of the third finger on the basal phalanx. Overall, then, it seems that 

the macaque has tendons inserting upon digits II-V from the communis group and from the individual proprius 

extensors mentioned above. A suggested hypothesis for this difference may be the habitual quadrupedalism of 

the macaque, with the constant extension of the wrist and fingers. 

Hartman, C.G. & Straus, W.L. Jr. (1933). The Anatomy of the Rhesus Monkey. Baltimore: Williams & Wilkins 

Co.; pp. 101-105, 116-120, 128-131. 

______________________________ 


Papio hamadryas: 

______________________________ 

In P. hamadryas the shoulder and forelimb is subject mostly to compressive forces associated with terrestrial 

weight bearing and locomotion. Locomotor patterns involve the movement of the upper limb in the sagittal 

plane, with no need for extensive rotation and abduction of the arm. P. hamadryas differs from Homo mainly in 

the musculature related to shoulder/arm mobility and scapular rotation. For terrestrial quadrupedal primates 

like P. hamadryas, the musculature shows efficient positioning for producing a greater degree of protraction (vs. 

rotation) of the scapula and retraction and protraction (vs. abduction) of the humerus. In addition, there is less 

demand on the pronators, supinators, and wrist deviators in P. hamadryas, who do not extensively use these 

muscles for climbing and limb grasping. 

Muscles that move the arm and pectoral girdle: 

P. hamadryas possess four muscles in this region that are not found in Homo. 1) Panniculus carnosus: This 

muscles overlies the latissimus dorsi. Muscle fibers arise along the gluteal and lateral thoracic region, are 

thickest ventrally, and form a tendon that attaches to the proximal part of the humerus, deep to the pectoralis 

musculature. The panniculus carnosus, relatively large in P. hamadryas, allows the hamadryas baboon to move 

its skin and aids the pectoralis major in depressing the arm when it is in an elevated position. 2) Pectoralis 

abdominus: This muscles originates from the sheath of the rectus abdominus and inserts into the proximal 

humerus. This relatively large muscle in P. hamadryas adducts and medially rotates the humerus. 3) 

Atlantoscapularis anterior: This muscle originates on the ventral part of the atlas and inserts into the cranial 

margin of the acromion and lateral scapular spine. 4) Atlantoscapularis posterior: This muscle originates at 

the dorsal part of the transverse process of the atlas and inserts at the vertebral border of the scapula between the 


24


spine and superior angle. The levator scapulae muscle of Homo is represented by these last two muscles. These 

muscles move the scapula cranially (anterior) and move the superior angle of the scapula laterally (posterior). 

In addition to the four muscles described above, there are a number of differences between P. hamadryas and 

Homo musculature in the superior back and shoulder region. 1) Latissimus dorsi: The origin is the same as 

Homo, but in addition to its insertion on the humerus, P. hamadryas displays a cranial portion of the muscle that 

attaches to the teres major, which helps produce more efficient retraction of the humerus in the sagittal plane. 

In addition, Homo also has more extensive posterior attachment at the iliac crest, lumbodorsal fascia, and on the 

inferior angle of the scapula. 

2) Triceps brachii: The triceps is divided into three heads: long, lateral and medial. The attachment of the 

long head differs from Homo. In P. hamadryas, the long head arises from an extensive area along the axillary 

border of the scapula (it is limited to the infraglenoid tubercle of the scapula in Homo). The three parts 

converge about mid-shaft to form a broad tendon and continues to receive fibers from the humerus nearly as far 

as the olecranon fossa in P. hamadryas. Again, the extensive attachment of the long head affords more 

retraction of the humerus and extension of the forearm for quadrupedal locomotion. 

3) Deltoid: The deltoid is composed of three parts: clavicular, acromial, and spinal, and is easily divisible into 

its three parts in P. hamadryas. The clavicular portion has an extensive attachment to the clavicle and limits the 

pectoralis major to the region of the sternoclavicular joint. The cephalic vein intervenes between the caudal 

border of the deltoid and the cranial border of the pectoralis major, but the division is not a precise as in Homo. 

The attachment of the deltoid on the humerus (on the deltoid turberosity) is more proximal in P. hamadryas than 

in Homo and brachiating primates. In the terrestrial quadrupedal P. hamadryas, it is a powerful protractor and 

retractor of the arm. The more distal placement in brachiators is interpreted as offering a greater mechanical 

advantage in lifting the humerus as they move through the trees. Homo also displays a more brachiator like 

pattern in the ability to lift the humerus. 

4) Rhomboids: The rhomboideus major and minor are not as easily divisible in P. hamadryas as in humans. 

In Homo, these muscles adducts and rotates the scapula, and scapular rotation is not as important for P. 

hamadryas' terrestrial lifestyle. 


Figure 7: Baboon right shoulder: (A) atlantoscapularis anterios, (B) infraspinatus, (C) teres minor, 

(D) triceps brachii, (E) teres minor, (F) latissimus dorsi, (G) dorsoepitrochlearis. 

25


Muscles that move the forearm: 

P. hamadryas possess one muscle in this region that is not found in Homo. 1) Dorsoepitrochlearis: This 

muscle is a part of the triceps that has become secondarily attached to the latissimus dorsi. The muscle arises 

near beginning of the tendon of the latissimus dorsi, accompanies the long head of the triceps to the elbow and 

passes to the medial epicondyle of the humerus. The action of this muscle is to extend the forearm. 

In addition to dorsoepitrochlearis, there are a number of differences between P. hamadryas and Homo arm 

musculature. 1) Biceps brachii: This muscle, a flexor and supinator of the forearm, has similar attachments in 

both Homo and P. hamadryas. However, the relative proportions of the biceps and triceps differ between the 

two - P. hamadryas has relatively larger triceps and Homo has relatively larger biceps. This reflects the need in 

terrestrial quadrupedal locomotion for powerful extension (retraction) of the arm at the shoulder. 

2) Anconeus lateralis: In humans, this muscle is simply called anconeus. This muscle, which originates on 

the posterolateral side of the elbow and attaches just below the elbow on the lateral ulna, is relatively larger in 

P. hamadryas. Anconeus lateralis aids in extending the elbow. 

3) Pronator teres: P. hamadryas has only one head of origin (medial epicondyle of the humerus), while 

humans have an additional origin on the medial border of the ulna. 

Mucles that move the wrist and hand: 

P. hamadryas possess four muscles in this region that are rare or absent in Homo. 1) Extensor digiti quatri 

proprius: This muscle is rarely found in humans, and when it occurs, it is normally referred to as extensor 

digiti angularis. This muscle originates on the lateral epicondyle of the humerus and inserts into the dorsal 

surface of the proximal phalanx of digit IV. Its action is to extend the fourth metacarpophalangeal joint. 2) 

Extensor digiti tertii proprius: This muscle is usually absent in humans, but when it does occur, it is referred 

to as extensor digiti medius. It originates in common with the extensor digit secundi proprius on the dorsal 

surface of the ulna and attaches to the dorsum of the proximal phalanx of digit III, which it extends. 

In addition to the two muscles described above, there are a number of differences between P. hamadryas and 

Homo musculature of the forearm. 1) Brachioradialis: The proximal attachment of this muscle is higher 

(more proximal) in P. hamadryas than Homo; it extend up to the insertion of the deltoid.). Brachioradialis is a 

forearm flexor. 

2) Palmaris longus: This muscle, frequently absent in Homo, originates from the medial epicondyle of the 

humerus and attaches into the palmar aponerosis. 

3) Flexor digitorum superficialis: Both P. hamadryas and Homo have origins of flexor digitorum 

superficialis on the medial epicondyle of the humerus, but Homo also has an ulnar and radial head. Both 

species show attachments to the middle phalanges of digits II-V. 

4) Extensor digitorum: The origin is similar to Homo, but in P. hamadryas, the insertions are in the sides of 

the proximal phalanges and bases of middle phalanges of digits II-V. This contrasts humans which have 

insertions into the dorsal surface of all three phalanges of digits II-V. 

5) Extensor digiti quinti proprius: This muscle, which extends the fifth metacarpophalangeal joint is 

commonly called extensor digiti minimus in humans. 

6) Extensor digiti secundi proprius: In humans, this muscle is called extensor digiti indicus. It originates 

on the distal aspect of the dorsal surface of the ulna in common with the extensor digiti tertii proprius and 

attaches onto the proximal phalanx (dorsal surface) of digit II, which it extends. 

Extensor pollicus brevis and flexor pollicus longus, each normally a differentiated muscle in Homo are not 

found in P. hamadryas. 


26


______________________________ 

SPIDER MONKEY 

Ateles 

______________________________ 

As with most other primates, the musculoskeletal anatomy of the genus Ateles greatly resembles that of the 

modern humans, genus Homo. The primary differences lie in the relative sizes of specific muscle groups and in 

the origins and insertions of the various muscles. Also, certain muscles found in Ateles (and other nonhuman 

primates) are lacking in Homo while other muscles distinct in humans are either fused or undifferentiated in the 

spider monkeys. 

As a whole, the upper limb, back, and shoulder regions in Ateles are very similar to the corresponding regions 

in Homo. The muscles are well differentiated and well developed for suspension. The shoulder, elbow, and 

wrist joints are very flexible and have the pronounced ability to support the mass of the animal in a hanging 

position below limbs as well as above the limb. The muscles and joints are well adapted to provide for minor 

modifications in the suspensory position of the animal. This is particularly important in a genus whose 

members use their tails and both the upper and lower limbs in varying combinations while both in locomotion 

and nonlocomotory behavior. The muscles and joints of the lower limb have similar abilities and functions with 

regard to suspensory behavior. However, the lower limb is used much less for locomotion and therefore the 

relative sizes of the muscles are smaller than in most primates, including humans. The muscles are also fused in 

many instances and have origins and insertions on the tendons of other muscles. 

Overall, the musculature of the upper limb and the back and shoulder regions is much more developed and 

differentiated than in the lower limb, hip, and thigh regions due to the emphasis placed upon locomotion using 

the upper body. The propulsion normally provided by the hindlimbs is instead accomplished using the tail. 

This includes the "leaping" used to cross gaps in the trees, where the power arises from the tail. Thus, the 

hindlimb musculature is less developed. Correspondingly, the intermembral index for Ateles is 105, reflecting 

the practice of semi-brachiation (arboreal quadrupeds normally have shorter upper limbs in order to lower the 

center of gravity close to the support). The brachial index is high and thus evident of the increased use of the 

forearms for suspension. 

Back and Shoulder: 

With the exception of the pectorals, the muscles of these regions are well developed and closely correspond to 

those in humans. However, those muscles which elevate the arm are relatively larger as are those which 

function to retract the arm. Those muscles which are relatively larger or have more extensive origins and 

insertions include Latissimus dorsi, Trapezius, Serratus anterior, Teres major, Atlantoscapularis, and the 

rhomboids. Latissimus dorsi differs considerably in its increased thickness over that in humans and its 

increased iliac insertion. Interestingly, spider monkeys have an iliac origin more extensive than that of humans 

and less than that of the great apes while the true brachiators, genus Hylobates, have no insertion onto the ilium. 

Another differenence in this muscle is the lack of a small slip of origin on the point of the scapula. Trapezius 

differs from its human morphology in the increased size but decreased vertebral attachment. The muscle 

originates most inferiorly on the vertebrae attached to the true ribs rather than the floating ribs. Serratus 

anterior, Teres major, Atlantoscapularis, and the rhomboids have similar origins and insertions to humans 

but are relatively thicker and larger. 

Other differences in the back and shoulder region between Ateles and Homo are the decreased size of the 

pectoral muscles, the absence of Serratus posterior inferior, and the additional origin of Pectoralis minor on 

the humeral head in addition to the coracoid process of the scapula. 


27


Upper Arm and Shoulder: 

The morphology of the upper arm musculature is largely similar to that of Homo. The major difference lies in 

the more prominent muscles which extend the forearm. Triceps brachii differs in that the medial head is 

attached more proximally to the humerus. Additionally, the long head of Triceps is attached to Teres major by 

connective tissue and extends more anteriorly to the medial side of the proximal humerus. Triceps brachii is 

complemented by the presence of an additional muscle not present in humans, Dorsoepitrochlearis. This 

muscle originates from the tendon of Latissimus dorsi and inserts into the humerus on the medial epicondyle. 

Among the flexors of the forearm, few differences arise between the morphology of Ateles and that of Homo. 

Biceps brachii differs from its human morphology only in the attachment to the Pronator teres by thick 

connective tissue. Brachialis morphology differs from that in humans only in the more proximal humeral 

origin. Brachioradialis inserts more distally onto the styloid process of the radius. Additionally, this muscle is 

much larger in Ateles. 

Forearm: 

The forearm region of the spider monkey is largely the same as that of humans with the important exception 

that no pollex is present in Ateles. This lack of an opposable thumb is thought to have arisen to reduce 

interference during brachiation. Interestingly, the absence of a pollex also occurs in the genus Colobus while 

gibbons and siamangs do retain the thumb. This would seem to refute the theory that a reduced or absent pollex 

is an adaptation to brachiation. The other major difference in the forearm region is the large size of many of the 

flexors, extensors, and rotators in the spider monkey due to the increased use of the forearm during locomotion. 

Within the hand region, the four remaining digits form functional hooks for suspending below branches. 

The flexors of the hand in Ateles differ in few ways from those in Homo. Flexor carpi ulnaris originates 

normally but remains attached to the ulna along its entire length. This tendonus attachment is fused with that of 

Flexor digitorum superficialis from the common flexor origin to the distal end of the ulna. Flexor digitorum 

profundus has a morphology which differs from the human form due to the division of the muscle into two 

major parts which are connected at their tendons. The portion originating from the radius inserts into the 

second digit. The portion originating from the olecranon process of the ulna inserts into the phalanges of the 

second through fifth digits. Finally, the absence of a pollex has resulted in the absence of Flexor pollicus 

longus. 

The extensors of the hand are largely the same with one major exception regarding supination of the hand. 

Extensor carpi ulnaris has an additional origin from the olecranon process of the ulna. Extensor digiti 

minimi has an additional insertion into the ulna near the wrist. Extensor indicus originates along virtually all 

of the ulnar border and inserts into the second through fifth digits. Finally, the major distinction regarding the 

extensors deals with the lack of the pollex. Extensor pollicus longus and Extensor Pollicus Brevis are both 

absent while an extensive abductor has become the predominant muscle in the region. Abductor pollicus 

originates from the radius and ulna very proximally near the elbow and inserts onto the metacarpal of the first 

digit (the only remnant of the pollex). The attachments to the radius and ulna extend along the entire lengths of 

the two bones, thus providing an extremely large and mechanically efficient supinator of the hand. This 

increased ability to laterally rotate the hand assists in suspensory type locomotion and posturing. 

