Why can weasels climb?
Biologists at the Museum für Naturkunde Berlin and the Royal Veterinary College have investigated how mammals evolve limbs for climbing, digging, and swimming by studying the internal structure of limb bones in the weasel family
Fundamental to the evolutionary success of mammals have been specializations for different locomotor habits or limb functions, such as climbing, running, digging and swimming. However, our anatomical understandings of such specializations have overwhelmingly focused on the external dimensions of the limb skeleton, namely the lengths and diameters of its constituent bones. The internal dimensions of the limb skeleton – which determine a bone’s ability to structurally withstand the forces acting on it as a limb functions – have received considerably less attention in the study of mammalian locomotion and evolution. To determine whether differences in the cross-sectional structure of limb bones relates to specialized locomotor habits in mammals, we studied cross-sectional shape in the forelimb bones of Mustelidae, a functionally diverse lineage of mammals that includes weasels, otters, badgers, and martens. We found that otters and badgers have the limb bones with the most robust cross-sections among mustelids, and this fits our prediction that otters and badgers have more robust cross-sections due to their respective needs of counteracting buoyancy during swimming and withstanding mechanical stresses incurred during digging. Though this association between bone cross-sectional structure and limb function would appear to be adaptive, models of the past evolution of these traits suggest that the evolution of these traits may not be consistently adaptive. This is in contrast to the likely adaptive evolution of the external dimensions of the mustelid limb skeleton. Thus, our results suggest that during the evolution of mammals, specializing an organ (i.e., a limb) for a given function does not necessarily entail a single mode of evolution governing all of the organ’s constituent traits.
Kilbourne BM & Hutchinson JR. 2019. Morphological diversification of biomechanical traits: mustelid locomotor specializations and the macroevolution of long bone cross-sectional morphology. BMC Evolutionary Biology 19: 37 (https://doi.org/10.1186/s12862-019-1349-8).