Trabecular architecture and joint loading of the proximal humerus in extant hominoids, Ateles, and Australopithecus africanus

Objectives: Several studies have investigated potential functional signals in the trabecular structure of the primate proximal humerus but with varied success. Here we apply for the first time a ìwhole-epiphysesî approach to analysing trabecular bone in the humeral head with the aim of providing a more holistic interpretation of trabecular variation in relation to habitual locomotor or manipulative behaviors in several extant primates and Australopithecus africanus. Materials and Methods: We use a ìwhole-epiphysisî methodology in comparison to the traditional volume of interest (VOI) approach to investigate variation in trabecular structure and joint loading in the proximal humerus of extant hominoids, Ateles and A. africanus (StW 328). Results: There are important differences in the quantification of trabecular parameters using a ìwhole-epiphysisî versus a VOI-based approach. Variation in trabecular structure across knucklewalking African apes, suspensory taxa, and modern humans was generally consistent with predictions of load magnitude and inferred joint posture during habitual behaviors. Higher relative trabecular bone volume and more isotropic trabeculae in StW 328 suggest A. africanus may have still used its forelimbs for arboreal locomotion. Discussion: A whole-epiphysis approach to analysing trabecular structure of the proximal humerus can help distinguish functional signals of joint loading across extant primates and can provide novel insight into habitual behaviors of fossil hominins