YOUNG, J.W.; Stony Brook University: Mechanical and Ecological Pressures on Muscle Lever Arm Growth in Primates

To offset well-established volume/area relationships limiting muscle force, larger animals walk on extended limbs and show allometric increases in extensor muscle lever arm lengths. However, this pattern might not apply to ontogenetic increases in size. Although immature animals have weak extensor muscles, they adopt crouched postures and experience relatively greater substrate reaction forces that increase the extensor force required for locomotion. Such deficits might compromise their ability to forage, evade predators and keep pace during group travel. Previous studies of non-primate mammals have shown that extensor muscle lever arms are proportionally longer early in ontogeny and decrease during growth to compensate for these restrictions. These patterns might characterize primate ontogeny as well. However, growing primates also face unique mechanical pressures, including strong positive allometry during limb growth and compliant gait kinematics. These pressures might necessitate relatively longer extensor muscle lever arms as size increases. This study tested these two contrasting predictions by examining patterns of relative muscle lever arm growth in two primate species, Cebus albifrons and Cebus apella. Lengths of the forearm and the lever arms of elbow flexor and extensor muscles were measured in a longitudinal series of radiographs. In contrast to previous studies of non-primate mammals, all lengths scaled to body mass with positive allometry. Forearm and extensor lever arm lengths increased at a faster rate in C. albifrons; C. apella had relatively longer flexor lever arms throughout development. Such differences are likely linked to differences in the locomotor repertoires of the two species. These findings suggest that lever arm growth in these primates responds primarily to mechanical, not ecological, pressures.