Identification of functionally-related adaptations in the trabecular network of the proximal femur and tibia of a bipedally-trained Japanese macaque

The axial and appendicular skeleton of Japanese macaques (Macacca fuscata) trained to adopt ipedal posture and locomotion display a number of functionally-related external and internal macro- and micro-morphological changes, including site-specific cortical and trabecular bone adaptations. In this study we use high-resolution microtomography scanning to analyse the 3D distribution of trabecular architecture of the proximal femur and proximal tibia of Sansuke, a male individual trained in bipedal performances for eight years, as well as five wild individuals. The distribution and architecture of trabecular bone in the femoral head of Sansuke is distinct from that found in wild M. fuscata individuals, with a unique bone reinforcement around the region of the fovea capitis. Conversely, wild individuals exhibit two pillar-like, high-density structures (converging in an inverted cone) that reach distinct regions of the posterior and anterior surfaces of the femoral head. For Sansuke’s proximal tibia, contrary to previous observations from the cortico-trabecular complex distribution at the plateau, our results do not show a more asymmetric distribution between medial and lateral condyles with a medial reinforcement. Additionally, relative bone volume in this region is not significantly higher in Sansuke. However, we observed a slightly more medially placed bone reinforcement in the lateral condyle compared to the wild individuals as well as a slightly higher trabecular bone anisotropy in the medial than in the lateral condyle not observed in the wild individuals. These analyses provide new evidence about the nature and extent of functionally-related adaptive arrangements of the trabecular network at the coxofemoral and the knee joints in individuals recurrently experiencing atypical load.