Special Issue Survey of Cetaceans in Captive Care

2. Whales, Dolphins, and Porpoises: Presentation of the Cetaceans 295of 500 Hz to 100 kHz, whereas humans hear wellfrom about 200 Hz to 17 kHz. The majority ofodontocetes have peak sensitivities in the ultrasonicranges, although most have moderate sensitivityfrom 1 kHz to 20 kHz and no acute hearingbelow 500 Hz (< 80 dB re 1 µPa). Models indicatethat the mysticetes’ functional hearing range commonlyextends down to 20 Hz. Several species areexpected to hear well into infrasonic frequencies,from 20 Hz to 30 kHz (Ketten, 1998).The odontocete larynx does not possess vocalcords. Instead, sounds are produced by nasalsacs in the blowhole region and controlled byorgans called monkey lips (Cranford et al., 1997;Degollada et al., 1998). The fatty melon abovethe skull acts as an acoustical lens focusing andamplifying these sounds into a beam. Receivedsounds are transmitted to the inner ear by thebones and fatty channels of the lower jaw. Manyodontocetes communicate with whistles andburst-pulse sounds. Whistles are continuous, frequency-modulatedpure tones, varying in intensityand pattern, with one or more harmonics. The frequencyof bottlenose dolphin whistles is generallybetween 4 and 24 kHz (Herman, 1980). Dolphinscan whistle and echolocate at the same time. Burstpulsesounds are another type of communicationsounds. They are broadband signals resemblingmoans, trills, grunts, squeaks, and creaking doors.Sperm whales, members of the porpoise family,and members of the river dolphin family appearto produce only clicks and bursts of clicks, whichmay function for both echolocation and communication(Evans, 1987). Acoustical interference canhave a dramatic effect on the behaviour and physiologyof captive animals. Loud, human-made,underwater noises within their hearing frequenciescan adversely affect cetaceans (Richardson et al.,1995). Acoustical interference, which cetaceanscannot escape in captivity, can cause endocrinechanges, increased aggression, decreased appetite,and irreversible hearing loss (Stoskopf & Gibbons,1994). The quality of their acoustical environmentin human care is critical and often has beenneglected. It is important to suppress human-madebackground noise, and to design a habitat thatreduces sound reverberation (see Chapter 5 fordetails).CognitionSenses are used by animals to understand their environment.The cognitive characteristics of dolphinsand whales are reflected in how the information isselected, encoded, stored, analysed, and retrievedby various sensory receptors. Little is known aboutbaleen whales’ cognitive abilities, but those of thebottlenose dolphins have been studied for manyyears. The bottlenose dolphin is a quick learner inmost types of auditory tasks, including those havingintricate conceptual demands. Its auditory memoryis very faithful (Herman, 1980). The dolphin iscapable of forming and generalising concept rules.Dolphins also seem proficient at imitative behaviours,and they are able to perform both vocal andmotor mimicry, demonstrating that they can learn byobservation. Despite the fact that attempts to demonstratethe existence of a natural language havefailed, dolphins may be capable of comprehendinga simple artificial language. Gestural language comprehensionseems to be better if the visual informationmanipulated is dynamic (Herman, 1980, 1986,1989). The extensive development of the dolphin’sbrain and the resulting cognitive skills have evolvedfrom the demands of social living, and accordingto Herman (1989), they require “the acquisitionand use of knowledge to facilitate an exchange ofinformation within a mutually dependant social network.”In other words, cetaceans may have developedtheir complex cognitive abilities because ofthe need for high flexibility and “communality” intheir social organisation and behaviour (Johnson &Norris, 1986).Social Life and BehaviourWhales and dolphins are essentially social animals.Group size varies greatly among species, beinganywhere from two to several thousand individuals;however, common social patterns and behaviourshave been identified. Living in groups hasseveral advantages: it maximises foraging, helpswith defence against predators, brings individualstogether for reproduction, and increases the efficiencyof calf rearing (Evans, 1987). Some species,such as the killer whale, show stronger social cohesionthan others. Cetaceans do not form stable longtermmale-female bonds, as wolves do for example.The basis of the social unit is the reproductivefemale; most of the stable and long-lasting socialinteractions that are ongoing within a group arebetween adult females and their offspring (Sweeney,1990). In bottlenose dolphins, multigenerationalfemale bands tend to be composed of females whoare related or who share long histories of associationwithin a common home range. Females at thesame stage of their reproductive cycle tend to swimtogether. Female cetaceans care for their offspringuntil they are weaned. Bottlenose dolphin calvestypically remain with their mother for three to sixyears (Wells & Scott, 1999). A mother and youngpair may remain together on an intermittent basisfor long periods. Bottlenose dolphins may return toeach other in times of stress many years after theoffspring have reached adulthood (Norris & Dohl,1980). Mothers continue to care for their younguntil the following pregnancy and some even longer.Other females sometimes assist the mother during