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

The relationship between thumb reduction and relative carpal volume in African colobines


1Interdepartmental Doctoral Program in Anthropological Sciences, Stony Brook University, 2Anatomy, Midwestern University-Downers Grove, 3Cell and Neurobiology, University of Southern California

Thursday 8:45-9:00, Ballroom B Add to calendar

The African colobus monkeys are unique among catarrhine primates in having a reduced, almost vestigial pollex, with pollical metacarpals that are relatively shorter and more gracile than those of Asian colobines and cercopithecines. The developmental mechanisms for pollical reduction have been indirectly linked with differential Hox gene expression patterns in the distal part of the autopod (i.e., digits). Whether similar developmental mechanisms, and corresponding phenotypes (e.g., carpal size), are present in the proximal part of the autopod (i.e., carpals) remains unclear. This is due in part to the limited data available on carpal size and proportions for most primates. Because the pollex is functionally ‘linked’ with the trapezium and other radial-side carpals, it is reasonable to hypothesize that these bones might be similarly reduced in overall size and robusticity. That is, the radial-side carpals should constitute a smaller proportion of the overall wrist volume in these monkeys with vestigial thumbs. To test this hypothesis, carpal volumes (derived from microCT scans) of African colobines were compared to those of Asian colobines and cercopithecines (n>50). Results show that the trapezia of African colobines are smaller when scaled to total carpal volume compared with those of the outgroup taxa. However, other radial-side carpals do not show this effect. These results indicate probable developmental integration between digital and trapezial size and morphology, and may hint at some degree of modularity in the hand-wrist complex or other functional constraints that limit the downstream effects of pollical reduction on the rest of the wrist.

Funding for this research was provided in part by the Turkana Basin Institute (to SLC), the Wenner-Gren Foundation (to CMO) and The Leakey Foundation (to BAP).

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