The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)


Hand postures during vertical clinging and grasping: implications for digit length in primates

LAURA E. JOHNSON1 and DANIEL SCHMITT2.

1Department of Anatomy, Ross University Medical School, 2Department of Evolutionary Anthropology, Duke University

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The presence of a grasping clawless hand presents biomechanical challenges when clinging to a vertical substrate in modern and fossil primates. Little is known about how primates solve this problem and whether hand postures are influenced and constrained by habitual locomotor mode and associated hand anatomy, body size, and trunk size. This study tested the hypotheses that hand posture should vary with: 1) locomotor mode of the species, 2) substrate size, and 3) body size. To study this, individuals of eight primate species from the Duke Lemur Center, ranging in size from 150g-4000g, with differing locomotor modes were prompted to vertically cling and grasp on clear PVC pipes of three sizes. Postures were filmed with two cameras, in order to record the angle between the tip of digit two, the wrist, and the tip of the pollex, as well as the position of the pollex relative to the substrate and other digits. We found that hand postures on vertical substrates vary with locomotor mode and substrate size but not body mass. Vertical clingers and leapers were more likely to have wider-angled hand grips on larger substrates compared with active arboreal quadrupeds, but slow arboreal quadrupeds had the widest hand grip angles. Additionally, as substrate size increased, most individuals in this sample decreased the engagement of the pollex with the substrate. When analyzed in the context of theoretical models of vertical clinging, these data can explain why many arboreal primates have reduced thumbs and animals, such as Ateles lost their thumbs.

Grant support: Sigma Xi G20101015154193, NSF-BCS 1155981

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