Biomechanical consequences of rapid evolution in the polar bear lineage

Abstract

The polar bear is the only living ursid with a fully carnivorous diet. Despite a number of well-documented craniodental adaptations for a diet of seal flesh and blubber, molecular and paleontological data indicate that this morphologically distinct species evolved less than a million years ago from the omnivorous brown bear. To better understand the evolution of this dietary specialization, we used phylogenetic tests to estimate the rate of morphological specialization in polar bears. We then used finite element analysis (FEA) to compare the limits of feeding performance in the polar bear skull to that of the phylogenetically and geographically close brown bear. Results indicate that extremely rapid evolution of semi-aquatic adaptations and dietary specialization in the polar bear lineage produced a cranial morphology that is weaker than that of brown bears and less suited to processing tough omnivorous or herbivorous diets. Our results suggest that continuation of current climate trends could affect polar bears by not only eliminating their primary food source, but also through competition with northward advancing, generalized brown populations for resources that they are ill-equipped to utilize.

Figure 1. Time-calibrated ursid phylogeny used for assessing rates of cranial shape evolution.

Node numbers correspond to those used for evolutionary contrasts (see Table 1).

Figure 2. Performance metrics assessed for four different bite positions in the polar bear (blue symbols) and brown bear (red symbols) FE models.

The X axis corresponds to bite point, with anterior bites towards the left and posterior bites to the right. Panel A shows bite forces, panel B shows cranial strain energy. Note that bite forces are similar in both species for all bites, while strain energies are uniformly lower in the brown bear.

Figure 3. FE models showing von Mises stresses in the polar bear (left) and brown bear (right) skulls during bilateral canine biting (A,E), and unilateral PM4 (B,F), M1 (C,G), and M2 (D,H) biting.