______________________________ 

ORANGUTAN 


______________________________ 

Back and shoulder: 

Basically, most of the back and shoulder musculature in the orangutan are identical to those found in humans. 

The differences arise in the extent of origins and insertions of muscles and the resulting activity differences. 


28


Non-human hominoids have two muscles that are not found in humans: the atlantoclavicularis and pectoralis 

abdominis (the latter of these is also found in other primates). The atlantoclavicularis originates on the 

spinous process of C2 and inserts on the lateral margin of the clavicle. It functions as a clavicular elevator. (On 

this particular specimen, due to an autopsy, the clavicles were broken and the muscles around them damaged, so 

this particular muscle is not clearly defined.) The pectoralis abdominis originates on the caudal portion of the 

rib cage and inserts onto the humerus near the insertion point of the other pectoralis muscles, and it functions as 

an arm flexor. (Again, due to the autopsy, most of the pectoralis muscles have been damaged and the rib cage 

cut so this particular muscle could not be found in my specimen.) 

The muscles discussed in this paragraph have different functions or attachments than their counterparts in 

humans. The trapezius has a more extensive (thicker) cranial portion than humans, as well as originating from 

more thoracic vertebrae (on this particular specimen, the trapezius originated from ~T8/9) than in humans. The 

serratus anterior has a large caudal portion with muscle fibers arising from a greater number of ribs with the 

most inferior fibers oriented almost directly craniocaudally. The size and orientation of the fibers indicate that 

the serratus anterior functions as a powerful scapular elevator/rotator in the orangutan. The latissimus dorsi 

is a laterally expanded muscle in the orangutan with the lateral fibers originating directly on the iliac crest. This 

muscle functions as an arm retractor and helps in forward movement/propulsion of the lower limb when 

climbing (or in suspensory postures) due to the direct attachment to the iliac crest. Both the supraspinatus and 

infraspinatus are relatively larger muscles in the orangutan, contributing to the power/shoulder mobility seen 

in this species. The large teres minor seems to have a slightly different function in orangutans, based upon 

EMG studies; it does not function in arm elevation; rather, it is an arm retractor. The deltoid is a large muscle 

which helps in maintenance of arm elevation. The subscapularis in orangutans functions as a strong medial 

arm rotator. The rhomboids have an extended origination in the orangutan when compared to humans. They 

originate from more thoracic vertebrae (past T1) than in humans. The atlantoscapularis anterior and posterior 

(found in non-hominoids) are represented in the orangutan as levator scapulae (the form seen in humans). For 

the most part, the muscles that are larger in the orangutan can be associated with demands related to bodily 

support during suspensory and/or climbing activities. 

Upper arm: 


Figure 8: The 

orangutan arm. 

(A) biceps 

brachii, (B) 

brachialis, (C) 

brachioradialis. 

Similar to the back and shoulder musculature, the muscles found in the upper arm in the orangutan are mostly 

identical to humans, with main distinctions lying in the attachments, size and sometimes the functions of 

muscles. The orangutan, unlike humans, has a dorsiepitrochlearis. The dorsiepitrochlearis originates from 

29


the tendon of the latissimus dorsi and inserts onto the medial epicondyle of the humerus and functions as an 

arm fascia tensor. 

The orangutan (like most apes) demonstrates larger and more powerful arm flexors and supinators. The 

robusticity in these muscles (when compared to humans) relates to the inclusion of the forearms in positional 

and locomotor behavior. The radioulnar joint allows for a great range of supination and pronation of the 

forearm in orangutans (almost as much as what is seen in humans). The arm flexors (brachialis, 

brachioradialis and biceps brachii) as a group are more powerful than the extensors (triceps brachii and 

anconeus). The arm pronators (pronator teres and pronator quadratus) are also rather large muscles in the 

orangutan. These differences can be noticed through the relative sizes of those muscles in the orangutan. The 

brachioradialis has a higher insertion in the orangutan than non-hominoids. 

Elbow and forearm: 

Much like the previous two muscular groups within this section, the orangutan shares much of its musculature 

with humans. Again, there are some differences which are noted below. The orangutan (like all non-human 

primates) lacks a distinct flexor pollicis longus tendenous insertion onto the distal thumb phalanx which is seen 

in humans. The flexor digitorum superficialis of the orangutan has three heads (radial, ulnar and humeral) 

like the form in humans, unlike the form in other primates. The orangutan also lacks an extensor digiti quarti 

proprius, which is found in other primates. 

The flexor digitorum profundus has two bellies. The radial belly (flexor digitorum radialis) originates from 

the ventral portion of the proximal radius and usually has tendenous attachments onto digits I - III (thumb and 

index). The ulnar belly (flexor digitorum ulnaris) originates from the medial and ventral portions of the 

proximal ulna and interosseus and inserts onto digits IV-V. The radial belly is known as the flexor pollicis 

longus in humans (and can be called that in apes, but they have different tendenous attachments). Similar to 

most of the arm musculature, the long flexors of the orangutans’ hands are better developed and more powerful 

than in humans, due to the orangutans postural and locomotor behaviors (suspension and climbing). 

Aiello L and C Dean (1990) An Introduction to Human Evolutionary Anatomy. New York: Academic. 

Larson SG (1994) Functional morphology of the shoulder in primates. In DL Gebo (ed.): Postcranial 

Adaptations in Nonhuman Primates. DeKalb: NIU Press. pp.45-69. 

Tuttle RH and GW Cortright (1988) Positional behavior, adaptive complexes, and evolution. In JH Schwartz 

(ed.): Orangu-utan Biology. New York: Oxford. pp.311-330. 

______________________________ 

COMPARATIVE MUSCULATURE 

The Forelimb 

______________________________ 

The locomotor adaptations of primates are reflected in the orientations, proportions, and functions of the 

musculature in the shoulder and forelimb. We dissected nine primate species: Saquinis oedipus, Saimiri 

scuireus, Ateles geoffroyi, Galago crassicaudatus, Macaca fascicularis, M. mulatta, Papio hamadryas, Pongo 

pygmaeus, and Homo sapiens. These primates represent the range of locomotor patterns from arboreal 

quadrupeds (S. oedipus, S. scuireus, G. crassicaudatus, M. fascicularis), to terrestrial quadrupeds (M. mulatta, 

P. hamadryas), to semi-brachiators and quadrumanus, suspensory climbers (A. geoffroyi, and P. pygmaeus, 

respectively), to bipeds (H. sapiens). This section will discuss the differences in musculature of the primate 

shoulder and forelimb, and will relate the differences to each primate’s respective locomotor category. 


30


Muscles which move the shoulder: trapezius, latissimus dorsi, levator scapulae (hominoids), 

atlantoscapularis anterior and posterior (non-hominoids), rhomboids, supraspinatus, infraspinatus, teres 

major, and minor, subscapularis, serratus anterior, pectoralis major, minor, and abdominalis, and the 

deltoid. 

A major distinction between hominoids and non-hominoids in the shoulder is the latter’s lack of a levator 

scapulae. Non-hominoids, all primates that are not apes or humans, posses the atlantoscapularis anterior and 

posterior. The muscles seem to be functionally equivalent (both levator scapulae, and the atlantoscapularis 

muscles assist in the elevation of the scapula); thus, perhaps the atlantoscapularis muscles are 

symplesiomorphies for all lower primates. A second distintion between hominoids and non-hominoids is the 

latter’s possession of a pectoralis abdominalis. This muscle aids in the adduction of the upperlimb, and is not 

present in humans or the orangutan. 

The latissimus dorsi, trapezius, and rhomboids in arboreal and terrestrial quadrupeds tend to be reduced in 

size in comparison to humans and the orangutan. However, the latissimus dorsi in G. crassicaudatus was 

noted to be more developed than in humans. The reduction in the size of these muscles appear to be related to 

quadrupedal adaptations. A quadruped lacks the need to habitually raise its arms over its head, and the muscles 

(latissimus dorsi, trapezius, rhomboids) which assist in the rotation and elevation of the scapula and arm are 

correspondingly reduced. The relative robustness of the latissimus dorsi in G. crassicaudatus is most likely 

related to the galago’s ability to engage in vertical clinging and leaping as well as in quadrupedalism. Arboreal 

and terrestrial quadrupeds also appear to have more robust development in the teres major, pectoralis major, 

and serratus anterior. The teres major and the pectoralis major adducts and medially rotates the humerus in 

the shoulder joint; these motions keep the forelimb close to the body in the para-sagittal plane. A quadruped’s 

limbs are always oriented in the para-sagittal plane. The enlargement of the serratus anterior is related to its 

function as a sling to support the ribcage and thorax in quadrupeds. 

The shoulder musculature in A. geoffroyi and P. pygmaeus is proportionally larger than in humans and the 

quadrupedal primates. It was noted in the Ateles and Pongo sections that the shoulder muscles were extremely 

robust, and that their size was related to the expanded range of motion, and strength required by semibrachiating 

and quadrumanus, suspensory climbing. The orangutan, and the spider-monkey are similar to 

humans in that the deltoid in all three taxa cannot be separated into three (spinous, acromial, clavicular) distinct 

sections. The deltoid in the quadrupedal primates could be divided into three sections. 

Muscles which move the arm: the tricep and bicep groups. 

The most distinctive difference between the primates studied was the possession of a dorso-epitrochlearis in 

all the primates except the human. The dorso-epitrochlearis assists the triceps group in extending the forearm. 

In the smaller arboreal quadrupeds (S. oedipus, S. scuireus, and G. crassicaudatus) the triceps and biceps 

groups were proportionally larger than in humans. The larger arboreal quadruped (M. fascicularis), and the 

larger terrestrial quadrupeds (M. mulatta, and P. hamadryas) were disinguished by having very robust triceps in 

comparison to their moderately reduced biceps groups. In all of these quadrupedal primates, the triceps is 

enlarged for the rapid and powerful extension of the forearm during locomotion. The larger biceps group in the 

smaller arboreal quadrupeds may be indicative of their occasional leaping behavior. 

Ateles also has well developed triceps and biceps muscles. Remember that Ateles is known for its eclectic 

locomotor behavior, and perhaps the well developed triceps and biceps in the species is related to the use of 

many locomotor behaviors. P. pygmaeus has proportionally huge biceps muscles. The hypertrophy of the 

biceps group in the orangutan is expected because the orangutan constantly hangs, and lifts its own body weight 

in suspensory behavior. 

The flexors in P. pygmaeus are proportionally much larger than any primate discussed here. Again, the 

development of the flexors in the orangutan is due to the orangutans suspensory climbing behavior. Ateles also 

displays some hypertrophy of its flexors. The arboreal and terrestrial quadrupeds also have more developed 

flexors in comparison to their extensors. 


31


The shoulder and upper limb in primates differs in proportion from humans. The muscular proportions relate to 

the locomotor adaptations of primates. Quadrupeds have reduced shoulder musculature, enlarged triceps, and 

more developed flexors in comparison to humans. Ateles has more developed shoulder and arm musculature, 

and the orangutan exhibits muscular hypertrophy in all areas of the shoulder, and upper limb except for the 

extensors of the hand. When examining muscles of any animal, always relate what you see in the animal to the 

locomotor behavior of the animal. 

______________________________ 

COMPARATIVE OSTEOLOGY 

The Shoulder 

______________________________ 

The shoulder joint is comprised of three bones, the clavicle, scapula and humerus. Between these bones there 

are four articulations: (1) glenohumeral (shoulder), (2) sternoclavicular, (3) acromioclavicular (scapula and 

clavicle) and (4) scapulothoracic. The last one is not a joint proper since the two bony regions never articulate 

directly; however, the scapula does move in relation to the thoracic vertebrae. Each of the four articulations can 

move independently, but act together to produce overall shoulder movements. 

The clavicle is an S-shaped bone which provides the only bony articulation between the upper limb and the 

body. The superior surface of the clavicle is relatively smooth, with the only morphology being the deltoid 

tuberosity. This tuberosity is a roughened, raised area on the anterior surface of the clavicle which depicts the 

most medial attachment of the deltoid muscle. The inferior surface of the clavicle displays more morphological 

landmarks. This area provides attachment points for shoulder stabilizing ligaments and the subclavius muscle 

(which also functions in joint stability). 


Figure 9: Primate scapulae. Clockwise from upper left: Saguinus, Galago, Saimiri, Macaca 

fascicularis, M. Mulatta, Papio, Ateles, Homo. 

32


The scapula is an irregularly shaped bone which can be roughly divided into two areas: (1) scapular blade and 

(2) glenoid. Viewed from the dorsal surface, the most notable feature on the scapula is the spine. It runs 

horizontally across the bone from the medial (vertebral) border and ends in the acromion process. The region 

above the spine is called the supraspinatus fossa and provides the origin for the supraspinatus muscle. The 

region below the spine is called the infraspinatus and provides the origin for the infraspinatus muscle. The 

ventral portion of the scapula is concave and is called the subscapular fossa. The muscles which are responsible 

for upward and downward scapular rotation insert along the blade edges. The glenoid region contains two main 

features: (1) coracoid process and (2) the glenoid fossa. The coracoid process projects laterally in front of the 

glenoid fossa. The coracoid provides area for ligament and muscular attachment. The ligaments aid in joint 

stability and the muscles in arm movement. The glenoid fossa is a shallow concave region which articulates 

with the humeral head. 

The proximal humeral anatomy includes the head, greater and lesser tubercles (tuberosities) and the bicipital 

groove. The head is rounded and articulates with the glenoid fossa to form the shoulder joint proper. The 

greater and lesser tubercles provide attachment area for the insertion of the rotator cuff muscles. The bicipital 

groove separates the greater and lesser tubercles. 

Locomotor Behavior: 

The shoulder joint in most primates provides an integral aspect of locomotion. Many of the contrasts in the 

anatomy are made using the dichotomy of suspensory verses quadruped. For this section, the only primate 

which displays any suspensory morphology is Ateles. The most contrast within the shoulder morphology 

should be between both Macaca species and Papio verses Ateles. Saguinus, Galago and Saimiri should look 

more like the quadrupeds. 

