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

Distal radioulnar joint morphology of short-tailed semi-terrestrial cercopithecines and its implications for the evolution of hominoid taillessness


Department of Anthropology, University of Toronto

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Semi-terrestrial cercopithecines are an ideal group with which to test hypotheses concerning the evolution of hominoid taillessness due to the frequent reduction in tail length that has occurred in parallel within this group. It has previously been proposed that the loss of the tail among the earliest hominoids was associated with an increase in forelimb mobility (Kelley, 1997) related to a greater reliance on forearm pronation-supination, which would have served as a functional correlate for the tail in maintaining balance (Larson and Stern, 2006). If this hypothesis is correct, I predict that short-tailed semi-terrestrial cercopithecines will exhibit a greater range of pronation-supination than their long-tailed relatives.

To test this hypothesis, five indices were calculated from seven skeletal measurements that reflect the range of rotation at the distal radioulnar joint. Eight semi-terrestrial cercopithecine species (n=70) were included in this analysis, and were divided into two groups (short tail, n=32; long tail, n=38) based on relative tail length and published observations of tail use during locomotion (Larson and Stern, 2006). An analysis of variance was used to compare the functional morphology of the distal radioulnar joint between these two groups.

The results indicate that semi-terrestrial cercopithecines with a reduced tail exhibit a significantly greater mediolateral expansion of the radial facet around the ulnar head (p=0.000). This morphology is indicative of a greater range of forearm pronation-supination (O’Connor, 1975), and therefore these results provide quantitative support for the relationship between tail loss and greater forelimb mobility.

This study was funded by the Ontario Graduate Scholarship, School of Graduate Studies Travel Grant (University of Toronto), General Motors Women in Science and Mathematics Award, and the Department of Anthropology Research Grant (University of Toronto).

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