Transitions Between Foot Postures Are Associated with Elevated Rates of Body Size Evolution in Mammals

Terrestrial mammals have evolved various foot postures: flat-footed (plantigrady), tiptoed (digitigrady), and hooved (unguligrady) postures. Although the importance of foot posture on ecology and body size of mammalian species has been widely recognized, its evolutionary trajectory and influence on body size evolution across mammalian phylogeny remain untested. Taking a Bayesian phylogenetic approach combined with a comprehensive dataset of foot postures in 880 extant mammalian species, we investigated the evolutionary history of foot postures and rates of body size evolution, within the same posture and at transitions between postures. Our results show that the common ancestor of mammals was plantigrade, and transitions predominantly occurred only between plantigrady and digitigrady and between digitigrady and unguligrady. At the transitions between plantigrady and digitigrady and between digitigrady and unguligrady, rates of body size evolution are significantly elevated leading to the larger body masses of digitigrade species (∼1 kg) and unguligrade species (∼78 kg) compared with their respective ancestral postures [plantigrady (∼0.75 kg) and digitigrady]. Our results demonstrate the importance of foot postures on mammalian body size evolution and have implications for mammalian body size increase through time. In addition, we highlight a way forward for future studies that seek to integrate morphofunctional and macroevolutionary approaches.