General scapular shape is different when comparing Ateles to more traditional quadrupeds. These differences 

are related to scapular rotation. More suspensory primates have wider ranges of motion at the shoulder. In 

order to facilitate more power within shoulder rotation the insertions for the trapezius and serratus anterior are 

further apart. Osteologically, this muscular difference is reflected in elongation of the vertebral border. The 

relative sizes of the supra- and infraspinatus fossae differ between quadrupeds and more suspensory primates. 

In suspensory primates, these fossae are larger, due to craniocaudal widening. 

Another feature of the scapula which displays different morphology between suspensory and quadrupedal 

primates is the degree of lateral projection of the acromion. In more suspensory primates, the acromion projects 

further laterally than in quadrupeds. The degree of acromion projection has been linked with deltoid muscle 

function in glenohumeral elevation. Other features of the glenoid region which displays different morphology 

between suspensory and quadrupeds are the shape and orientation of the glenoid fossa. Quadruped primates 

have an elongated glenoid fossa in the cranio-caudal dimension. This cranio-caudal elongation allows a better 

range of motion within the parasagittal plane. The cranial margin of the glenoid fossa in quadrupeds also 

displays lipping, which prevents glenohumeral dislocation during extreme humeral retraction. The glenoid 

fossa in suspensory primates is more ovate, which allows for a greater range of motion. The glenoid fossa in 

suspensory primates also is more cranially oriented to facilitate a larger range of motion. 

In order for the arm to function normally, the humerus needs to face forward. In suspensory primates, the 

thoracic cavity is medio-laterally broadened and the scapulae rest on the back. In order for the humeral head to 

properly articulate with the scapula, either the arms face laterally or the head faces medially. Suspensory 

primates display medial torsion of the humeral head. Quadrupeds’ humeral heads face directly posteriorly. In 

suspensory primates, the humeral head is also more spherical, allowing for circumduction. In quadrupeds, the 

humeral head is flatter and narrow. 

Another region of contrast between suspensory and quadrupedal primates is the relative heights of the humeral 

head and greater tubercle. In suspensory primates, the greater tubercle is small in comparison with the humeral 

head. In quadrupeds, the greater tubercle projects above the humeral head. The differences in the relative 

heights of these two structures relates to the mechanical advantage of the supraspinatus. Increasing the size of 

the greater tubercle increases the lever arm, thus reducing the amount of force needed from the supraspinatus 


33


during arm elevation. In suspensory primates with lower greater tubercle height, larger supraspinatus muscles 

compensate for the lack of mechanical advantage. 

______________________________ 


The Elbow Joint 

______________________________ 

The elbow joint is defined as the articulation among the distal humerus, the proximal radius and the proximal 

ulna. This joint allows for flexion and extension as well as pronation and supination. Aiello and Dean (1990) 

describe the elbow as a third class lever, indicating that it is most efficient at moving often and fast but is only 

able to displace a small amount of weight. 

The morphology of the three bones that comprise the elbow joint will be briefly reviewed here. The distal 

humerus has both a medial epicondyle and a lateral epicondyle. The medial epicondyle is where almost all of 

the wrist flexors originate as well as some finger flexors. The lateral epicondyle is the source for the wrist 

extensors and some finger extensors originate here also. The trochlea is located on the anteriomedial aspect of 

the distal humerus and is the location where the ulna articulates with the distal humerus. The capitulum can be 

found on the anteriolateral aspect of the distal humerus. The capitulum is usually rounded and is the point of 

articulation between the humerus and the radial head of the radius. The olecranon fossa is located on the 

posterior aspect of the distal humerus. This is the location where the olecranon process of the ulna contacts that 

distal humerus. 

The two major features of the proximal radius are 1) the head of the radius and 2) the tuberosity of the radius. 

The radial head is the part of the radius that articulates with the capitulum of the humerus. The radial tuberosity 

is the point of insertion of the biceps brachii muscle. 

The proximal ulna has the following features: 1) olecranon process, 2) trochlear notch, 3) coronoid process, 4) 

radial notch. The olecranon process is located on the most proximal portion of the ulna. The olecranon process 

in the point of insertion for the triceps brachii muscle, and in some primates it is also the location where the 

dorsoepitrochlearis muscle inserts. The trochlear notch of the proximal ulna articulates directly with the 

trochlea of the distal humerus. The coronoid process of the ulna marks the most distal portion of the trochlear 

notch. The radial notch of the proximal ulna is the point of articulation between the ulna and the radial head. 

Locomotor Behavior: 

As is evident from the anatomical description above, locomotor adaptations are reflected in the osteology of the 

elbow region. Generally speaking primates that engage in suspensory locomotion have relatively large and 

medially extensive medial epicondyles. This is the result of having large and powerful wrist flexors. Note the 

medial epicondyle morphology of Ateles. This taxon primarily moves about by suspensory locomotion. 

Arboreal quadrupeds also have a large and medially projecting medial epicondyle, but it is not as robust the 

medial epicondyle possessed by suspensory primates. The hands and wrists of arboreal quadrupeds are often in 

different degrees of pronation and supination. Their large medial epicondyle provides leverage for the flexors 

of the wrists and hands in all degrees of pronation and supination. Observe the medial epicondyle in the 

following arboreal quadrupeds: Galago crassicaudatus, Saguinus oedipus, Saimiri and Macaca fascicularis. 

Contrast these species with the suspesory primates. Because the hand of a terrestrial quadruped is pronated 

while in locomotion, their medial epicondyle is pointed dorsally and does not have an extensive medial 

projection. This orientation and morphology of the medial epicondyle aids the wrist and hand flexors while the 

hand is in pronation. Note the medial epicondyle of Macaca mulatta and Papio hamadryas and compare these 

species to the primates that engage in arboreal quadrupedalism and suspension. 


34



Figure 10: Primate humerii. From left: Saguinus, Galago, Saimiri, Macaca fascicularis, M. 

Mulatta, Papio, Ateles, Homo. 

The relative sizes of the capitulum and trochlea of the distal humerus can also be informative about the 

locomotor adaptations of terrestrial quadrupeds. Terrestrial quadrupeds have a relatively large capitulum and a 

narrow trochlea. The capitulum, oriented distally, transmits force to the radius, a major weight bearing bone. 

Note the capitulum size and orientation in both Macaca mulatta and Papio hamadryas. Contrast the articular 

region of the distal humerus of these terrestrial quadrupeds with this same region of the arboreal quadrupeds. 

The morphology of the olecranon process of the ulna and the olecranon fossa of the distal humerus reflects 

locomotor adaptations of different groups of primates. Suspensory primates have short olecranon processes. 

Note the condition of this feature in Ateles. Arboreal quadrupeds have a long olecranon process and a shallow 

olecranon fossa. The long olecranon process allows for more powerful forces to be exerted by the triceps 

brachii muscle and the dorsoepitrochlearis muscle. The shallow olecranon fossa of arboreal quadrupeds reflects 

the fact that the forearm of these primates is seldom fully extended. For the most part, the arboreal quadrupeds' 

forearm remains slightly flexed during locomotion. Note the olecranon fossa and olecranon process on the 

following primates: Galago crassicaudatus, Saguinus oedipus, Saimiri and Macaca fascicularis. The 

morphology of the olecranon process of terrestrial quadrupeds is indicative of their mode of locomotion. The 

process extends posteriorly to allow for maximum leverage of the triceps while the forearm is straight as 

opposed to flexed (as it is commonly in arboreal quadrupeds). The olecranon fossa of the distal humerus is 

relatively deep in terrestrial quadrupeds. Again, this is the result of a forearm that is commonly fully extended. 

Note these two features on the following two terrestrial quadrupeds: Macaca mulatta and Papio hamadryas. 

Contrast this region among these three locomotor categories. 

Fleagle, JG (1988). Primate Adaptation and Evolution. San Diego: Academic Press. 

Rose, MD (1988). Another look at the anthropoid elbow. Journal of Human Evolution 17:193-224. 

Senturia, SJ (in press). The morphometry and allometry of the primate humerus. Primates. 

Szalay, FS and Dagosto, M (1980). Locomotor adaptations as reflected on the humerus of Paleogene primates. 

Folia Primatologica 34:1-45. 

35

Comparative Primate Anatomy Tail, Hip & Hindlimb 

______________________________ 

INTRODUCTION 

Homo sapiens 

______________________________ 


SECTION 3: 

TAIL, HIP AND HINDLIMB 

The muscles of the hip and thigh in humans can be divided in a fashion similar to those in the shoulder and 

pectoral girdle. The five different muscle actions include flexion, extension, abduction, adduction, and lateral 

rotation. The flexors are the iliopsoas, rectus femoris, tensor fascia latae, gluteus maximus, and the sartorius. 

Extension is controlled by the biceps femoris, semimembranosus, semitendinosus, and the gluteus maximus. 

The abductors include the gluteus medius and gluteus minimus, while the adductors are made up by the 

adductor magnus, adductor longus, adductor brevis, pectineus, and the gracilis. Lateral rotation is achieved 

through the actions of the piriformis, obturator externus, obturator internus, superior gemellus, inferior 

gemellus, and quadratus femoris. 

Iliopsoas 

--proximal attachment - psoas major attaches at sides of T12 to L5 vertebrae and intervertebral discs between 

them, iliacus attaches at the iliac crest, the iliac fossa, the ala of the sacrum and the anterior sacroiliac ligaments 

--distal attachment - psoas major inserts in the lesser trochanter of the femur, iliacus inserts at the tendon of the 

psoas major and body of the femur inferior to the lesser trochanter 

--main actions - flexes thigh at hip joint and acts as a stabilizer of the joint 

Rectus Femoris 

--proximal attachment - anterior inferior iliac spine, groove superior to acetabulum 

--distal attachment - base of patella, via patellar ligament to tibial tuberosity 

--main actions - extends leg at knee joint, steadies hip joint and helps iliopsoas to flex thigh 

Tensor Fascia Latae 

--proximal attachment - anterior superior iliac spine, anterior part of external lip of iliac crest 

--distal attachment - iliotibial tract that attaches to lateral condyle of tibia 

--main actions - abducts, medially rotates, flexes thigh, helps to keep knee extended, and steadies trunk on thigh 

Gluteus Maximus 

--proximal attachment - external surface of ala of ilium, including iliac crest, dorsal surface of sacrum and 

coccyx, and sacrotuberous ligament 

--distal attachment - iliotibial tract (inserts into lateral condyle of tibia), gluteal tuberosity of femur 

--main actions - extends thigh and assists in its lateral rotation, also steadies thigh and assists in raising trunk 

from a flexed position and stabilizes it 

Sartorius 

--proximal attachment - anterior superior iliac spine and superior part of notch inferior to it 

36


--distal attachment - superior part of medial surface of tibia 

--main actions - flexes, abducts, and laterally rotates thigh at hip joint 

Biceps Femoris 

--proximal attachment - long head is at ischial tuberosity, short head begins at lateral lip of linea aspera and 

lateral supracondylar line 

--distal attachment - lateral side of head of fibula, tendon is split at this site by fibular collateral ligament of 

knee joint 

--main actions - flexes leg and rotates it laterally, extends thigh (when starting to walk) 

Semimembranosus 

--proximal attachment - ischial tuberosity 

--distal attachment - posterior part of medial condyle of tibia 

--main actions - extend thigh, flex leg and rotate it medially, when thigh and leg are flexed, extends trunk 

Semitendinosus 


--distal attachment - medial surface of superior part of tibia 

--main actions - same as semimembranosus 

Gluteus Medius 

--proximal attachment - external surface of ilium between anterior and posterior gluteal lines 

--distal attachment - lateral surface of greater trochanter of femur 

--main actions - abducts and medially rotates thigh, steadies pelvis 

Gluteus Minimus 

--proximal attachment - external surface of ilium between anterior and inferior gluteal lines 

--distal attachment - anterior surface of greater trochanter of femur 

--main actions - same as gluteus medius 

Adductor Magnus 

--proximal attachment - inferior ramus of pubis, ramus of ischium (adductor part), and ischial tuberosity 

--distal attachment - gluteal tuberosity, linea aspera medial, supracondylar line (adductor part), and adductor 

tubercle of femur (hamstring part) 

--main actions - adducts thigh, its adductor part also flexes thigh, and its hamstring part extends thigh 

Adductor Longus 

--proximal attachment - body of pubis inferior to pubic crest 

--distal attachment - middle third of linea aspera of femur 

--main actions - adducts thigh 

Adductor Brevis 

--proximal attachment - body and inferior ramus of pubis 

--distal attachment - pectineal line, proximal part of linea aspera of femur 

--main actions - adducts thigh and somewhat flexes it 

Pectineus 

--proximal attachment - pectineal line of pubis 

--distal attachment - pectineal line of femur 

--main actions - adducts and flexes thigh 

Gracilis 

--proximal attachment - body and inferior ramus pubis 

--distal attachment - superior part of medial surface of tibia 

--main actions - adducts thigh, flexes leg, and helps to rotate it medially 


37


Piriformis 

--proximal attachment - anterior surface of sacrum, sacrotuberous ligament 

--distal attachment - superior border of greater trochanter of femur 

--main actions - laterally rotates extended thigh and abducts flexed thigh, steadies femoral head in acetabulum 

Obturator Externus 

--proximal attachment - margins of obturator foramen, obturator membrane 

--distal attachment - trochanteric fossa of femur 

-- main actions - laterally rotates thigh, steadies head of femur in acetabulum 

Obturator Internus 

--proximal attachment - pelvic surface of obturator membrane and surrounding bones 

--distal attachment - medial surface of greater trochanter of femur 

--main actions - laterally rotates extended thigh and abducts flexed thigh, steadies femoral head in acetabulum 

Superior Gemellus 

--proximal attachment - ischial spine 


--main actions - same as obturator internus 

Inferior Gemellus 



--main actions - same as obturator internus and superior gemellus 

Quadratus Femoris 

--proximal attachment - lateral border of ischial tuberosity 

--distal attachment - quadrate tubercle on intertrochanteric crest of femur, inferior to it 

--main actions - laterally rotates thigh and steadies femoral head in acetabulum 

The muscles that move only the lower leg are divided into flexors and extensors. The muscle responsible for 

flexion is the popliteus. The extensor group is made up of the vastus lateralis, vastus medialis, and vastus 

intermedius. 

