Microanatomical features of bovids long bones: What are the effects of mass and habitat?

Bovids are a valuable group for studying limb long bone adaptations due to differences in size and the environment that the nearly 280 species inhabit. Their analysis can thus shed light on the constraints that occur during locomotion, which affect muscles and both the outer and inner bone structure. Nevertheless, no study has directly looked at the bone inner structure within bovids. This study is the first to analyse the inner structure of stylopod and zeugopod bones within bovids of different masses and habitats using both qualitative and quantitative approaches. Both thickness maps and virtual sections show remarkable differences in the heaviest individuals, which display a thicker compact bone layer, mostly at flexor muscle insertions associated with highly anisotropic trabeculae expending into the metaphysis. These qualitative results are supported by quantitative analyses that highlight a significant link to mass. By allowing greater dissipation of forces, these adaptations increase bone resistance to the associated higher compressive load and thus contact and ground reaction forces. As anticipated, the stabilising effect related to mass is evident in the forelimb and seems higher than that in the hindlimb. The influence of habitat is more evident in the hindlimb, but these observed variations are more subtle than those previously observed in the forelimb. Species that live in highly covered or mountainous areas, where a greater manoeuvrability is required, tend to display a globally higher bone volume fraction associated with the insertion of muscles increasing limb stability (e.g., adductores). While the gross shape of these bones shows a greater similarity between heavy-bodied and open-habitat bovids, the microanatomical features reveal a stronger similarity between heavy-bodied bovids and those from covered and mountainous habitats than with open-habitat species.