Anatomical correlates of cursoriality are compromised by body size and propensity to burrow in a group of small mammals (Lagomorpha)

Abstract

Highly cursorial animals are specialised for fast, sustained running via specific morphological adaptations, notably including changes in limb segment length and mechanical advantage. Members of the order Lagomorpha (hares, rabbits and pikas) vary in cursorial ability; hares are generally highly cursorial, rabbits more frequently saltate, and pikas predominantly trot. Previous investigations of lagomorphs have identified anatomical trends correlated with this ‘cursoriality gradient’, however, the phylogenetic sampling of such investigations has been limited to three American species, namely the American pika (Ochotona princeps), brush rabbit (Sylvilagus bachmani), and black-tailed jackrabbit (Lepus californicus). Here, we expand the phylogenetic sample and body size range by including novel data from Australian samples of the European rabbit (Oryctolagus cuniculus) and European hare (L. europaeus), alongside unpublished data on the Eastern cottontail (S. floridanus). X-ray Computed Tomography and digital landmarking were used to capture proportions within the appendicular skeleton of ~ 40 specimens of each European species. In doubling the number of species studied, we find the previously-identified morphological gradients associated with cursorial behaviour are complicated when evaluated in the larger sample. The relative length and joint velocity of limbs was found to be lower than predicted in European rabbits and hares. Furthermore, we present a novel assessment of morphological integration in the lagomorph appendicular skeleton, finding between-limb covariation patterns that are generally similar to those of other mammals. Broadly, these results suggest cursoriality is only one of many selective forces driving lagomorph skeletal evolution, with variations in body size and fossoriality potentially having measurable impacts.