Variation in trabecular bone microarchitecture across rhesus macaque (Macaca mulatta) load-bearing joints

Globally, human population structure is quickly trending older, increasing the prevalence and systemic burden of age-related skeletal disorders such as osteoporosis. Osteoporosis is characterized by the loss of bone mass, including trabecular bone tissue, leading to skeletal fracture. Although clinically important, fundamental questions remain about normal trabecular bone variation and age-related bone loss. In this study, we use free-ranging rhesus macaques (Macaca mulatta) from the Cayo Santiago Field Station to explore variation in trabecular bone structure. We measured several aspects of trabecular bone structure (maximum and mean bone volume fraction, degree of anisotropy, and trabecular thickness) across the elbow (humerus, ulna, radius), hip (proximal femur), and knee (distal femur, tibia). Analyses of covariance models assessed factors influencing bone structure, including body mass, demography (age, sex, matriline), as well as indices of sociality and early life adversity. Point cloud models of prime and postprime age groups visualized age-related differences in bone structure. We observed significant variation in trabecular bone morphology (max and mean bone volume fraction, degree of anisotropy, and trabecular thickness) across both bones and joints. Sex influenced trabecular thickness, with thicker trabeculae in males. Max and mean bone volume fraction as well as trabecular thickness were positively associated with body mass. Age was associated with significantly lower values of mean bone volume fraction, specifically in the hind limb. We observed significant bone loss specifically in the femoral head and neck. There were no associations of trabecular bone structure with either sociality or early life adversity in this sample. This study provides a comprehensive view of trabecular bone variation by region, sex, mass, and age contextualized by social factors.