Functional morphology of trabecular bone in the calcaneus of African apes

The relationships between whole-bone trabecular structure and mechanical loading have been well-documented in the human calcaneus. In contrast, these relationships have not been thoroughly investigated in most nonhuman primates. A recent analysis of gorillas found that calcaneal trabecular architecture varies among species generally along locomotor and ecological lines. Gorillas are the most terrestrial of the nonhuman great apes, however, which limits our comparative context to understand trabecular bone functional adaptation in both the human foot and that of closely related extant and extinct taxa. Here we analyze whole-bone calcaneal trabecular variation among a sample of both Pan (n = 16) and Gorilla (n = 22) to continue exploring the factors governing trabecular bone adaptation in the foot, as the two genera differ in both degree of arboreality and adopt variable foot postures during locomotion. Calcaneal trabecular architecture was quantified from micro-CT scans. An initial three-dimensional geometric morphometric sliding semilandmark analysis was run to position 150 volumes of interest within each bone. Trabecular thickness (Tb.Th), trabecular spacing (Tb.Sp), and bone volume fraction (BV/TV) were calculated in ImageJ and MATLAB for each VOI. Parameter distributions were summarized using principal component analysis and visualized using color maps. Non-parametric MANOVAs designed for high-dimensional data were run to test for significant differences in each parameter between genera. Pan and Gorilla significantly differ in whole-bone Tb.Th, Tb.Sp, and BV/TV (p < 0.001 for all analyses). Both African apes exhibit relatively higher Tb.Th and BV/TV in the anterior half of the calcaneus relative to the posterior half; however, the anteroposterior difference is exaggerated in Gorilla. This likely reflects a more consistent loading pattern in Gorilla as a consequence of their relatively more frequent use of terrestrial behaviors. Pan exhibits relatively higher Tb.Th and BV/TV in the calcaneal tuberosity compared to Gorilla, which may be indicative of a higher magnitude and frequency of forces from the triceps surae muscle complex for vertical climbing in Pan.