Popliteus 

--proximal attachment - lateral surface of lateral condyle of femur, lateral meniscus 

--distal attachment - posterior surface of tibia, superior to soleal line 

--main actions - weakly flexes knee and unlocks it 

Vastus Lateralis 

--proximal attachment - greater trochanter and lateral lip of linea aspera of femur 

--distal attachment - base of patella and via patellar ligament to tibial tuberosity 

--main actions - extends leg at knee joint 

Vastus Medialis 

--proximal attachment - intertrochanteric line and medial lip of linea aspera of femur 

--distal attachment same as vastus lateralis 

--main actions - same as vastus lateralis 

Vastus Intermedius 

--proximal attachment - anterior and lateral surfaces of body of femur 

--distal attachment - same as vastus lateralis and medius 

--main actions - same as vastus lateralis and medius 

Muscles responsible for movement of the foot are divided into five categories bases on the movements they 

control. Dorsiflexion (extension of the foot) is controlled by the tibialis anterior, extensor hallicus longus, 


38


extensor digitorum longus, and peroneus tertius. Plantar flexion (flexion of the foot ) is achieved by the 

gastrocnemius, soleus, and plantaris. The muscle responsible for inversion is the tibialis posterior. The muscles 

involved in the action of eversion are the peroneus longus, peroneus brevis, and sometimes the peroneus tertius. 

Flexion of toes is controlled by the flexor hallicus longus and the flexor digitorum longus. Extension of the toes 

is the responsibility of the extensor hallicus longus and extensor digitorum longus. 

Tibialis Anterior 

--proximal attachment - lateral condyle, superior half of lateral surface of tibia 

--distal attachment - medial and inferior surfaces of medial cuneiform bone, base of first metatarsal bone 

--main actions - dorsiflexes and inverts foot 

Extensor Hallicus Longus 

--proximal attachment - middle part of anterior surface of fibula, interosseous membrane 

--distal attachment - dorsal aspect of base of distal phalanx of great toe 

--main actions - extends great toe and dorsiflexes foot 

Extensor Digitorum Longus 

--proximal attachment - lateral condyle of tibia, superior three-fourths of anterior surface of fibula, and 

interosseous membrane 

--distal attachment - middle and distal phalanges of lateral four digits 

Peroneus Tertius 

--proximal attachment - inferior third of anterior surface of fibula, interosseous membrane 

--distal attachment - dorsum of base of fifth metatarsal bone 

--main actions - dorsiflexes foot and aids in eversion of it 

Gastrocnemius 

--proximal attachment - lateral head attaches at lateral aspect of lateral condyle of femur, medial head originates 

at popliteal surface of femur superior to medial condyle 

--distal attachment - posterior surface of calcaneus via tendo calcaneus 

--main actions - plantarflexes foot, raises heel during walking, flexes knee joint 

Soleus 

--proximal attachment - posterior aspect of head of fibula, superior fourth of posterior surface of fibula, soleal 

line, medial border of tibia 

--distal attachment - same as gastrocnemius 

--main actions - plantarflexes foot, steadies leg on foot 

Plantaris 

--proximal attachment - Inferior end of lateral supracondylar line of femur, oblique popliteal ligament 

--distal attachment - same as gastrocnemius and soleus 

--main actions - weakly assists gastrocnemius in plantarflexing foot and flexing knee joint 

Tibialis Posterior 

--proximal attachment - interosseous membrane, posterior surface of tibia inferior to soleal line, posterior 

surface of fibula 

--distal attachment - tuberosity of navicular, cuneiform, and cuboid bones, bases of second, third, and fourth 

metatarsal bones 

--main actions - plantarflexes and inverts foot 

Peroneus Longus 

--proximal attachment - head and superior two-thirds of lateral surface of fibula 

--distal attachment - base of first metatarsal bone, medial cuneiform bone 

--main actions - eversion, steadies leg on foot 

Peroneus Brevis 


39


--proximal attachment - inferior two-thirds of lateral surface of fibula 

--distal attachment - dorsal surface of tuberosity on the lateral side of base of fifth metatarsal bone 

--main actions - everts foot and weakly plantarflexes it 

Flexor Hallicus Longus 

--proximal attachment - inferior two-thirds of posterior surface of fibula, inferior part of interosseous membrane 

--distal attachment - base of distal phalanx of great toe 

--main actions - flexes great toe at all joints, plantarflexes foot, supports longitudinal arch of foot 

Flexor Digitorum Longus 

--proximal attachment - medial part of posterior surface of tibia, inferior to soleal line, and by a broad 

aponeurosis to fibula 

--distal attachment - bases of distal phalanges of lateral four digits 

--main actions - flexes lateral four digits and plantarflexes foot, supports longitudinal arch of foot. 

Aiello, Leslie and Dean, Christopher (1990). An Introduction to Human Evolutionary Anatomy. Academic 

Press: London. 

Moore, Keith L. (1992). Clincally Oriented Anatomy. Williams and Wilkins: Baltimore. 


40


______________________________ 



______________________________ 

The muscles of the deep back and tail, thigh, and leg vary between tamarins and humans both in the types of 

muscles themselves and in the relative proportions of the shared muscles. Both unique muscles and shared, but 

different, muscles can be explained in terms of the functional anatomy of the tamarin. 

The tamarin tail has three unique muscles not found in humans. The sacrococcygeus dorsalis lateralis, 

sacrococcygeus dorsalis medialis , and intertransversarius dorsalis medialis. While the tamarin tail is not 

prehensile, it is still quite muscular, no doubt because of its use in balance. All of the erector spinae muscles are 

well-developed because they not only extend the spinal column but also aid in extending the tail. The tail 

muscles are also quite well-developed, although, for obvious reasons, they are not comparable tothe human tail! 

The muscles that move the hip and thigh in the tamarin were hard to view in this specimen. Both of the femora 

were removed post-mortem--a procedure which destroyed most of the muscles of the medial thigh and pelvis, as 

well as damaging any muscles which attach to the femur. I will do my best to describe fully all of the 

interesting features in the tamarin thigh, and will note when they are presumed but unobservable. 

The major differences in the observable muscles of the tamarin thigh involve relative proportions of these 

muscles. Gluteus maximus takes a very different form in the tamarin relative to the human condition. It is 

divided into two distinct parts--gluteus maximus proprious and ischiofemoralis. Both arise via aponeurosis 

from the distal sacrum and upper caudal vertebrae. Gluteus maximus proprious inserts into the iliotibial tract. 

It is a thin, small muscle compared with its counterpart in humans, and functions as an abductor and lateral 

rotator. Ischiofemoralis inserts along the lateral length of the femur. It is quite robust in the tamarin and 

functions as an extensor of the hip joint. Gluteus medius is quite robust in tamarins, as are both the flexors and 

extensors of the hip and thigh. 

These differences in the tamarin musculature are not surprising when you take into account the differences 

between bipedal and quadrupedal locomotion. Humans have extremely large gluteus maximus muscles which 

are used in bipedal locomotion such as climbing and running, as well as in balancing the trunk. The tamarin 

gluteus maximus, because of the 'relatively flexed' angle of the hip joint in quadrupeds, functions somewhat 

differently. Although gluteus maximus proprious is homologous woth the human gluteus maximus, it is 

relatively underdeveloped, as its function of abduction and lateral rotation are not very important in tamarin 

locomotion. The robustness of the ischiofemoralis, however, is a function of the constant tension of the hip 

flexors of the quadrupedal tamarin. Additionally, in humans gluteus maximus is much larger than gluteus 

medius. In tamarins, gluteus medius is quite robust compared with gluteus maximus because they are also 

primary extensors of the hip. In fact, the robustness of all of the extensors and flexors results from the fact that 

these muscles are continuously active in a standing quadruped. 


Figure 11: The tamarin lower leg. 

Note that the leg (including the 

primary calf muscles, in tweezers) is 

less robust relative to the forelimb. 

41


The long muscles that move the lower leg and foot are the same in tamarins and humans with one exception. 

Abductor hallicus longus originates as part of the tibialis anterior and inserts on the base of the first metatarsal. 

It functions to abduct the big toe in the tamarin. 

The other muscles in the leg of the tamarin, while remarkably well defined, are similar to the muscles in the leg 

of humans. It is interesting to note that overall the muscles of the leg in tamarins are relatively less developed 

than those of the arm. This is no doubt a reflection of the fact that tamarins spend much of their time climbing 

upwards--an activity which requires very strong arms. 

______________________________ 



______________________________ 

The musculature of the lower limb of G. crassicaudatus is typical of a saltatory primate. 

Deep back and tail: 

The deep back muscles found in G. crassicaudatus are virtually identical to Homo. They are well developed. 

Leaping primates have relatively long lumbar regions that allow for flexion and extension of the back during 

take-off and landing, and these muscles aid in that process. 

Unlike Homo, the galago has a tail. This important appendage is used for balance, and is especially important 

for maintaining control in an arboreal environment. Sacrococcygeus dorsalis lateralis, sacrococcygeus 

dorsalis medialis, and intertransversarius dorsalis coccygeus encircle the length of the caudal vertebrae in 

the tail. These muscles are uniformly well developed, indicating that the tail is used frequently and for 

balancing when the animal is in any position. These muscles do not provide any clues as to the specific mode 

of arboreal locomotion. 

Hip and thigh: 

All muscles found in the hip and thigh of Homo are also found in G. crassicaudatus with one exception: 

tensor fascia latae. No trace of this muscle could be found in the galago. In addition, the galago has a clear 

division of the psoas muscle into a major and minor which is not found in humans. No functional differences 

are created by this division. 

Arboreal quadrupeds often locomote with their hindlimbs abducted in order to bring their centers of gravity 

closer to the support. Leapers prefer to keep their hindlimbs in a parasagital plane. By limiting motion to 

flexion and extension, leaping primates are able to move with more control during the take-off and landing of 

powerful jumps. With this in mind, the musculature of the hindlimb of G. crassicaudatus resembles that 

expected in a leaping primate. 

All muscles responsible for extension of the hip and knee are enormous. Most notably, the vastus lateralis and 

the sartorius are incredibly robust. All of these muscles are used in the powerful takeoffs, as well as in 

support upon landing. Muscles used for abduction and adduction are uniformly robust. This indicates that G. 

crassicaudatus does not seem to carry its hindlimbs in the abducted posture of an arboreal quadruped. Rather, 

these muscles are used to stabilize the leg in a parasagital plane during takeoff and landing. Robust flexors are 

also present and are necessary to counteract the tremendous force of take-offs and landings. 


42


Lower leg and foot: 


Figure 12: Galago superficial lateral thigh. (A) vastus lateralis, (B) 

sartorius, (C) gluteus maximus, (D) gastrocnemius, (E) peroneus 

longus, (F) semimembranosus. 

All of the muscles of the lower leg and foot of G. crassicaudatus are well developed. They, too, are 

responsible for providing power and control during leaps. Of note, plantaris is a well developed muscle, with a 

fleshy muscular belly from origin to insertion. This is drastically different from its form in Homo where there 

is very little muscle, and a very long tendon. The tibialis posterior, responsible for inverting the foot, is equal 

in mass to the peroneus longus and the peroneus brevis, which are responsible for everting the foot. This 

symmetry is necessary for stabilizing the foot while leaping. These muscles are also important in that they 

allow the foot to be placed on unstable, angled supports found in an arboreal environment. 

All those muscles found in the lower leg and foot of Homo are present in G. crassicaudatus. The galago, like 

most non-human primates, also has an abductor hallucis longus muscle. This muscle originates from the 

lateral tibial condyle to the tibial tuberosity, and inserts at the base of the phylanx of the first digit. 

Stevens, J.L., Edgerton, V.R., Haines, D.E., and D. M. Meyer (1981). An Atlas and Source Book of the Lesser 

Bushbaby, Galago senegalensis. Boca Raton, Florida: CRC Press, Inc. 

43


______________________________ 


Saimiri 

______________________________ 

In many respects, the muscular anatomy of the hind limb of Saimiri is similar to Homo. The differences 

between these two genera can best be understood through their locomotor behavior. Saimiri is an arboreal 

quadruped that commonly leaps while Homo is an obligate biped. 

The hip and thigh: 

Generally, the muscles of the hip and thigh of Saimiri originate and insert in the same manner as in Homo. The 

main difference between these two genera is the remodeling of the pelvis in Homo for bipedality. The gluteal 

muscles in Saimiri reflect this taxon's adaptation to quadrupedal locomotion. The gluteus superficialis muscle 

has a rather long insertion on the posterior lateral aspect of the proximal half of the femur. This muscle 

functions both as a hip extensor and as an abductor of the thigh. The gluteus superficialis muscle plays an 

important role in quadrupedal running and may aid in leaping. The gluteus medius muscle originates at the 

gluteal plane of the ilium and inserts on the caudal edge of the trochanter of the femur. It functions as a medial 

rotator of the thigh at the hip joint. The origin and insertion of the tensor fascia latae muscle is essentially the 

same in Saimiri as it is in Homo except that in Saimiri it is fused to the gluteus superficialis muscle. In 

addition, the tensor fascia latae muscle is relatively more robust in Saimiri. The main reason for the large size 

of this muscle is that Saimiri often runs and this muscle aids in protraction of the hip, as well as an abductor of 

the thigh at the hip joint. 

The flexors of the hip in Saimiri are similar to those of Homo with one exception. The sartorius muscle is 

relatively larger and more robust in Saimiri, attatches to the gracilis muscle, and has a long insertion on the 

proximal half of the tibia. In addition to being a hip flexor and protractor, the sartorius is also a lateral rotator 

of the leg. 

The main difference between Saimiri and Homo in the leg extensor group involves the relative size of the 

vastus lateralis. In Saimiri this muscle is a relatively robust and very powerful leg extensor. The vastus 

lateralis is important in leaping. This anatomical feature fits with the general profile of the squirrel monkey 

which commonly leaps. 

Some muscles of the posterior aspect of the thigh in Saimiri differ from Homo. The biceps femoris muscle is 

very well developed and has an extensive attachment on the lateral proximal half of the tibia. It acts as a 

powerful knee flexor which is important in both quadrupedal running and leaping behaviors. The other muscles 

of the posterior thigh are well developed as well, but none compare to the relative size of the biceps femoris. 

The adductor group of Saimiri is notable for one major reason: the relative size of the gracilis muscle. In 

Homo this muscle is, as the name implies, rather small. However, Saimiri posesses a powerful gracilis muscle 

that attaches with the sartorius to form a common tendon which inserts on the medial proximal half of the tibia. 

The gracilis acts as both an adductor of the thigh and a flexor and medial rotator of the leg. The tendonous 

insertion of the gracilis/sartorius, coupled with the tendonous insertion of the biceps femoris muscle, helps to 

form a sling. This supportive structure may limit the adduction and abduction at the knee joint and aid in 

keeping the motion at the knee restricted to the parasagittal plane. 

