Substrate Size and Primate Forelimb Mechanics: Implications for Understanding the Evolution of Primate Locomotion

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Did the anatomical and locomotor specializations of primates evolve in response to requirements of locomotion and foraging on thin branches? Laboratory studies of primates and other mammals provide data suggesting that as substrate size decreases primates will protract their arms to a greater degree, lower the center of gravity by increasing elbow flexion, and decrease forelimb substrate reaction forces. I tested these hypotheses by calculating maximum arm protraction, shoulder height, elbow flexion, and substrate reaction forces during stance phase in 5 species of Old World monkeys walking on a flat runway and raised poles of varying diameters. As substrate size decreased most subjects increased elbow flexion and lowered their shoulder height. Three of the 5 species lowered peak substrate reaction forces as substrate size decreased but, only 2 of the species increased arm protraction as substrate size decreased. These results reject the hypothesis that arm protraction is a function of branch size, but provide stronger support for the notion that branch size influences elbow flexion, shoulder height, and peak substrate reaction forces in some primates. The fact that biomechanical expectations are met in some (but not all) cases and some (but not all) species suggests that the topic is quite complex and requires further study. Nonetheless, preliminary data suggest that biomechanical accommodations to substrate size may have played a role in the early differentiation of primates from other mammals.