Trabecular structure of the proximal and distal ulna in extant primates

Trabecular bone (re)models in response to localized mechanical loading, potentially enabling inferences about the positional behavior of extinct taxa. However, the trabecular architecture of the ulna remains understudied in extant primates, particularly regarding whether it reflects habitual joint posture and loading. Across a broad sample of anthropoid primates, including apes, atelids, and cercopithecoids, we quantified the relative bone volume fraction (rBV/TV) of trabeculae in the proximal and distal ulna and applied a two-block partial least squares analysis between rBV/TV and locomotor frequencies to explore the relationship between trabecular bone and locomotor behavior. In the distal ulna, high rBV/TV values at the tip of the styloid process in non-hominoids are consistent with its known role in load transmission through the ulnocarpal articulation. In hominoids, the absence of direct articulation with the carpus offloads internal stresses, weakening the functional signal in the trabeculae, which is consistent with the lack of covariation between rBV/TV and the quantified locomotor behaviors. In contrast, proximal ulnar trabecular structure covaries with the frequency of quadrupedalism and captures clearer functional signals; arboreal monkeys display mediolateral stress patterns consistent with mobile, semipronated elbow use, whereas terrestrial monkeys show compressive patterns linked to semiflexed, pronated elbow use. Symphalangus show dense trabecular reinforcement around the triceps insertion related to forelimb overhead reaching, while hominid trabecular patterns appear related to more versatile locomotor behaviors, with Gorilla and Pongo showing evidence of extended-elbow loading. Overall, our results indicate that the proximal ulna more accurately reflects loadings related to locomotor behavior than the distal ulna, providing a valuable framework for inferring locomotor behaviors in fossil primates.