Adaptation to graviportality in Rhinocerotoidea? An investigation through the long bone shape variation in their hindlimb

Abstract

Abstract Weight support is a strong functional constraint modelling limb bones in heavy quadrupeds. However, the complex relations between bone shape, mass, size and body proportions have been poorly explored. Rhinocerotoidea is one of the groups showing the highest body mass reached by terrestrial mammals through time. Here, we explore the evolutionary variation of shape in hindlimb stylopod and zeugopod bones and its relationship with mass, size and gracility in this superfamily. Our results show that bones undergo a general increase in robustness towards high masses, associated with reinforcements of the main muscle insertions. The shape of the femur, carrying a marked phylogenetic signal, varies conjointly with mass, size and gracility, whereas that of the tibia appears related to gracility and mass only. The shape of the fibula does not vary according to that of the tibia. Moreover, congruent variation of shape between the distal part of the femur and the complete tibia underlines the potentially strong covariation of the elements constituting the knee joint. These results, coupled with those previously obtained from forelimb study, allow a better comprehension of the relationship between bone shape and mass among Rhinocerotoidea, and a refining of the concept of ‘graviportality’ in this superfamily.