How do forelimb long bones adapt in rhinoceroses? An in-depth examination of their microanatomy

Rhinoceroses, as the second heaviest extant land mammals, show a skeleton highly modified for weight bearing. However, the limbs of these animals, which are still capable of galloping, do not have the columnar organisation of the more massive graviportal taxa, such as elephants. In this context, studying how the limb bones manage to meet their functional requirements can provide a better understanding of how bones adapt to biomechanical constraints. This study provides a detailed investigation of the inner structure of the three forelimb long bones of the five modern rhino species. For that, it resorts to the combination of two approaches for in-depth descriptions and comparisons of the microanatomical organisation along the entire volume of the bones: 2D longitudinal and transverse sections and a recent 3D approach with a new methodology for visualising and quantifying trabecular bone density and anisotropy in addition to compact bone distribution throughout the bone. By comparing the microanatomical features with the forces exerted on the limb long bones of a rhinoceros at rest obtained through musculoskeletal modelling, this study seeks to highlight the extent to which the microanatomy within each entire bone and among these three long bones with varying shapes and functions reflects the biomechanical stresses imposed on them. The sample is of unprecedented size for studying the microanatomy of such large animals. This study originally shows how microanatomy varies within the bones, between the different limb bones and between the species in regard to the forces exerted on these bones, thereby improving our understanding of the bone form-function relationships. This study highlights the strong correspondence between the forces acting on the bones and their microanatomical structure and thus highlights the great potential of detailed study of bone microanatomy to improve our understanding of musculoskeletal adaptation in extinct taxa and, consequently, our palaeoecological inferences.