Pongidae

Abstract: The chimpanzee (Pan troglodytes, Pongidae) among all other living species, is our closest relation, with whom we last shared a common ancestor less than five million years ago. These African apes make and use a rich and varied kit of tools. Of the primates, and even of the other Great Apes, they are the only consistent and habitual tool-users. Chimpanzees meet the criteria of working definitions of culture as originally devised for human beings in socio-cultural anthropology. They show sex differences in using tools to obtain and to process a variety of plant and animal foods. The technological gap between chimpanzees and human societies living by foraging (hunter-gatherers) is surprisingly narrow, at least for food-getting. Different communities of chimpanzees have different tool-kits, and not all of this regional and local variation can be explained by the varied physical and biotic environments in which they live. Some differences are likely customs based on non-functionally derived and symbolically encoded traditions. Chimpanzees serve as heuristic, referential models for the reconstruction of cultural evolution in apes and humans from an ancestral hominoid. However, chimpanzees are not humans, and key differences exist between them, though many of these apparent contrasts remain to be explored empirically and theoretically.

Abstract: It has long been known that primate species differ greatly in the weight of their testes relative to body weight1. Recently it has been suggested that among the three species of Pongidae (the great apes), the disparity in testes weights is associated with their different breeding systems2–4. Male gorillas and orangutans copulate infrequently, and when a female comes into oestrus she normally mates with only one male. However, in the chimpanzee, several males mate frequently with the oestrous females, so that each male has to deposit enough sperm to compete with the presence of sperm from other males. For the chimpanzee, therefore, we hypothesize that selection will favour the male that can deposit the largest number of sperm; thus the volume of spermatogenic tissue and hence testis size is far greater in the chimpanzee than in the gorilla or orangutan. If this is correct, it implies that primates in which more than one male mates with each oestrous female should have larger testes relative to their body weight than those with single-male breeding systems. We have tested this prediction across a wide range of primates, and the results support the hypothesis. The relative size of testes may, therefore, provide a valuable clue to the breeding system of a primate species.

Abstract: The social organization of spider monkeys (Ateles geoffroyi) and chimpanzees (Pan troglodytes) appear remarkably similar. In this paper, field studies of these two species were used to (1) test a model of ecological constraints on animal group size which suggests that group size is a function of travel costs and (2) assess ecological and social factors underlying the social organization of these two species. Spider monkeys were studied over a 6-year period in Santa Rosa National Park, Costa Rica, and chimpanzees were studied for 6 years in Kibale National Park, Uganda. Adults of both species spent their time in small subgroups that frequently changed size and composition. Thus, unlike most primate species, spider monkeys and chimpanzees were not always in a spatially cohesive social group; each individual had the option of associating in subgroups of a different size or composition. Both species relied on ripe fruit from trees that could be depleted through their feeding activity. However, spider monkey food resources tended to occur at higher densities, were more common, less temporally variable, and did not reach the low levels experienced by chimpanzees. Analyses of the relationship between subgroup size and the density and distribution of their food resources suggested that travel costs limit subgroup size. However, these ecological factors did not influence all age/sex classes equally. For example, the number of adult males in a subgroup was a function of food density and travel costs. However, this was not the case for female chimpanzees, suggesting that the benefits of being in a subgroup for females did not exceed the costs, even when ecological conditions appeared to minimize subgroup foraging costs. Therefore, it seems likely that social strategies influenced the relationship between food resource variables and subgroup size.

Abstract: Female chimpanzees often forage alone and do not display obvious linear dominance hierarchies; consequently, it has been suggested that dominance is not of great importance to them. However, with the use of data from a 35-year field study of chimpanzees, high-ranking females were shown to have significantly higher infant survival, faster maturing daughters, and more rapid production of young. Given the foraging behavior of chimpanzees, high rank probably influences reproductive success by helping females establish and maintain access to good foraging areas rather than by sparing them stress from aggression.