Ontogeny of knuckle-walking hand postures in African apes

Abstract

Examination of the ontogeny of hominoid hands indicates that the majority of differences observed among adult bonobos, chimpanzees, and gorillas in the gross proportions of the hand are allometric due to ontogenetic scaling (Inouye, 1992). However, ray segment lengths depart from this pattern, resulting in relatively shorter fingers in the gorilla hand compared to the hands of chimpanzees and bonobos of comparable sizes. Furthermore, as adults and throughout ontogeny the fifth metacarpal is shorter than the second, third and fourth metacarpals in bonobos and chimpanzees, whereas gorillas have metacarpals which are more similar in length (Susman, 1979; Inouye, 1992). In addition, qualitative descriptions of knuckle-walking (Tuttle, 1967, 1969a,b,c; Tuttle & Basmajian, 1978) suggest that chimpanzees and gorillas use different hand postures and appear to bear their weight on different digits. Therefore, ontogenetic, descriptive kinematic data was taken from videotapes of knuckle-walking by chimpanzees, bonobos, and gorillas, and analysed to determine if differences among the African apes in length of the fifth digit was meaningfully correlated with variation in hand positions in load-bearing postures. Results indicate that throughout ontogeny there is no significant change in the frequency of use of digits II-V during knuckle-walking in all the African ape species. During ontogeny, chimpanzees and bonobos use their fifth digits consistently less often than the other digits when knuckle-walking, and comparisons between chimpanzees and bonobos reveal no significant difference in the frequencies of digits used during knuckle-walking. Gorillas, in contrast, use all four digits consistently in knuckle-walking postures throughout ontogeny, and comparative analyses reveal that the gorilla pattern is significantly different from the chimpanzee/bonobo pattern as adults and throughout ontogeny. Thus, metacarpal lengths and the comparative lengths among manual rays of the same hand are not a product of simple ontogenetic scaling between chimpanzees/bonobos and gorillas and instead, appear to be related to interspecific differences in hand postures used during knuckle-walking. These data, when considered with previous allometric data on other postcranial dimensions, indicate that differences between Pan and Gorilla in proportions of the hand may not be related to differences in weight distribution between forelimbs and hindlimbs or to differences in the frequency of knuckle-walking behavior. Rather, differences between Pan and Gorilla hand proportions are more likely related to kinematic differences in knuckle-walking behavior, increased efficiency of knuckle-walking in the latter, or perhaps to other locomotor behaviors, such as climbing.