The last important point regarding the lower limb of Saimiri concerns the muscles involved in plantarflexion of 

the foot at the ankle joint. The three muscles of the posterior compartment of the leg (gastrocnemeous, soleus, 

plantaris) join together at about the middle of the tibia in a common tendon that inserts on the calcaneous. It is 

important to note the difference in calcaneus length between Homo and Saimiri. The calcaneous of Saimiri is 

relatively longer than that of Homo and this length is important for Saimiri's common leaping behavior. This 


44


long calcaneous coupled with well developed muscles of the posterior compartment of the leg, helps to make 

Saimiri a powerful leaper. 

Deep back: 

The muscles of the erector spinae group (iliocostalis, longissimus, spinalis) are considerably well developed in 

Saimiri. The relative size of these muscles can best be understood when one considers the leaping behavior of 

this primate. One way for a leaper to increase its power at take off is by flexing and then rapidly extending the 

back. The erector spinae group serves as a powerful back extensor. 

Hartman, CG & WL Strauss (1961). The Anatomy of the Rhesus Monkey (Macaca mulatta). New York: Hafner 

Publishing. 

Stern, JT (1971). Functional myology of the hip and thigh of cebid monkeys and its implications for the 

evolution of erect posture. Bibliotheca Primatologica 14. 

______________________________ 

LONG-TAILED OR CRAB-EATING MACAQUE 


______________________________ 


Erector spinae 

As a member of the erector spinae group, the iliocostalis muscles aid in the stabilzation of the head, trunk, and 

tail; extension, lateral flexion, and rotation of the spinal column are also aided by this muscle. The origin and 

insertion of the thoracis portion possess no distinguishable differences between Macaca fascicularis and 

humans. However, the lumborum portion in the long-tailed macaque has a more extensive origin on the 

superior portion of the iliac crest than in humans. The insertion of the lumborum remains the same in Macaca 

fascicularis and in humans. The most medial muscles of the erector spinae group, longissimus 

lumborum/cervicis/capitis do not differ from their orientation in humans. The third muscle group of the 

erector spinae group is the spinalis group (spinalis lumborum/thoracis/cervicis). The lumborum portion of the 

spinalis group is absent in humans, and originates from the lumbodorsal aponeurosis and inserts on the spinous 

processes of the lumbar vertebrae. The thoracic portion also arises from the lumbodorsal aponeurosis and 

inserts on the spinous processes of the more distal cervical vertebrae, and on the more proximal thoracic 

vertebrae. The cervical portion originates on the spinous processes of the first few thoracic vertebrae and 

inserts on the spinous processes of the axis. Comparitively, the entire spinalis muscle group in humans has 

moved cranially in humans; thus the loss of the lumborum section is explained by the cranial migration of the 

muscle. 

Intrinsic muscles of the tail 

Macaca fascicularis, the long-tailed macaque, does not posses a prehensile tail. The tail functions as a counter 

weight during locomotion for the long-tailed macaque. Humans, though we nolonger have a tail, do posses the 

vestigal muscles sacrococcygeus anterior, and the pubo-iliocaudal muscles which once controlled the flexion 

and extension of a tail. 

Flexors of the tail 


45


Flexor caudae longus originates from the ventral surface of the sacrum, from the transverse processes of the 

most distal lumbar vertebrae, and inserts on the ventral surface of the caudal vertebrae. Flexor caudae brevis 

shadows the flexor caudae longus, and originates and inserts just beneath it. Pubocaudalis originates from the 

inner surface of the pubic symphysis and part of the ilium to insert on the lateral and ventral surfaces of the 

caudal vertebrae. Iliocaudalis originates from the inner surface of the ilium to insert with the pubocaudalis on 

the caudal vertebrae. The ischiocaudalis lies superiorly to the iliocaudalis and inserts on the ventral surface of 

the transverse processes of the first three to five caudal vertebrae. These muscles depress, abduct, and flex the 

tail. 

Extensors of the tail 

Caudal extensions of the longissimus group 

Extensor caudae lateralis arises from the metapophyses of the second to fifth lumbar vertebrae, and it inserts 

on the dorsal aspects of the spinous processes of the caudal vertebrae. Abductor caudae medialis originates 

from the aponeurosis of the extensor caudae lateralis and the sacrum to insert on the base of the transverse 

processes of the most proximal caudal vertebrae. Abductor caudae lateralis originates from the sacrum and 

from from the proximal caudal vertebrae. It inserts on the lateral processes of the most distal caudal vertebrae. 

These muscles extend and abduct the tail. 

Muscles of the hip: 

The internal muscles of the hip (iliopsoas major, iliopsoas minor, iliacus, gluteus minimus, piriformis, and 

gemullus) do not differ from their orientation in humans. However, the following muscles of the gluteal region 

do show discernable differences. 

Gluteal group 

Fascia lata: This tough, fibrous sheet covers the quadriceps femoris group. Proximally, it derives from the 

abdominal and dorsal fascia as well as from the distal portion of the tensor fascia lata. Distally, the fascia lata 

attaches to the patellar region and with the fascia of the leg. When the tensor fascia lata is flexed, the fascia 

lata is pulled and aids in the abduction of the thigh. 

Tensor fascia lata: This muscle originates from the distal and medial portion of the gluteus maximus, and 

unlike the rhesus macaque does not have any attachments with the iliac spine. Distally, the tensor fascia lata 

inserts into the fascia lata. 

Gluteus maximus: Unlike in humans, the gluteus maximus is not the largest muscle of the gluteal group in 

macaques. The glutues maximus originates from the transverse processes of the most proximal caudal 

vertebrae, and from the dorsal fascia covering the sacrum. The muscle inserts at three locations: 1. laterally, on 

the more proximal portion of the linea aspera of the femur for roughly a half an inch; 2. medially, directly into 

the fascia lata covering the most proximal portions of the rectus femoris; 3. most medially, into the tensor 

fascia lata. When flexed, the gluteus maximus retracts, abducts, and laterally rotates the thigh. 

Gluteal medius: The gluteus medius is the largest of the gluteal group, and it covers most of the external iliac 

fossa. The gluteus medius originates from the iliac fossa and the outer portion of the iliac crest. It inserts upon 

the lateral surface of the greater trochanter, and assists in the retraction and abduction of the thigh. 

Obturator internus (greater and lesser): This muscle lies superiorly to the ischial callous and originates from 

the interior surface of the pelvis along the obturator foramen. The greater portion inserts into the obturator 

fossa at the medial base of the greater trochanter, while the lesser portion inserts on the caudal border of the 

obturator tendon. The action of the muscle is the same as in humans. 


46


Quadratus femoris: The origin and insertion of the quadratus femoris is the same in Macaca fascicularis as 

it is in humans. The muscle acts as an adductor and lateral rotator in the bipedal locomotor patterns of humans. 

In the quadruped, the quadratus femoris acts as in abductor and a lateral rotator. 

Muscles of the thigh: 

Flexor (posterior) group 

Humans have two heads, the long and short, of the biceps femoris. However, the macaques posses only a long 

head which originates from the lateral portion of the ischial tuberosity, and inserts on the fascia lata 

(anteriorly), and into the aponeurosis of the proximal half of the leg (distally). The action of the biceps femoris 

is the same in the long-tailed macaque as it is in humans. Proportionally, the biceps femoris is much larger in 

the long-tail than in humans. The massiveness of the muscle can be correlated to the quadrupedal locomotor 

pattern of the macaque. The flexion of the thigh would push the quadruped forward. Semimembranosus 

proprius originates from the inferior lateral border of the ischial tuberosity lateral to the origin of the 

semimembranosus accessorius, and medial to the semitendinosus. The semimembranosus proprius inserts 

by a short tendon onto the medial aspect of the tibial tuberosity. When flexed, the semimembranosus 

proprius aids in the flexion, and medial rotation of the leg. Semimembranosus accessorius originates from 

the inferior portion of the ischial tuberosity and inserts on the shaft of the femur medially to the linea aspera. 

Semimembranosus accessorius aids in retraction and adduction of the thigh. Humans only have a proprius 

head of the semimembranosus. The accessorius head is believed to be included into the adductor magnus. 

The orientation of the semitendinosus does not differ from humans. 

Adductor (medial) group and flexor (posterior group) 

The adductor muscles of the thigh (gracilis, adductor magnus, adductor longus, adductor brevis, pectinius, 

and satorius) diplay no differences between humans and the long-tailed macaque. The extensor muscles of the 

thigh (satorius, vastus lateralis, rectus femoris, vastus medius, vastus intermedius, and popliteus) also do 

not display any significant differences from their orientation in humans. The rectus femoris in humans tends to 

have a second head originating from the borders of the acetabulum; the second head is missing in Macaca 

fascicularis. 

Muscles of the leg: 

Flexor (posterior) group 


Figure 13: M. 

fascicularis thigh. 

(A) rectus femoris, 

(B) vastus 

intermedius, (C) 

vastus lateralis. 

Neither the gastrocnemius, or the soleus displays any difference from their orientation in humans. The 

plantaris is proportionaly larger in Macaca fascicularis than in humans. The size difference may be related to 

the plantar flexion of the foot during locomotion. The remainder of the flexors (flexor digitorum longus, 

flexor hallicus longus, and tibialis posterior) show not differences from their orientation in humans. 

47



Figure 14: M. 

fascicularis medial 

thigh. (A) adductor 

magnus, (B) 

adductor longus. 

Extensor (anterior) group 

Unlike humans, the tibialis anterior in Macaca fascicularis possess two heads, and it does not insert solely on 

the first cuneiform. The whole muscle originates from the lateral condyle of the proximal tibia. The medial 

head passes beneath the superior extensor retinaculum, and it inserts on the medial plantar surface of the first 

cuneiform. The smaller, lateral head of the muscle passes underneath the superior extensor retinaculum, and 

inserts upon the medial plantar surface of the first metatarsal. The medial head is synonymous with the 

abductor hallicus longus in humans. The tibialis anterior (both portions) assist in dorsi flexion and inversion 

of the foot, and in abduction of the hallux. The origin of the peroneal longus is the same in Macaca 

fascicularis as it is in humans. In the long-tailed macaque, the peroneus longus inserts upon the lateral plantar 

surface of the base of the first metatarsal bone. At times, at least in Macaca mulatta, an additional slip of the 

peroneal longus may arise from the tendon to insert on the first metatarsal. In both macaques and humans, the 

peroneus longus aids in eversion and plantar flexion. The additional slip in Macaca mulatta aids in flexsion of 

the hallux. The peroneal brevis and the extensors of digits (extensor digitorum longus, extensor hallicus 

longus, extensor digitorum brevis, and extensor hallicus brevis) show no differences from their arrangement 

in humans. 

Agur,M.R. (1991) Grant’s atlas of anatomy. Gardner, J.N. (ed.) Williams and Wilkins. Baltimore, Md. 

Aiello, L., and Dean, C. (1990) An introduction to evolutionary anatomy. Acedemic Press. San Diego, Ca. 

Berringer, O.M., Browing, F.M., and Schroeder, C.R. (1968) An atlsas and dissection manual of rhesus monkey 

anatomy. Animal Laboratory Aids. Tallahassee, Fl. 

Hartman, C.G., and Straus, W.L. (1933) The anatomy of the rhesus monkey Macaca mulatta. Williams and 

Wilkins. Baltimore, Md. 

48


______________________________ 



______________________________ 

Deep back musculature: 

The deep back muscles--the spinalis, longissimus, and iliocostalis--exhibit no great differences from the 

corresponding muscles in Homo. This is suprising since these muscles aid in maintaing posture, and the 

macaque’s quadrupedal posture is extremely different from the erect posture of humans. However, in noting the 

comparative anatomy of the deep back muscles, Hartman and Straus (1933) note the the complex exhibits “such 

a degree of variation” that it is difficult to ascribe any single morphology to a primate species (127). 

Muscles of the hip: 

The differences in hip musculature from Homo are slight, and mainly concern origin/insertion sites. The psoas 

minor, often absent in Homo, originates from the lumbar vertebrae bodies, passing over the ventral and medial 

aspects of the psoas major to insert upon the iliopubic region of the pelvis. The gluteus maximus does not 

have an iliac origin, as in Homo, but arises from the upper lumber vertebrae transverse processes, and from the 

dorsal fascia over the sacrum. The gluteus medius also differs from that in Homo in this origin from the dorsal 


Figure 15: Rhesus monkey 

thigh. Reproduced from 

Hartman & Strauss (1933). 

49


fascia. Thus, these latter two muscles can be said to have a more extensive origin in the macaque than in 

humans. Moreover, the tensor fascia lata is more developed in the macaque than humans. This is to be 

expected in a quadrupedal animal, for these muscles function in the retraction and abduction of the thigh, i.e. 

greater control of thigh movement. One should note, however, that the large size and extensive origins of the 

gluteal muscles permit such controlled and powerful movement mainly in the parasagittal plain. The psoas 

major, gluteus minimus, and piriformis are all similar the corresponding muscles in Homo. 

The gemelli, rather than separated into a superior and inferior component as in Homo, is a continuous muscle 

sheet in the rhesus macaque. The obturator extenus and internus, and quadratus femoris are similar to 

Homo. The main difference in the above is in muscle function. In Homo, they serve shiefly as adductors and 

lateral rotators of the thigh, whereas they are abductors of the thigh in macaques, due to its quadrupedal posture. 

Muscles of the thigh: 

First, the muscles which flex the leg upon the thigh and retract the thigh differ in interesting ways from Homo. 

These muscles, commonly referred to as the hamstring group, are especially important in quadrupadlism for 

propulsion from the hindlimbs and movement in the parasagital plane. 

The biceps femoris has a short and long head in humans; the former is absent in the macaque. Rather, 

additional fibers arise from the main mass caudalward by deeper fasciculi to fuse in an aponeurosis over the 

anterior leg, while the former broad sheet attaches to the fascia lata. The insertion differs from Homo, in 

which both the long and short heads insert onto the proximal head of the fibula. Therefore, the macaque biceps 

femoris differs significantly from Homo in that its insertion spans the length from the distal femur to the middle 

leg region. Such a broad insertion, and correspondingly large muscle, function in the powerful flexion of the 

leg and, when the knee joint is fixed, retraction of the thigh--the chief movements of the hindlimb in 

quadrupedal locomotion. 

