Sternum Morphology and Locomotor Adaptation in Anthropoid Primates

Abstract

Trunk shape varies among anthropoid primates and is thought to reflect locomotor adaptation. Hominoids, who are orthograde, below‐branch primates that rely on suspensory behaviors, have broader, deeper rib cages than do pronograde quadrupedal anthropoids. Apes have more cranially‐oriented glenohumeral joints than do humans, thought to orient the joint for use in suspensory postures during locomotion. The sternum represents a key part of the thoracic skeleton and contributes to overall rib cage morphology, yet variability in sternal shape across primates is undocumented.In particular, hominoids should be expected to have a relatively broader manubrium and sternum than do monkeys. Further, apes should have more cranially‐oriented sterno‐clavicular joints than do humans, reflecting the higher position of the shoulder joint indicated by scapular morphology. In this study, we examined proportions of the manubrium and sternum in 199 anthropoid primates by taking linear measurements of the sternum from a series of calibrated digital photographs using the ImageJ software package. We also measured orientation of the sterno‐clavicular joint across hominoids.Results indicate that, as expected, manubrium and sternum breadth are absolutely and relatively greater in hominoids than in both Old and New World monkeys, correlated with the transversely broader ribcages of hominoids. The semisuspensory New World monkeys Lagothrix and Ateles show similarities to hominoids in manubrial proportions, supporting a hypothesized link between thoracic breadth and suspensory locomotion. In terms of sterno‐clavicular orientation, humans have more laterally‐facing articular surfaces than most apes. Contrary to expectations, Symphalangus resembled humans in average angle of the sterno‐clavicular joint, suggesting a different configuration of the pectoral girdle in these apes that warrants further investigation. Overall, manubrial and sternal proportions appear related to locomotor adaptation in anthropoid primates, but relations between thoracic and pectoral morphology may be more varied than previously appreciated.