Humans and platyrrhines share a pattern of articular variance

Previous work has identified a pattern of articular shape variation in modern human limb joints, in which convex joint surfaces exhibit constrained variance relative to their more variable concave conarticulars. This pattern is notable given that joint morphogenesis is influenced by local biomechanical environments, which vary substantially across joints and throughout development. Whether this biased variance pattern reflects generalizable developmental or biomechanical constraints, or instead represents a human-specific phenomenon, remains unclear. Here, we test the generalizability of this pattern by examining the humeroulnar joint in New World monkeys (Platyrrhini), a diverse radiation of primates that retain many aspects of the ancestral anthropoid bauplan while exhibiting wide variation in body size, locomotor behavior, and limb-loading regimes. We analyzed three-dimensional geometric morphometric landmark data from the humeroulnar joints of 84 adult platyrrhines. Phylogenetic comparative analyses identified moderate but significant phylogenetic signal in both distal humeral and proximal ulnar articular shape, and two-block partial least squares analysis demonstrated strong humeroulnar covariance. Mean pairwise landmark distance distributions were used to compare relative variance between conarticular surfaces across the pooled sample and within groups. Distal humeri consistently exhibited significantly lower morphological variance than associated proximal ulnae, indicating that this pattern is not unique to humans, despite differences in posture, gait, and locomotor loading. These results demonstrate that biased constraint of convex articular surfaces may be conserved across a wide range of locomotor strategies. This suggests the involvement of shared underlying developmental or biomechanical mechanisms in joint morphogenesis and highlights the utility of comparative variance analyses for inferring processes of morphogenesis.