Forelimb mechanics as a function of substrate type during quadrupedalism in two anthropoid primates

Abstract

During the past century, many anthropologists have proposed that hominoid orthograde locomotion arose in an arboreal quadrupedal ancestor with highly mobile, low weight-bearing forelimbs. However, no quantitative data comparing kinematic and kinetic aspects of forelimb use during arboreal and terrestrial quadrupedalism have been available to evaluate such theories. In this preliminary study, a spider monkey and a baboon were videotaped in three planes while walking quadrupedally on an instrumented runway and a raised instrumented horizontal pole. Forelimb angles and substrate reaction force resultants were calculated for each animal on each substrate. The quantitative data presented here support previous models for the evolution of primate locomotion that were based on theoretical biomechanics and qualitative or anecdotal evidence. In addition, this study has revealed several previously undocumented accommodations to "arboreal" quadrupedal locomotion in these two primates. While walking on the pole, compared to travel on the ground, (1) both animals adopted a "crouched" forelimb posture, but only the spider monkey abducted its arm and ulnar deviated its hand; (2) both subjects have lower resultant forces on the forelimb due to lower absolute force magnitudes and changes in the timing of component peaks; and (3) both animals reduce and reorient transverse forces. Similar accommodations to arboreal travel by both subjects appear to be mechanical requirements of arboreal locomotion. However, differences may be due to morphological differences between the subjects, or to their divergent phylogenetic history. These results are used to explore potential explanations for the morphological differences between arboreal and terrestrial primate quadrupeds in terms of bone and joint strain and to evaluate models of primate locomotor evolution.