Positional behavior in Pan troglodytes at the Mahale Mountains and the Gombe Stream National Parks, Tanzania.

Chimpanzee anatomy and body proportions are unusual among the primates, but most of their unique anatomical traits are related to a few general features. Chimpanzees have long arms and fingers, mobile shoulders and wrists, a shallow, wide, funnel-shaped thorax and large arm-raising muscles. Keith asserted that these features were adaptations to brachiation. Others maintain that these traits are primarily adaptations for efficient climbing, and that brachiation (sensu stricto) was unimportant in the evolution of ape anatomy. This first quantitative study of chimpanzee positional behavior attempts to resolve this issue. Without a clear underst and ing of chimpanzee positional behavior and its relationship to morphology the reconstruction of the early hominid lifeways is impossible. Adult wild chimpanzees were studied at the Mahale Mountains and Gombe Stream National Parks in Tanzania. Over a one year period, 784 hours of observation were logged, during which data were collected on chimpanzee positional behavior and its ecological and social contexts yielding 16303 samples. A further 83 hours of observation were made on Papio anubis, resulting in 2087 samples. Data were analyzed by site, season, context, stratum size, social rank, body size and sex. Chimpanzees were found to be more arboreal than baboons, but were not "brachiators" (sensu stricto). Sitting represented 70% of all chimpanzee positional behavior, knucklewalking, 15.7%, armhanging 4.4% and climbing 0.9%. Despite substantial habitat differences between Mahale and Gombe, positional behaviors were very similar. Armhanging was a frequent and crucial feeding behavior, and chimpanzees were shown to armhang more often the smaller the superstrate. Baboons did not armhang. It is concluded that the mechanical dem and s of armhanging and vertical climbing, not brachiation (sensu stricto), are responsible for the evolution of the unusual features of chimpanzee morphology. In the context of this study Australopithecus afarensis positional behavior was reconstructed as arboreal armhanging and climbing, and terrestrial bipedalism. The evolution of bipedality is most likely to have evolved as an adaptation to feeding on small, evenly distributed and abundant fruits.PhDPhysical anthropologyUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/162321/1/9001647.pd