Swimming and running: a study of the convergence in long bone morphology among semi-aquatic mustelids (Carnivora: Mustelidae)

The shape of long bones is linked to a species' ecology and is thought to reflect the constraints imposed by locomotion. The evolution of the shape of the long bones in semi-aquatic mustelids has likely been shaped by the divergent mechanical properties of both water and land. Adaptation to a semi-aquatic lifestyle occurred independently in otters (Lutrinae) and minks (Mustelinae). Here we test the idea that these similar locomotor constraints led to morphological convergence between minks and otters, and between European and American minks. We use 3D geometric morphometrics to quantify shape differences in the humerus, radius, ulna, femur, tibia and fibula of ten species of mustelids belonging to the Lutrinae and Mustelinae subfamilies. Our results show convergence in all bones between the European and American minks, but this convergence is significant only for the humerus. We suggest that this strong convergence in humeral shape between the two minks results from functional demands on the forelimb as it produces most of the thrust when swimming in minks. The American minks show a slight but nonsignificant convergence with the Lutrinae for the shape of the ulna, femur and tibia. The sea otter (Enhydra lutris) shows an ulnar and radial shape that diverges from that observed in the other Lutrinae, possibly due to the strong manipulative abilities and unique locomotor mode of this species. In contrast to our initial hypothesis, bone shape in minks does not converge significantly with that of otters. Otters show a large variety of shapes suggesting that a semi-aquatic lifestyle can comprise a wide array of locomotor behaviours.