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Characteristics of the growth of chimpanzee and the implications for its evolution.

Yuzuru Hamada
(Section of Morphology, Primate Research Institute, Kyoto University)

     Growth pattern reflects the phylogenetic position, ecology, and social system of a given species. Compared with macaques, great apes and humans, chimpanzees (Pan troglodytes) have such growth characteristics as follows: relatively longer infantile period, that is, the period before weaning; shorter juvenile period: and longer post-adolescent growth period, that is, the period from the end of adolescent rapid growth to the end of growth. They do not also show the adolescent growth spurt in linear physical dimensions. Thus the same relationships between somatic growth and reproductive development found in macaques and humans does not always hold for all catarrhine primates. I give here an example of the discussion of the evolution of chimpanzee's longer infantile period. It is true at the comparison with gorilla and orangutan. Ecological and social factors are considered to have influenced the length of infantile period. First factor is the care of infant. Infant chimpanzee and orangutan is cared only by its mother. On the other hand, gorilla infant is also taken care of by adult male gorilla, and human infant is by grandmother and/or the other member(s) of the society. Second factor is the feeding habit. Chimpanzees are fruit eater. As they tend to live in a relatively dry forest, the distribution of fruit trees is patchy and the variation of yield is wide with time. Although orangutan is fruit eater, it inhabits the dense forest. Gorilla takes fibrous plant parts more than chimpanzee does. Humans eat variety of food items and shared foods. The variation of yield with time in the three anthropoids is less than that in chimpanzee.
     Facts described above would give an impression that only chimpanzee has specialized its infant growth. However, another idea is also possible, that the all subject species have specialized with different fashion. Taking all growth characteristics into consideration, I will examine the evolution in growth.