The 81st Annual Meeting of the American Association of Physical Anthropologists (2012)

The relationship between distal humeral morphology and tail length within the genus Macaca


Department of Anthropology, University of Toronto

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Members of the genus Macaca exhibit the widest range of tail lengths of any primate taxon, and therefore serve as an ideal group to test hypotheses concerning the evolution of tail loss in hominoids. Hominoid taillessness has been hypothesized to have evolved in association with greater forelimb mobility that would have replaced the role of the tail in maintaining arboreal balance (Kelley, 1997). Although macaques have adapted to a life of greater terrestriality compared with early hominoids, they continue to frequently engage in arboreal behaviour. If the relationship between tail loss and forelimb mobility is valid, shorter-tailed macaques should possess greater forelimb mobility relative to longer-tailed macaques.

To test this hypothesis, 19 skeletal measurements reflecting the degree of mobility at the distal humerus were compared for eight macaque species (n=34) exhibiting a wide range of relative tail lengths. Based on the results of an analysis of variance, a significant difference was found in male macaques (n=19) between articular surface width and tail length (p=0.047), suggesting that tailless macaques possess the greatest width, followed by short-tailed, and long-tailed species. In addition, a least squares regression comparing species means of relative tail length with the mean articular surface width of male macaques resulted in a negative correlation (r2=0.403; m=-2.924) that approached significance (p=0.090). Because larger articular surfaces are indicative of greater joint mobility (Rose, 1988), these results provide the first quantitative support for the relationship between tail loss and greater forelimb mobility hypothesized by Kelley (1997).

This study was funded by the Department of Anthropology Research Grant, University of Toronto.

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