In addition, the semitendinosus and semimembranosus aid in the same two move-ments--leg flexion and thigh 

retraction. The latter muscle differs from Homo in that it is composed of two parts: 

1) semimembranosus proprius--arises from the border of the ischial tuberosity between the 

semitendinosus and the semimembranosus accessorious (see below), to insert upon the medial tibial 

tuberosity. 

2) semimembranosus accessorious--runs medial to the above to insert upon the femoral shaft down to 

the medial condyle. This portion of the muscle is part of the adductor magnus in Homo. 

It should be kept in mind that, due to their medial insertions, these two muscles may act as adductors of the leg 

and thigh in addition to leg flexors. 

Considering the adductor muscles, the gracilis, adductor longus, pectineus, adductor brevis, and obturator 

externus are similar to that of Homo. The adductor magnus--a separate muscle in the macaque--is fused with 

the semimembranosus accessorius in Homo. A functional explanation for this separation in the rhesus monkey 

may be that adduction of the thigh is needed near-contantly while locomoting. Two muscles inserting 

separately on the femur may adduct the thigh more efficiently than one large muscle with a single insertion. 

There are no great differences from Homo in the leg extensors--sartorius, rectus femoris, vastus lateralis, 

vastus medialis, and vastus intermedius. 

Muscles of the leg (movement of the foot): 

The muscles involved in plantar flexion--gastrocnemius, soleus, plantaris, politeus, peroneotibialis, flexor 

digitorum fibularis and tibialis, and tibilias posterior--are mostly similar to Homo; differences, which are 

minor, are within the breadth of chance variation. Of note, a tibial and fibular head of the soleus is present in 

Homo, whereas the macaque has only a fibular head. The rhesus plantaris is relatively larger than that in Homo, 

presumably due to the propulsive force that the macaque may derive through plantar flexion during locomotion. 


50


No major differences could be discerned from the muscles of dorsiflexion--tibialis anterior, extensor hallicis 

longus, extensor digitorum longus, peroneus longus, and peroneus brevis. However, the macaque does have 

a peroneus digiti quarti, a digital extensor with a tendinous attachment to the fourth digit at its terminal 

phalanx. 

Agur, M.R. (1991) Grant’s atlas of anatomy. Gardner, J.N. (ed.) Baltimore: Williams and Wilkins Co. 

Hartman, C.G. & Straus, W.L. Jr. (1933). The Anatomy of the Rhesus Monkey. Baltimore: Williams & Wilkins 

Co. 

______________________________ 


Papio hamadryas: 

______________________________ 

As with the shoulder and forelimb, the hamadryas baboon's lower limb is subject mostly to compressive forces 

associated with terrestrial weight bearing and locomotion. Locomotor patterns involve the movement of the 

lower limb in the sagittal plane, with no need for extensive rotation and abduction. For terrestrial quadrupedal 

primates like P. hamadryas, the musculature shows efficient positioning for retraction of the thigh coupled with 

prevention of passive leg extension. In addition, the ability to resist flexion of the thigh and extension of the leg 

would be important in decelerating the limb at the end of the recovery stroke in running. 

Muscles that move the hip, thigh, and leg: 

P. hamadryas possess two muscles in this region that are absent in Homo. 1) Scansorius: This inconsistent 

muscle derives from the ventral fibers of the gluteus minimus, which may be segmented to form a separate 

division often referred to as scansorius. It originates from the caudal surface of the ilium, inserts onto the 

greater trochanter, and acts to abduct and internally rotate the thigh. 2) Semimembranosus accessorius: In 

Papio the semimembranosus is composed of two parts, a proper and accessory. In Homo, only 

semimembranosus proprius is present. Both muscles in Papio arise from the ischial tuberosity, but the proper 

part attaches to the medial border of the tibia and the accessory portion to the medial shaft of the femur, as far 

as the medial condyle. In Homo, the accessory part is united with the adductor magnus, resulting in a 

compound muscle. The action of the semimembranosus muscles is to extend the hip and flex the knee. 

In addition to the two muscles describe above, there are a number of differences between P. hamadryas and 

Homo in the hip and thigh region. 1) Tensor fascia latae: In P. hamadryas, this flexor and medial rotator of 

the hip and thigh is relatively thick and powerful. Because of its increased mass, tensor fascia latae is well 

suited to aid in the rapid recovery of the hindlimb in quadrupedal running; its action is added to the force of the 

other protractors and consequently the limb can be decelerated and recovery effected more quickly. As in 

Homo, this muscle inserts as the iliotibial tract. However, in humans the tract is limited to the more lateral 

aspect of the thigh, while in Papio, the iliotibial tract covers the majority of the anterior and lateral thigh with 

the exception of sa (rtorius and rectus femoris. Finally, the muscle is fused with the upper portion of the gluteus 

maximus. 

2) Gluteus maximus and medius: The origin of gluteus maximus is similar to Homo, but the insertion is 

unique. The anterior fibers insert into the tensor fascia latae on the lateral side of the thigh. The femoral 

insertion involves the posterior fibers, which attach to the bone by a weak tendon. The gluteus maximus is a 

powerful extensor of the thigh in humans, while more of an abductor in Papio. The origin and insertion of the 

gluteus medius is similar in Papio and Homo, but in Homo, this muscles has become an efficient abductor by 

virtue of the rotation of the iliac blade and its increased fore and aft extension. 


51


3) Quadriceps femoris (rectus femoris, vasti lateralis, medialis, intermedialis). In Homo, rectus femoris has 

two heads of origin, while in Papio there is only one distinct origin that is caudal to the anterior inferior spine. 

However, more distally, the muscle may be divisible into two parts. This muscle, a thigh flexor and leg 

extensor is relatively large in leaping primates, as are the vasti, also leg extensors. 

4) Semitendenosis and gracilis: Gracilis is a thin, wide muscle, especially in the cranial half. Both the 

gracilis and the semitendenosis have a low insertion on the proximomedial tibia, aiding in the flexor movement 

for quadrupedal locomotion. 

5) Biceps femoris: Only the long head of this muscle is present in Papio; it is often referred to as flexor cruris 

lateralis. It originates on the ischial tuberosity of the ischium and inserts onto the leg as far down as a third to a 

half on the tibia. The well developed and wide attachment on the leg endows the muscle with greater 

effectiveness in flexion of the leg. This would be useful in resisting passive leg extension during the propulsive 

stoke of quadrupedal locomotion and in decelerating the leg at the end of the recovery stroke. 

Muscles that move the ankle and foot: 

P. hamadryas possess two muscles in this region that are absent in Homo. 1) Peroneus digiti quinti: This 

muscle originates on the upper posterolateral edge of the fibular shaft and inserts onto the lateral side of the 

distal phalanx of digit V. Its action is to evert the foot. 

2) Peroneotibialis: This muscle, lying deep to the popliteus, attaches on the head of the anteromedial surface 

of the fibula and inserts onto the proximal third of the posterolateral surface of the tibia. It aids in holding the 

head of the fibula in place, important for stabilization during quadrupedal locomotion. 

In addition to the two muscles describe above, there are a number of differences between P. hamadryas and 

Homo in the leg. 1) Peroneus longus and peroneus brevis. In Papio, peroneus longus originates on the lateral 

fibular head and shaft (the condition in Homo), but also on the lateral tibial condyle. Its insertion on Papio is 

only to the base of the hallucal metatarsal, and not also to the cuneiform I, as in Homo In Papio, these muscles 

are strong flexors of the hallux. 

2) Gastrocnemius and Soleus. These two muscles plantar flex the foot at the ankle. In Papio, the 

gastrocnemius remains fleshy throughout much of its length, and soleus has a single head of origin from the 

fibula. In Homo, the soleus also originates from the popliteal line of the tibia. 

3) Popliteus: This muscle is well developed in Papio. Its proximal portion (lateral epicondyle of the femur) 

runs horizontally, and the distal portion travels a relatively oblique course (vs. Homo) and inserts of the medial 

condyle of the tibia and posteromedial tibial shaft. Its action is to help maintain the integrity of the knee joint. 

4) Flexor digitorum tibialis and flexor digitorum fibularis. These muscles are know as flexor digitorum 

longus and flexor hallucis longus, respectively, in Homo. Papio and Homo have similar insertions on the 

posteromedial third of the tibial shaft for flexor digitorum tibialis, but in Papio, tendons insert into the bases of 

the distal phalanges of all digits, contrasting Homo, whose insertions are digits II-V. The tendons of flexor 

digitorum fibularis insert onto the distal phalanges of digits I, II and IV in Papio, while onto the base of the 

distal phalanx of the hallux only in Homo. The more extensive tendonous attachments in Papio provide more 

mobility of the digits than in Homo. 


52


______________________________ 

SPIDER MONKEY 

Ateles 

______________________________ 

Tail and Spinal Musculature: 

The primary distinction between the Atelines and most other primates is the possession of a prehensile tail. 

This ability to use the tail as a fifth limb has resulted in a number of modifications of the musculoskeletal and 

nervous systems. Essentially, a major increase in the number (33) of caudal vertebrae over many other 

primates has occurred along with increased innervation of the tail. However, this “increase” may actually be a 

retained ancient feature due to the caudal portion of the spinal cord developing in utero as a primtive feature 

with regard to ontogenetics. This innervation of a large number of segmentally arranged muscles has taken on 

the form of a brachial plexus with each muscle being stimulated by mutiple nerves. Thus, the tail in Ateles can 

be considered a true “limb” due to the “plurisegmental innervation” (Chang). 

In Ateles, the muscles of the lower back are essentially the same as in humans. The difference lies in the 

continuation of the erector spinae distally along the dorsal side of the tail in the basal region closest to the body. 

The ventral basal region is characterized by locomotor muscles which in Homo have become associated with 

maintaining the positions of the visceral organs in the pelvic region. These muscles in the spider monkey have 

become responsible for flexion and lateral movement of the tail, and include Ischio-caudalis and Ilio-caudalis. 

The more distal muscles of the tail in Ateles are those which move the tail in the parasagittal and lateral planes 

via flexion, extension, and rotation. The dorsal muscles are more prominent in the proximal regions of the tail 

with the ventral muscles becoming relatively larger more distally. There are five muscles which are 

segmentally arranged. These “muscles” are actually series of muscles which are repeat at each vertebral 

segment. Abductor caudae internus et externus originate from the anterior transverse process on the ventral 

side. The muscle has two insertions. One insertion lies on the dorsal surface of the anterior transverse process 

of the distal vertebra once removed (the muscle bridges one vertebral segment). The smaller insertion is on the 

subjacent vertebra into the posterior transverse process. Flexor caudae lateralis originate along the ventral 

surface of the vertebra and insert into subcutaneous fascia. This series of muscles is very well-developed. 

Flexor caudae medialis has two portions. One segment originates from the distal ventral portion of one 

vertebra and inserts into the proximal ventral surface of the adjacent distal vertebra, The other segment of 

Flexor caudae medialis inserts into the anterior ventral process of the distal vertebra once removed from the 

origin vertebra to which it attaches to the proximal ventral portion and the ventral medial muscular septum. 

Extensor caudae medialis originate dorsally from the proximal half of the caudal vertebrae and insert dorsally 

into the proximal vertebral process of the vertebra once removed. Extensor caudae lateralis originate between 

the transverse and dorsal processes of a vertebra along the lateral surface. This series of muscles inserts into 

vertebrae as much as ten segments away via long tendons which cover the dorsal surfaces of the extensor 

muscles. 

In general, the muscles of the hindlimbs in Ateles are smaller and less specialized than in other primates 

(including Homo). This applies to both the flexors and extensors of the thigh and lower leg. This relative 

unspecialization of the hindlimb musculature reflects the decreased importance of the lower limbs in 

locomotion and the increased use of the prehensile tail in Ateles. The crural index is close to 100 and differs 

from the high brachial index due to the increased importance of forelimb suspension in both Ateles and other 

prehensile tailed genera. 

Hip and Thigh: 

The muscles of the hip and thigh are greatly reduced compared to both humans and the nonhuman quadrupedal 

primates. Additionally, the hip joint is more mobile and is functionally like a shoulder joint, reflecting the use 


53


of the hindlimbs in suspension. The hip joint acts as a rotator cuff and is designed to support the mass of the 

animal and permit small movements which would stabilize the animal in a variety of suspensory positions. 

The use of the hip joint as a rotator cuff is reflected in the appearance of the Gluteus maximus as a Deltoidlike 

muscle. The Gluteus maximus (superficialis) functions to move the leg in a wide variety of ways much 

like the rotator cuff muscle but the size of the muscle is greatly reduced. According to one source (Stern), the 

Gluteus maximus is relatively smaller in Ateles than in any other Cebid. This muscle differs from its human 

form in a number of ways, most noticeably in the weaker development of the posterior portion and the lack of 

clear differentiation between the anterior and posterior portions as a whole. No clear distinction exists between 

the proximal and distal portions (Gluteus maximus proprius and ischiofemoralis, respectively). With respect 

to bony attachments, the origin is largely from the gluteal fascia (the vertebral origin is greatly reduced). The 

attachment to the ischial tuberosity is somewhat extensive. Finally, this muscle is connected by a thick layer of 

connective tissue to the Gluteus medius and to the Vastus lateralis by the descending tendon of the Gluteus 

maximus. This cohesion is also present in other prehensile tailed genera and in man, but is uncommon among 

the other primates. Overall, the Gluteus maximus has reduced its retraction abilities and supplemented its 

ability to rotate, protract, and abduct the thigh. 

Another major difference in the hip and thigh region when compared to the human anatomy is the apparent lack 

of the Tensor fasciae femoris in the Ateles specimen studied. According to the literature (Stern), part 2 of this 

muscle should have been present yet it is likely that this muscle is not well differentiated and was included in 

the above description of the Gluteus maximus. 

While much of the gluteal region is characterized by undifferentiated musculature, one interesting exception is 

the existence of a separate Piriformis which is not integrated into the Gluteus medius. This division also 

occurs in humans but is rare among the Cebids and even within the genus Ateles. Another muscle in this region 

is also clearly divided and separate from the gluteals. Scansorius (not present in Homo) originates from the 

lateral part of the ilium and inserts into the femur slightly distal to the insertion of the Gluteus medius. This 

muscle serves as a fine control of thigh movement via flexion, medial rotation, and abduction. Thus, despite 

decreased muscle mass and less clearly defined hip musculature, certain muscles are present and well 

differentiated in Ateles and permit the complex motor control necessary for suspension. The only muscles not 

present are the Tensor fasciae femoris and the Superior gemellus. 


Figure 16: Ateles gluteal region. (A) gluteus medius, (B) semitendinosus, (C) tail. 

