Ontogeny of locomotion in mountain gorillas and chimpanzees.

The African apes are a group of closely related taxa that differ considerably in body size. In spite of the large body size difference, the African apes are similar in many aspects of their morphology; it has been suggested that most of their differences result from selection for these body size differences (Shea, 1988). The influence of body size on locomotion has been well-documented, but what is less clear, is whether these behavioral differences occur throughout ontogeny because few studies have directly addressed the influence of ontogeny (and changing body size) on locomotion. This study documents the ontogeny of mountain gorilla locomotion and compares it with that of chimpanzees in order to consider how changing body size during ontogeny influences locomotion in the two species. Results indicate that gorilla locomotor development is greatly accelerated compared with chimpanzees, and that much of the interspecific variation in age can be explained by body size. When chimpanzees and gorillas are at similar sizes (although widely disparate ages), they perform very similar locomotor activities. However, it is incorrect to view a gorilla as a faster growing and ultimately larger chimpanzee. Throughout ontogeny, gorillas have broader scapulae and relatively shorter phalanges and metacarpals than chimpanzees (Susman, 1979; Shea, 1981; Jungers & Susman, 1984; Inouye, 1992) which are associated differences in mountain gorilla and chimpanzee suspensory behavior; gorillas never show as high an incidence of suspensory behavior as chimpanzees during ontogeny.