

Cross-sectional geometry of chimpanzee finger bones


1Interdepartmental Doctoral Program in Anthropological Sciences, Stony Brook University, 2Cell and Neurobiology, University of Southern California

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During knuckle-walking, chimpanzee finger bones contact the substrate and sustain large mechanical loads. Digital flexor muscles are not recruited to moderate these loads during weight support. Manual pressure data suggest that the loads sustained by chimpanzee fingers during knuckle-walking are not evenly distributed across the digits. Higher pressures are experienced by digits 2 and 3 than by digit 5. Given that bone tissue has the capacity to adjust to its functional environment such that its morphology is “optimized” according to mechanical demand, the load variation implied by pressure data should have a predictable effect on the relative robusticity of chimpanzee finger bones, namely, the bones of digits 2 and 3 should exhibit greater robusticity than those of digit 5. Furthermore, knuckle-walking engenders larger bending moments in obliquely oriented proximal phalanges than in horizontally oriented middle phalanges, which should lead to relatively greater robusticity in proximal versus middle elements. To assess these predictions, we measured the mid-diaphyseal cross-sectional geometry of the proximal and middle phalanges of digits 2-5 in 20 wild-shot, adult chimpanzees using micro-CT. As predicted, we found that cortical bone area (scaled to body mass) and polar moment of area (scaled to the product of body mass and bone length) are consistently larger in (1) phalanges from digits 2 and 3 than from digit 5, and (2) proximal versus middle phalanges from the same digit. The results of this study provide support for the idea that diaphyseal bone morphology reflects, at least to some extent, habitual locomotor behavior.

Grant sponsor: L.S.B. Leakey Foundation.

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