Bear feet with Björn: tarsal evolution in the origin of polar bears

Polar bears (Ursus maritimus) diverged from brown bears (U. arctos) in the last one million years. Polar bears have a strikingly different external appearance because of their white pelts and large size, but osteological differences are thought to be limited to flatter crania and altered dentition, with little difference in the postcrania despite swimming frequently and living almost entirely on icy substrates. This paper shows that polar bears have substantial differences in their tarsal form and function from other bears, including their close relatives, the brown bears. Ankle gear ratio records the major functional transition from semi-cursorial hemicyonines to the more plantigrade locomotion of crown ursines. Analysis of tarsal morphology among seven extant ursines show that the arboreal species Helarctos malayanus, Melursus ursinus, Tremarctos ornatus, U. thibetanus, and U. americanus have morphologies that permit greater movement at the transverse tarsal and lower ankle joints, especially broader and more gently curved astragalocalcaneal and sustentacular facets on the calcaneum and their articular equivalents on the astragalus, as well as broader and more gently curved navicular facets on the astragalar head. U. arctos and U. maritimus, which are strongly terrestrial, have smaller sustentacular facets and pronounced interlocking between astragalus and calcaneum at the sustentaculum. Polar bears, however, differ from brown bears in that this interlocking is less tight and thus permits more movement at the lower ankle joint. The phylogenetic comparative analysis of shape shows that the divergence in ankle morphology of the polar bear from the brown is one the most rapid bursts of tarsal evolution in ursines.