54


The adductor group was difficult to dissect due to a previous necropsy done on the specimen, however it is 

fairly clear that no broad distinctions can be made between the individual muscles. Most noticeably, the 

Pectineus and Adductor longus are fused for much if not all of their lengths. This lack of differentiation and 

the decreased sizes of the adductor muscles of the thigh is due to the reduced use of the hindlimbs in Ateles. 

The difference is particularly profound when the morphology in this region is compared with that of a terrestrial 

quadruped, whose adductors are greatly enlarged in order to restrict mobility in the lateral plane and thereby 

increase the power generated for movement in the parasagittal plane. 

The final morphological differences between Ateles and Homo in the musculature of the thigh deal with the 

extensor muscles. The Semitendinosus originates as in humans yet inserts with Gracilis into a thick tendon 

which attaches to the medial portion of the tibia along much of the bone’s length. The Semimembranosus 

originates as in Homo but inserts both onto the medial epicondyle of the tibia and to capsule of the knee (to a 

popliteal ligament). Finally, Biceps femoris has a morphology differing from the human form due to the 

presence of a well developed second head which inserts into a thick fascia surrounding the (lower) leg. The two 

heads form a distinctive ‘X’ pattern which is readily identifiable. 

Lower Leg: 

The muscles of the leg have essentially the same morphology in both Ateles and Homo. The most profound 

distinction is the greater size of the Triceps surae group in humans due to bipedal walking. However, this 

muscle group as a whole is relatively large in spider monkeys and may be due in part to the practice of bipedal 

walking in this genus. Bipedal walking is more common in captivity and thus the particular specimen studied 

may have had slightly more developed calf muscles due to it being a zoo animal. The other differences of the 

Triceps surae when compared to human specimens were the increased size of the medial head relative to the 

lateral head of Gastrocnemius and extensive fibular origin of Soleus in Ateles. 

Few other differences can be noted between the human and spider monkey morphologies of the leg 

musculature. No Peroneus tertius could be located, in accords with its variable presence in humans. Popliteus 

inserted more distally in Ateles. This extended insertion gives a mechanical advantage in medial rotation of the 

leg, a useful feature for a suspensory primate. Flexor hallicus longus has more extensive origins along the 

fibula and interosseus membrane. Finally, Abductor hallicus longus originates from the tibia and interosseus 

membrane and inserts onto the base of the first metatarsal. This muscle acts as an abductor of the opposable 

hallux and is not present in Homo. 

Agur, M.R. Grant’s Atlas of Anatomy. Gardner, J.N. (ed.). Williams and Wilkins. Baltimore, MD (1991). 

Aiello, L. and C.Dean. An Introduction to Human Evolutionary Anatomy. Academic Press. San Diego, CA 

(1990). 

Bergeson, David. Personal communication and unpublished materials. 

Chang, H-T. and T.C.Ruch. “Morphology of the Spinal Cord, Spinal Nerves, Caudal Plexus, Tail Segmentation, 

and Caudal Musculature of the Spider Monkey.” Yale Journal of Biology and Medicine, 19 (1947), 345-77. 

Netter, F.H. Atlas of Human Anatomy. Colacino, S. (ed.) Ciba-Geiba Corporation. Summit, NJ (1989). 

Schon, M.A. The Muscular System of the Red Howling Monkey. Smithsonian Institution Press, Washington DC 

(1968). 

Stern, J.T. Functional Myology of the Hip and Thigh of Cebid Monkeys and its Implications for the Evolution of 

Erect Posture. Bibliotecha Primatologica 14. Karger. Basel (1971). 


55


______________________________ 

ORANGUTAN 


______________________________ 


The deep back muscles, collectively termed the erector spinae group includes the muscles iliocostalis, 

longissimus and spinalis. Collectively these muscles work in extension of the spinal column, ventral flexion of 

the back and help to laterally flex and rotate the spinal column. The iliocostalis can be found along the ribs, the 

spinalis along the spinal cord and the longissimus can be found between the other two. These muscles in the 

orangutan share the same functions, outlined above, as in the human. 

Orangutans, like all hominoids, lack tails and thus lack extensive representation of the caudal musculature. In a 

few individuals, some of the caudal musculature can develop, but only the most cranial aspects and only in a 

very rudimentary form. 

Hip: 

Unlike the shoulder and forearm musculature where the orangutan shared much of the muscular function with 

the humans, the lower limb diverges a good deal, mainly due to the specialized nature of the bipedal humans. 

This paragraph will discuss the gluteal group of muscles in the orangutan. Gluteus maximus in the orangutan 

is divided into two distinct muscles--the gluteus maximus proprius and the ischiofemoralis. The gluteus 

maximus proprius originates from the sacrum and inserts onto the fascia lata and proximal linea aspera of the 

femur. The ischiofemoralis takes it origin from the ischial tuberosity and inserts on the gluteal tuberosity to the 

middle of the femoral shaft. The gluteus maximus proprius functions as a thigh abductor and the 

ischiofemoralis functions as a thigh adductor. Together these two function as lateral rotators of the hip. The 

gluteus minimus, like the gluteus maximus, is divided into two distinct muscles in the orangutan. The gluteus 

minimus proprius takes its origin from the caudal half of the ilium and inserts along the greater trochanter. 

The scansorius originates from the ventral fibers of minimus and inserts along the greater trochanter also. Both 

minimus portions function as thigh abductors and lateral rotators. In the orangutan dissected for this class, all 

the deep gluteals (gluteus medius, gluteus minius proprius and scansorius) were fused into one muscle mass. 


Figure 17: Orangutan 

lateral thigh. (A) 

ischiofemoralis, (B) 

biceps femoris, long 

head (C) biceps 

femoris, short head, 

(D) semi-tendinosus. 

56


Pongo lacks a tensor fascia lata, but retains a lateral fascia. For all the muscles in the quadriceps femoris 

group, there is a tendency for them not to be greatly differentiated within the orangutan. The rectus femoris 

anatomy in the orangutan is similar to monkeys in that its origin is from the acetabular border of the ilium and 

inserts in a common tendon along the patella, it functions to extend the leg and flex the thigh. The sartorius 

also takes it origin from the acetabular iliac border and inserts onto the medial tibial shaft. In Pongo, it 

functions to flex the leg and thigh and rotates the thigh laterally (like in leg crossing motions). 

The leg extensors will be discussed in this paragraph. The biceps femoris has two distinct heads in Pongo. 

The long head originates from fibers of the ischiofemoralis. The short head takes its origin from the lower 

portion of the linea aspera and inserts onto the head and shaft of the fibula. It functions to extend the hip, flex 

the knee and rotate the leg laterally. 

The adductor group of muscles in the orangutan are powerful. The adductor magnus is a large muscle in the 

orangutan. It originates on the inferior ischiopubic ramus and ischial tuberosity and inserts onto the gluteal 

tuberosity, linea aspera and popliteal surface of the femur. It adducts the thigh and aids in flexion and lateral 

rotation of the thigh. The adductor brevis morphology in Pongo is similar to that in monkeys. Its origin is the 

superior pubic ramus and inserts along the proximal quarter of the linea aspera. It functions to adduct, flex and 

laterally rotate the thigh. 

For the lateral rotators of the thigh, most of the origin and insertion of the muscles are similar between humans 

and Pongo, the main differences lie in their functions. The obturator internus abducts and laterally rotates the 

thigh. The gemellis (the superior portion is smaller in orangutans than the inferior) abduct and laterally rotate 

the thighs. The quadratus femoris is another abductor and lateral rotator of the thigh. Obturator externus 

originates from the lateral border of the obturator foramen and inserts along the intertrochanteric fossa of the 

femur. It functions as an external rotator of the thigh. 

Like the arm musculature, the leg musculature in orangutans shows better development of flexors and adductors 

than extensors. The relative development of the flexors and adductors relate to the orangutan’s locomotion of 

suspensory quadrumanus climbing. 


Figure 18: 

Orangutan medial 

thigh. (A) sartorius, 

(B) gracilis, (C) 

adductor magnus. 

57


Muscles that move the lower leg: 

The quadriceps femoris group of muscles are the main muscles which move the lower leg in primates. The 

anatomy and function of the vastus intermedius is similar in most primates. The vastus group of muscles 

function to extend the leg at the knee. The popliteus anatomy is also similar in most primates, it functions to 

rotate and flex the knee. 

Muscles that move the foot: 

Among these muscles, this orangutan is missing three which can be found in other primates: the peroneus 

tertius, plantaris and peroneotibialis. The flexor hallucis longus (digitorum fibularis) and the flexor 

digitorum longus (tibialis) are both missing the tendon that inserts onto the hallux. The orangutan’s shortened 

hallux is positioned in a permamently abducted position, therefore the muscles that would function in hallucal 

grasping in the orangutan are the hallux adductors. For the other tendons associated with these muscles have 

strong insertions for strong grasping associated with the orangutan’s postural behavior. 

The anterior extensor group of foot muscles include the tibialis anterior, extensor digitorum longus and 

extensor hallucis longus. The extensor hallucis longus is reduced in the orangutan, due to the small size of 

the hallux. The tibialis anterior originates along the lateral condyle and lateral shaft of the tibia and inserts 

along the base of the medial cunieform and base of the first metatarsal. It functions to dorsiflex and invert the 

foot. The extensor digitorum longus originates along the lateral condyle of the tibia and along the head and 

anterior surface of the fibula its four tendons insert along the dorsum of the middle and distal phalanges of 

digits II-V. It functions in foot dorsiflexion and extension of the digits. 

The lateral extensor group includes the peroneus longus, peroneus brevis and peroneus digiti quinti (which 

is absent in humans). All the peroneus muscles function to evert the foot. The peroneus longus originates 

along the lateral fibular head and shaft and inserts onto the base of metatarsal I. The peroneus brevis originates 

along the lateral surface of the fibula and inserts onto metatarsal V. The small peroneus digiti quinti originates 

along the posterior surface of the proximal fibula and inserts along the lateral side of digit V. 

The posterior flexors include the gastrocnemius, soleus, popliteus and tibialis posterior in the orangutan. The 

soleus originates from the posterior surface of the head of the fibula, inserts along the calcaneus and is a larger 

muscle in the orangutan compared with humans. The popliteus functions to help rotate and flex the knee and it 

originates on the lateral epicondyle of the femur and inserts along the medial condyle of the tibia. The tibialis 

posterior shares its anatomy with most primates. It originates along the proximal portions of the posterior tibia 

and fibula and inserts along the base of the foot within both the tarsals and II-IV metatarsals. The adductor 

hallucis longus is relatively well developed in the orangutan when compared to the other hallux muscles. It 

originates along the medial side of the calcaneal tuberosity and inserts along the medial side of the proximal 

phalanx of the first digit. 

Aiello L and C Dean (1990) An Introduction to Human Evolutionary Anatomy. New York: Academic. 

Tuttle RH and GW Cortright (1988) Positional behavior, adaptive complexes, and evolution. In JH Schwartz 

(ed.): Orangutan Biology. New York: Oxford. pp.311-330. 


58


______________________________ 

COMPARATIVE MUSCULATURE 

The Hindlimb 

______________________________ 

Deep Back and Tail: 

The erector spinae musculature--including the spinalis, longissimus, and iliocostalis--is more developed in 

those primate species which have a tail (S. oedipus, G. crassicaudatus, M. fasicularis, and Ateles). In those 

primates with a small tail (M. mulatta) or no tail at all (P. pygmaeus), the deep back musculature does not differ 

much from Homo. This makes sense when locomotor patterns are taken into account. The smaller primates 

with tails engage in leaping, and the erector spinae groups aids in both flexion and extension of the back, both 

necessary movements for efficient leaping ability. In those primates without a tail and which do not leap, the 

erector spinae group mainly supports the trunk, and can aid in lateral rotation as well. 

The tail muscles are the most elaborate in Ateles, the only primate dissected in this group to have a prehensile 

tail. This “fifth limb” results in a different musculoskeletal and nervous system in the following ways: 

1) an increase in the number of caudal vertebrae 

2) an increase in innervation of the tail 

3) a continuation of the erector spinae muscles caudalward along the dorsal side of the tail. 

The main difference between Ateles and other primates with a tail is that six muscles repeat at each vertebral 

segment along the length of the tail--abductor caudae internus et externus, flexor caudae lateralis, flexor 

caudae medialis, extensor caudae medialis, and extensor caudae lateralis. This results in prehensility, and 

ability to finely manipulate the tail around objects. In the other tailed primates, the lack of prehensility results 

in more generalized musculature, with the three muscles of importance being sacrococcygeus dorsalis 

lateralis, sacrococcygeus dorsalis medialis, and intertransversarius dorsalis medialis. 

Hip and Thigh: 

The comparative anatomy of the hip musculature can be best analyzed by separating the primates dissected into 

two groups--the arboreal/terrestrial quadrupeds and leapers, and the more suspensory/semibrachiator types. 

First, the following quadrupedal species exhibit similar morphologies of gluteal muscles--S. oedipus, G. 

crassicaudatus, Saimiri, M. fascicularis, M. mulatta, and P. hamadryas. The main function of the gluteus 

maximus and medius in these species is thigh protraction and retraction for movement in the parasagittal plain. 

The respective sizes of the gluteus maximus and medius are roughly equal when compared to Homo, where the 

maximus is larger, reflecting is primary function as a hip extensor. Moreover, these species all have a robust 

tensor fascia latae, usually fused with the gluteus maximus, which covers the majority of the anterior and 

lateral thigh whereas it is restricted to the lateral thigh in Homo. The arrangement of the muscles in this way 

with their corresponding robust sizes enables these species to repeatedly flex and extend the hip during 

quadrupedal running and walking. 

The suspensory primates, P. pygmaeus and Ateles, exhibit different morphology in the gluteal muscles. In these 

two primates, the gluteus maximus is more reduced and generalized; it resembles and functions much like a 

deltoid in rotating the lower limb. In the orangutan, it is divided into the gluteus maximus proprius and 

ischiofemoralis, a thigh abductor and adductor, respectively. In both Pongo and Ateles, the gluteus minimus is 

divided into the gluteus minimus proprius and scansorius, both thigh abductors and lateral rotators. The 

scansorius, especially, enables fine control of thigh movement via flexion, medial rotation, and abduction. 

Thus, the hip musculature in these two primates differs from the above quadrupeds in that movement is not 

primarily limited to the sagittal plain; rather, all-around rotation is facilitated for the needed ability to reached 

and support the upper body on various substrates during suspensory locomotion and posturing. 

