Trabecular and Cortical Bone Morphology of the Distal Radius in Humans, Chimpanzees, Cercopithecids, and Anteaters.

Identifying indicators of locomotor differences in extant and fossil taxa has been an ongoing endeavor in functional morphology. However, overlap between locomotion and phylogeny in primates makes overall interpretation of morphology, function, and behavior difficult. The distal radius is a particularly crucial area to understand loading patterns in the forelimb. This study aims to quantify trabecular and cortical bone properties in humans (bipeds), chimpanzees (knuckle-walkers), cercopithecids (quadrupeds), and anteaters (quadrupeds) in order to discern different loading patterns in animals that vary in phylogeny and locomotion. Using a combination of whole bone analysis with point cloud correspondence registration and Bayesian mapping, we found locomotor signals in the trabecular and cortical bone properties. Importantly, we found a correspondence between cortical bone thickness and bone volume fraction in accordance with previously hypothesized patterns. Humans, unsurprisingly, have relatively low values for bone volume fraction and cortical bone thickness overall. Knuckle-walking chimpanzees have increased cortical and trabecular bone along the palmar articular surface, reflecting increased loading in that region when compared to palmigrade or digitigrade cercopithecids. Anteaters have increased cortical bone thickness and trabecular bone volume fraction on the medial side of the radius, which reflects their unique loading patterns. Overall, trabecular and cortical bone variation in the distal radius appears to reflect differences in loading patterns due to differences in locomotion. Whole bone analysis of both cortical and trabecular bone can provide valuable insights into the skeletal response to locomotor loading and can be useful for reconstructing behavior in the past.