To Hop or Not to Hop? The Answer Is in the Bird Trees.

Birds can use different types of gaits to move on the ground: they either walk, hop, or run. Although velocity can easily explain a preference for running, it remains unclear what drives a bird species to favor hopping over walking. As many hopping birds are relatively small and arboreal, we wanted to test the link between size, arboreality, and hopping ability. First, we carried out ancestral character state reconstructions of size range, hopping ability, and habitat traits on over 1000 species of birds. We found that both hopping ability and arboreality were derived and significantly correlated traits in avian evolution. Second, we tested the influence of hopping ability on the morphology of the lower appendicular skeleton by quantifying the shape differences of the pelvis and the three long bones of the hind limbs in 47 avian species with different habitats and gait preferences. We used geometric morphometrics on 3D landmarks, digitized on micro-computed tomography (micro-CT) and surface scans of the pelvis, femur, tibiotarsus, and tarsometatarsus. Locomotion habits significantly influence the conformation of the pelvis, especially at the origin of hip and knee muscle extensors. Interestingly, habitat, more than locomotion habits, significantly changed tarsometatarsus conformation. The morphology of the distal part of the tarsometatarsus constrains digit orientation, which leads to a greater ability to perch, an advantageous trait in arboreality. The results of this work suggest an arboreal origin of hopping and illuminate the evolution of avian terrestrial locomotion.[Anatomy; avian; gait; leg; lifestyle; pelvis; tree-dwelling.].