1Department of Anatomy, Case Western Reserve University School of Medicine, 2Department of Anthropology, Kent State University, 3Department of Orthodontics, Case Western Reserve University School of Dental Medicine
Saturday 2:00-2:15, Ballroom C
Second only to bipedalism, knuckle-walking is perhaps the most unusual means of terrestrial locomotion known in mammals, regularly practiced only by living chimpanzees, gorillas, and anteaters. While knuckle-walking was traditionally considered to be a shared primitive adaptation of extant African apes, it is now recognized to have been acquired independently by chimpanzees and gorillas. There are many similarities in body form among orangutans and African apes, yet orangutans lack any adaptations to knuckle-walking most likely due to their infrequent terrestrial travel. Several characters specific to the knuckle-walking phenotype have been identified, but a functional explanation of the adaptation remains elusive. Here, we describe a model of knuckle-walking that integrates various characters of the torso and forelimb anatomy shared by the African apes. We argue that the digital and wrist flexors act to ameliorate impact loading by eccentric and isometric contraction during terrestrial locomotion in large bodied hominoids and thereby reduce irreversible cartilage and bone damage in forelimb joints. The primary adaptive changes to knuckle-walking appear to be changes in the volume and geometry of the antebrachial musculature as well as the narrowing of the upper thoracic cage. Unlike the common assumption that metacarpal and carpal geometry plays a significant role in restricting motion during KW, the characteristic osteological signature of knuckle-walking most likely results from each chondral element’s loading history during ontogeny. These forelimb characters provide compelling functional explanations of the locomotor parallelisms in chimpanzees and gorillas.