Morphological Analysis of Long Bones in Semi-aquatic Mustelids and their Terrestrial Relatives

The locomotor environment and behavior of quadrupedal mammals exert functional constraints on their limbs. Therefore long bone shapes are thought to reflect at least partially the species' locomotor ecology. Semi-aquatic species move through two media with distinct density and viscosity and their locomotor apparatus should therefore reflect a trade-off between the divergent functional constraints it faces. Adaptation to a semi-aquatic lifestyle occurred independently in otters (Lutrinae) and minks (Mustelinae). Analyzing semi-aquatic mustelids and their terrestrial relatives, we investigate long bone shape diversity, describe changes in long bone shape associated with a semi-aquatic lifestyle, and discuss the functional consequences of these shape changes. The robustness of the otter bones is highlighted and its potential ballast role discussed. Large epiphyses are observed in otters but this trend seems associated with terrestrial more than with aquatic locomotion. Thus, the most aquatic species, Enhydra lutris, presents narrow knee articulations compared to similar sized less aquatic species. Enhydra lutris presents a fore- and hind limb shape that diverge from that in other otters. Minks show bone shapes similar to each other but only Neovison vison tends to differ from its terrestrial relatives. The evolution of limb shape in this group is strongly correlated with size, locomotor mode, and phylogenetic history, leading to a morphological pattern where the roles of each of these factors are difficult to disentangle.