Concerning the thigh muscles, clear differences can be related to locomotor patterns. In S. oedipus, the leg 

musculature is relatively less developed than that of the arm; this reflects the use of the arms in climbing. In the 


59


leaping primates, G. crassicaudatus and Saimiri, extremely robust leg extensors and flexors are evident (vastus 

lateralis, rectus femoris, and sartorius, especially). Rather than carry its hindlimbs in the abducted psoture of 

an arboreal quadruped, the muscles used for abduction and adduction are used to stabilize the leg in the 

parasagittal plain during takeoff and landing. The biceps femoris-gracilis/sartorius “sling” aids in leaping, 

also. In the non-leaping quadrupeds M. mulatta, M. fascicularis, and P. hamadryas, the biceps femoris is 

enormous and attaches halfway down the lateral tibia to function in leg flexion and thigh retraction when the 

knee joint is fixed. The adductor muscles are also robust in these three species, while the sartorius and gracilis 

are not so developed as in the leapers. Regarding the primates who engage in some suspensory locomotion, 

Ateles and Pong, the leg musculature shows better development of flexors and adductors than extensors; since it 

is unnecessary for these species to receive much propulsive force from the hindlimbs, these morphologies are to 

be expected. Of note, both the spider monkey and orangutan have two heads of the biceps femoris, whereas the 

other primates dissected do not. 

Leg: 

In most of the primates examined, the leg musculature showed minor differences from Homo; most involved 

slight variations in origins and insertion points. Of note, the abductor hallicus longus, absent in humans, 

originates as part of the tibialis anterior and inserts on the base of the first metatarsal for the primates dissected. 

In the leaping primates, the muscles involved in plantarflexion (plantaris, gastrocnemius, soleus) are 

especially robust for propulsive force. Additionally, muscles involved in inversion and eversion (tibialis 

posterior, peroneus longus, peroneus brevis) of the foot are equally developed to stabilize the foot while 

leaping. In the non-leaping quadrupeds, the plantaris and other muscles of the posterior leg compartment are 

robust, presumably for propulsion during quadrupedal running and walking. In Pongo and Ateles, the adductor 

hallucis longus is well developed for the grasping ability of the foot in suspensory posture and locomotion. 

Pongo does not have a pernoneus tertius, plantaris, or peroneotibilias, which are present in the other 

primates dissected. 

______________________________ 


The Hip Complex 

______________________________ 

The morphological variations in the hip complex found among the primates reflect their different locomotor 

behaviors. Particularly, differences relate to the degree of mobility and the amount of weight bearing required 

by different locomotor patterns. The skeletal terminology between the different species, however, is similar. 

The pelvis and hip complex refer to five elements: the ilium, ischium, and pubis make up the innominate, the 

sacrum articulates with two innominates to form the pelvis, and the head of the femur articulates with the pelvis 

to form the hip joint. The acetabulum is the meeting place of the ilium, ischium and pubis, as well as the place 

of articulation of the femoral head. The hamstrings (hip extensors) attach to the ischium, and anterior-inferior 

iliac spine provides origin for the rectus femoris, a quadriceps muscle that flexes the hip as well as extends the 

leg at the knee. The auricular surfaces of both the sacrum and iliac blade is the place of articulation between the 

two bones. 

The major mechanical issue faced by habitually bipedal primates (Homo sapiens) is balance, particularly from 

side to side, and supporting all the body weight on a single pair of limbs. Both the pelvis and femur show 

adaptations relating to bipedal locomotion. The iliac blade of humans is short, broad and mediolaterally 

oriented. This serves to bring the lesser gluteal muscles (gluteus medius and minimus) into a position at the 

side of the body, where they act as abductors to balance the trunk over the lower limb during bipedal 

locomotion. That is, during the phase of locomotion when only one leg is supporting the entire body weight, 

the lesser gluteals act to stabalize the trunk over the planted leg. This prevents the side to side lurching of the 

trunk during bipedal locomotion. This contrasts non-human primates, which possess longer, narrower and more 

posteriorly oriented iliac blades. Reinforcement of the anterior part of the blade against the pull of the gluteal 


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muscles is also needed; therefore, an iliac pillar (= a thickening of bone) runs down the blade from the iliac 

crest to the acetabulum to aid in this reinforcement. 

The ischium in humans is extended posteriorly. This provides greater leverage for the major hip extensors 

which move the lower limb behind the trunk. The human sacrum is relatively wider than non-human primate 

sacra and is positioned at a more acute angle to the lumbar spinal column. The greater width increases the 

distance between the two sacroiliac joints and positions them more vertically over the hip. This helps reduce 

stress in the pubic symphysis and transmits the weight over the lower limbs. The orientation provides increased 

leverage for the muscles of the back that balance the spine over the pelvis. In addition, the auricular surfaces on 

both the ilium and sacrum are relatively large, providing a greater area for transmitting the weight of the trunk. 

The morphology of the human femur also relates to weight bearing. The femoral head, which supports the 

weight of the entire body during locomotion, is relatively large. In addition, the femur is aligned obliquely, 

with the proximal ends much further apart than the distal ends (the valgus position). These adaptations permit 

the limb to be near the midline of the body (its center of gravity) during the phase of the walking cycle when 

only one limb is one the ground. 

Non-human primates contrast with the human morphology in the skeletal areas above. In addition, various 

primate show different morphologies relating to their major locomotor pattern - quadrupedalism (arboreal and 

terrestrial), leaping, and suspension. 

The locomotor pattern of terrestrial quadrupeds involves the restriction of lower limb movement to the 

parasagittal plane; i.e., simple fore-aft movement at the hip without extensive rotation or abduction. To provide 

powerful extension of the thigh at the hip, the ischium is relatively long compared to acetabular diameter and 

minimum width of the iliac blade. Also, the load arm of the hamstrings (based on the length of the ischium and 

the distance from the hip joint to the ground) is in a more advantageous position when the hip is flexed, as in 

terrestrial quadrupeds. Thus, the long ischium is adapted to powerful hip extension and not to increased range 

of motion. Related to this decreased range of motion is the depth of the acetabulum. The deeper the 

acetabulum, the more restricted is the movement of the hip joint. Habitually terrestrial quadrupeds have the 

deepest acetabulae. 


Figure 19: Primate hip bones. Counterclockwise, from bottom left: Homo, Ateles, Papio, 

Macaca mulatta, M. fascicularis, Saguinus, Galago, Saimiri. 

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The major concern for arboreal quadrupeds is balance and stability. These animals locomote on supports that 

are usually small compared to the animal and often unstable and uneven. One solution to this problem is to 

bring the center of gravity closer to the support. This is accomplished by using abducted and flexed limbs 

during locomotion. Thus the femoral neck is set at a high angle relative to the shaft. 

The auricular surfaces of quadrupedal animals are proportionately much smaller to the total iliac width than in 

humans. This is due to the fact the quadrupedal primates do not support the entire weight of the upper body on 

their pelves and lower limbs. In other words, the area required for weight transfer is not as large as humans. 

The narrower sacrum and less angulation to the vertebral column also reflect the different weight support 

system in quadrupedal animals. 

The hip complex of hindlimb suspensory primates is characterized by very mobile joints. Mobility at the hip 

joint is increased by the spherical head of the femur set on a highly angeled femoral neck. This permits extreme 

degrees of abduction. The extreme mobility is also reflected in the depth of the acetabulum; hindlimb 

suspensory primates have relatively shallow acetabulae, orangutans having the shallowest among monkeys and 

apes. 

In leaping primates, most of the propulsive forces come from a single, rapid extension of the hindlimbs with 

little or no contribution from the forelimbs. The major source of this propulsive force is hip extension. 

Therefore, leaping primates tend to have relatively long ischiums which increase the leverage of the hamstrings. 

The direction of ischial extension depends on the postural habit of the leaping species. The ischium extend 

distally in line with the ilium in primates that leap from a quadrupedal position. This enhances hip extension in 

the quadrupedal position when the hindlimb is at a right angle to the trunk. In contrast, the ischium usually 

extends posteriorly in primates that leap from a vertical clinging position, which increases the moment arm of 

the hamstrings. 

Leapers, in contrast to arboreal quadrupeds who use abducted and flexed limbs for balancing on small supports, 

restrict lower limb movement to simple flexion and extension. This restriction helps to avoid twisting and 

damaging of the joints during powerful take-offs, as well as providing greater mechanical efficiency. In this 

regard, the leapers more closely resemble terrestrial quadrupeds. Finally, the strongly developed anteriorinferior 

iliac spines are large in leapers because the rectus femoris is an important and well developed leaping 

muscle. 

Aiello, L and Dean, C. An introduction to human evolutionary anatomy. London: Academic Press Inc. 

Fleagle, JG. 1988. Primate adaptation and evolution. San Diego: Academic Press Inc. 

______________________________ 


Knee, Ankle and Foot 

______________________________ 

In this section I will give a general overview of the differences in skeletal morphology of the primate knee, 

ankle, and foot. This section is designed to alert the reader to a few specific differences that can be seen easily 

when viewing comparative photographs or when actually holding postcrania. This overview is, by no means, a 

complete discussion of primate skeletal adaptations to positional behavior at the knee, ankle, and foot. 

Several distinct features of the knee joint are adaptations to different types of position behavior. The carrying 

angle of the femur on the tibia can be used to discern between arboreal, terrestrial, and obligate bipedal animals. 

When the condyles of the femur extend distally to an equal length, as in Papio, the femur sits directly above the 


62


tibia at an angle of 180o . This is because terrestrial quadrupeds move in a hinge-like manner of flexion and 

extension in the parasagittal plane when locomoting. Arboreal quadrupeds tend to hold their femora at the hip 

in an abducted position in order to bring their centers of gravity closer to their supports. In order to bring their 

feet back underneath their bodies and upon the branches, the carrying angle of the femur on the tibia is reduced 

(when measured medially) to less than 180o . This is done by increasing the extent to which the lateral femoral 

condyle extends distally. This can be seen very easily in the Saimiri and less so in both species of Macaca. 

This variation among the arboreal primates reflects the tendency for Macaca to move, on occasion, in a 

terrestrial environment. The carrying angle of the femur on the tibia for Homo is greater than 180o (when 

measured medially), in response to the angle at which the femur leaves the hip joint. As with the arboreal 

primates, this carrying angle for Homo brings the feet underneath the body to facilitate locomotion. 


Figure 20: Primate femora. From left: Saguinus, Galago, Saimiri, Macaca fascicularis, M. 

mulatta, Ateles, Homo (top). 

The shape of the femoral condyles and of the tibial plateau are also interesting to note. These differences can be 

seen most easily in a comparison of Ateles and Galago, with the other species falling between them along a 

continuum. The knee of Ateles (and to a lesser extent Pongo) is used in a great deal of suspensory behavior, 

and therefore needs to be mobile. This is characterized by broad, shallow, round femoral condyles, as well as 

shallow groove in which the patella sits. The galago, however, must have controlled, powerful flexion and 

extension at the knee. Mobility at this joint would prove disastrous for a leaping primate. The morphology, 

therefore, serves to limit the possibility for twisting and dislocation. The femoral condyles are deep and 

compressed mediolaterally, creating a tight fit with a decreased range of motion. The patella has a prominent 

lateral ridge, which also serves to stabilize this joint. Papio and Macaca also have femoral condyles that are 

compressed mediolaterally as a result of repetitive movement in the parasagittal plane. 

The ankle (or talocrural) joint is a complex arrangement of several bones, at which point a variety of motions 

occur. The basic structure of the ankle consists of the calcaneus, upon which rests the talus. The calcaneus is 

the bone upon which the gastrocnemius, plantaris, and soleus tendons attach, thus forming the heel of the foot. 

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The talus articulates proximally with the distal end of the tibia. Distally, the talus articulates with the three 

cuneiform bones. The calcaneus articulates with cuboid distally. These four bones -- the cuboid and the three 

cuneiforms -- articulate with the metatarsals. 

The four basic movements at the ankle joint are inversion, eversion, plantarflexion, and dorsiflexion of the foot. 

Disproportionate amounts of movement in these four directions as well as the need to react to and produce 

differing amounts of force at particular angles (i.e., the actions of positional behaviors, especially locomotion) 

affects the shape of the bones. 

All primates that use their hindlimbs in locomotion need to be able to plantarflex powerfully in order to propel 

their bodies forward. Dorsiflexion must subsequently be controlled, and able to absorb the shock of the animal 

as it lands. Animals that leap (Galago, Saimiri, and Saguinus) need to increase the amount of force that can be 

generated upon take-off. There are two different morphologic adaptations to achieving this in the foot and 

ankle. Note in the galago an elongated distal portion of the calcaneus. This elongation increases the lever arm, 

with the tibia/talus joint acting as the fulcrum. Saimiri and Saguinus increase the amount of force that can be 

produced by increasing the length of the bones found distally, especially the metatarsals. This lengthening of 

the foot increases the load arm of this lever system, with the tibia/talus joing again acting as the fulcrum. 

Another difference in foot and ankle osteology can be seen when arboreal quadrupeds and terrestrial 

quadrupeds are compared. Arboreal supports are flexible, unpredictable, and encompass a variety of sizes and 

angles. In contrast, terrestrial animals move on the relatively flat, predictable surface of the ground. Arboreal 

animals must, therefore, be able to move their ankles and feet to fit their supports. Inversion and eversion of the 

ankle joint is especially important, and can be seen osteologically in the range of motion that this joint allows. 

Terrestrial animals have much more restricted movement at their ankle joints than do arboreal. 

Note, though, among arboreal species, the difference between Ateles and Galago. Ateles has the greatest range 

of motion at the ankle of all of the species considered here. That is because this animal uses hindlimb 

suspensory postures when feeding. The galago, at the other extreme, must have very controlled movements at 

the ankle joint during its very powerful take-offs and landings. It, therefore, sacrifices some flexibility at this 

joint in order to prevent twisting when leaping. Naturally, the musculature reflects these differences as well 

(see previous texts). 

Common to all non-human primates is the grasping (or prehensile) ability of the foot. This adaptation is clearly 

seen when viewing the osteology of the hallux (big toe). Notice the saddle-shaped articulation between the 

proximal end of metatarsal 1 and the distal end of the entocuneiform bone. Arboreal primates have a greater 

range of motion at this joint than do the terrestrial species. This, again, is due to the type of supports upon 

which the animals must maneuver. 


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A COMPARATIVE PRIMATE ANATOMY - University of Cape Town

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