Patterns of joint size dimorphism in the elbow and knee of catarrhine primates.

Abstract

Differences in body size between conspecific sexes may incur differences in the relative size and/or shape of load-bearing joints, potentially confounding our understanding of variation in the fossil record. More specifically, larger males may experience relatively greater limb joint stress levels than females, unless an increase in weight-related forces is compensated for by positive allometry of articular surface areas. This study examines variation in limb joint size dimorphism (JSD) among extant catarrhines to: 1) determine whether taxa exhibit JSD beyond that expected to simply maintain geometric similarity between sexes, and 2) test whether taxa differ in JSD (relative to body size dimorphism) with respect to differences in limb use and/or phylogeny. "Joint size" was quantified for the distal humerus and distal femur of 25 taxa. Analysis of variance was used to test for differences between sexes (in joint size ratios) and among taxa (in patterns of dimorphism). Multiple regression was used to examine differences in JSD among taxa after accounting for variation in body size dimorphism (BSD) and body size. Although degrees of humeral and femoral JSD tend to be the same within species, interspecific variation exists in the extent to which both joints are dimorphic relative to BSD. While most cercopithecoids exhibit relatively high degrees of JSD (i.e., positive allometry), nonhuman hominoids exhibit degrees of JSD closer to isometry. These results may reflect a fundamental distinction between cercopithecoids and hominoids in joint design. Overall, the results make more sense (from a mechanical standpoint) when adjustments to BSD are made to account for the larger effective female body mass associated with bearing offspring. In contrast to other hominoids, modern humans exhibit relatively high JSD in both the knee and elbow (despite lack of forelimb use in weight support). Estimates of BSD based on fossil limb bones will vary according to the extant analogue chosen for comparison.