Language and life history: A new perspective on the development ...

Response/Locke & Bogin: <strong>Language</strong> <strong>and</strong> <strong>life</strong> <strong>history</strong>particularly to our treatment of <strong>life</strong> <strong>history</strong>, <strong>and</strong> then turnour attention to replies that pertain more specifically tolanguage.R1. Life <strong>history</strong>Any theory of human biology <strong>and</strong> culture needs to explainhow Homo sapiens successfully combines delayed reproduction,helpless <strong>new</strong>borns, a short duration of breastfeeding,a vastly extended period of offspring dependency,an adolescent growth spurt in both sexes, <strong>and</strong> femalemenopause a decade or more before death. No otherprimate species shares all of these traits. Theory also isneeded to explain the extraordinary size of the humanbrain, in relation to overall body size, <strong>and</strong> the extraordinarycognitive-behavioral abilities of the brain, includingsymbolic language, theory of mind, kinship, marriage, religion,<strong>and</strong> even nihilistic philosophy. In our target article,we apply <strong>life</strong> <strong>history</strong> theory in an attempt to better underst<strong>and</strong>human nature regarding growth, development, <strong>and</strong>language.One commentator, Weisfeld, denies that human beingshave a unique or unusual <strong>life</strong> <strong>history</strong>; in fact he does notuse the phrase “<strong>life</strong> <strong>history</strong>” at all in his commentary. IfWeisfeld’s critique is correct, then the whole of our argumentfalls. But, his citations regarding human <strong>life</strong> <strong>history</strong><strong>and</strong> nonhuman primate growth are both selective <strong>and</strong>somewhat old. The most recent citation is from 1993,well before some of the most salient work on primate<strong>life</strong> <strong>history</strong> was published. We cite that more recentresearch in our target article, <strong>and</strong> we provide additionalreferences in this response (see below). Moreover,Weisfeld’s interpretations are idiosyncratic, do not reflectthe broad consensus of primate <strong>life</strong> <strong>history</strong> researchers, orare incorrect. The studies he cites (also cited by Ragir &Brooks) alleging that all primates have a skeletal, or evena body weight, growth spurt at the time of pubertyoriginated in the 1950s (Gavan 1953). Gavan’s originalpaper concludes that chimpanzees (a sample of 9 males<strong>and</strong> 7 females) do not have a pubertal growth spurt.Watts <strong>and</strong> Gavan (1982) reanalyzed the 1953 chimpanzeedata, along with <strong>new</strong> data for the rhesus monkey, <strong>and</strong>reported that a “very small” positive deviation in bonegrowth could be detected at the time of puberty. “Verysmall” means less than a 3.0-mm deviation, which occursduring one year, <strong>and</strong> the detection scheme requiredpolynomial regression fitted to the bone growth data.We have criticized the methods <strong>and</strong> statistics needed toshow these “spurts” (Bogin 1999a; 1999b). The humanadolescent growth spurt is measured in centimeters –with a mean peak velocity of about 7.5 cm per year forgirls <strong>and</strong> 9.0 cm per year for boys. The human spurttakes about eight years to complete. Leigh (1996; 2001)shows that, among 61 primate species, weight growthspurts vary greatly in magnitude <strong>and</strong> even in their existence– only one species of New World monkey may have aweight spurt; gibbons do not, <strong>and</strong> only males of thecommon chimpanzee do. Leigh could not confirm thatany nonhuman primate species has a skeletal growthspurt. Hamada <strong>and</strong> Udono (2002) published the first statisticallysound <strong>and</strong> methodologically rigorous longitudinalstudy of chimpanzee skeletal growth. They found thatchimpanzees reared within semi-natural social conditionsat a zoological park in Japan show no increase in longbone growth velocity at the time of puberty (Fig. R1).In contrast, chimpanzees used in medical research, whowere taken out of their social groups prior to puberty,stopped or greatly slowed growth during the time of experimentation.When returned to their social group afterexperimentation these chimpanzees experienced rapidgrowth, imitating a growth spurt. But this is not at allsimilar to the human adolescent growth spurt. Rather,Hamada <strong>and</strong> Udono (2002) explain that it is a type of“catch-up” growth following the stress of social separation<strong>and</strong> medical experimentation. Human beings show thisFigure R1. Model of distance (left) <strong>and</strong> velocity (right) curves for chimpanzee growth in body length by years of age. Data for thefigure come from the longitudinal study of captive chimpanzee growth conducted by Hamada <strong>and</strong> Udono (2002). The infancy,juvenile, <strong>and</strong> mature adult <strong>life</strong> stages are labeled. In the wild, weaning (W) usually takes place between 48–60 months of age(Pusey 1983). In captivity, female puberty (P), assessed by age at first maximal perineal swelling, takes place at a mean age of 7.95years (Littleton 2005).302 BEHAVIORAL AND BRAIN SCIENCES (2006) 29:3