Cross‐sectional geometry of chimpanzee (Pan troglodytes) finger bones is correlated with habitual load bearing of individual digits during knuckle‐walking

Abstract

Chimpanzee finger bones sustain large mechanical loads during knuckle‐walking on the ground. Digital flexors are not recruited to moderate these loads during weight support. Pressure data indicate that loads sustained by fingers are not evenly distributed across digits. Higher pressures are experienced by digits II and III than by digit V. Because bone tissue can 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 finger bones, i.e. the bones of digits II/III should be more robust than those of digit V. Also, knuckle‐walking postures engenders larger bending moments in obliquely oriented proximal phalanges than in horizontally oriented middle phalanges, which should lead to relatively greater robusticity of proximal elements. Here, we measured the midshaft cross‐sectional geometry of phalanges from digits II‐V in 20 wild‐shot, adult chimpanzees using μCT. As predicted, we found that cortical bone area (scaled to body mass) and polar moment of area (scaled to body mass × bone length) are consistently larger in (1) phalanges from digits II and III than from digit V, and (2) proximal versus middle phalanges from the same digit. These results provide support for the idea that diaphyseal bone morphology reflects, at least to some extent, habitual behavior.Grant Funding Source: N/A