Effect of mass and habitat on the shape of limb long bones: A morpho-functional investigation on Bovidae (Mammalia: Cetartiodactyla).

Abstract

Limb long bones are essential to an animal's locomotion, and are thus expected to be heavily influenced by factors such as mass or habitat. Because they are often the only organs preserved in the fossil record, understanding their adaptive trends is key to reconstructing the paleobiology of fossil taxa. In this regard, the Bovidae has always been a prized group of study. This family is extremely diverse in terms of both mass and habitat, and it is expected that their bones will possess adaptations to both factors. Here, we present the first 3D geometric morphometric study focusing on bovid limb long bones. We used anatomical landmarks as well as curve and surface sliding semi-landmarks to accurately describe the stylopod and zeugopod bones. We included 50 species from ten of the twelve currently recognized tribes of bovids, ranging from 4.6 to 725kg, and living in open plains, forests, mountains, or anywhere in-between. Shape data were correlated with the mean mass of the species and its habitat, even when taking into account the phylogenetic history of our sample. Bones pertaining to heavy species are more robust, adapted for a better repartition of stronger forces. Articulations are especially affected, being proportionally much larger in heavier species. Muscle insertion areas are unevenly affected. Insertion areas of muscles implied in body support and propulsion show a strong increase in their robustness when compared to insertion areas of muscles acting on the limb mostly when it is off the ground. Habitat influences the shape of the humerus, the radius-ulna, and the femur, but not of the tibia, whether the phylogeny is taken into account or not. Specific habitats tend to be associated with particular features on the bones. Articulations are proportionally wider in open-habitat species, and the insertion areas of muscles involved in limb extension and propulsion are wider, reflecting the fact that open habitat species are more cursorial and rely on fast running to avoid predators. Forest and mountain species generally present similar adaptations for increased manoeuvrability, such as a round femoral head, and generally have more gracile bones.