Do differences in skull morphology and bite performance explain dietary specialization in sea otters?

Abstract

Intraspecific studies of morphology and performance are essential for understanding the factors that enable resource partitioning within ecological communities. The sea otter (Enhydra lutris) is one of the few mammal species in which individual-level dietary specialization has been documented, making them an ideal system to investigate the morphological basis of food resource partitioning. Here, we test if differences in food resource use within and between sea otter subspecies can be explained by differences in ecologically relevant metrics of bite performance that are mainly the product of variation in size, cranial morphology, or a combination of these traits. We use geometric morphometrics to evaluate variation in cranium size and morphology, and 2-dimensional models to estimate bite performance differences between 2 sea otter subspecies that differ in dietary ecology: the northern sea otter (Enhydra lutris kenyoni, a facultative generalist) and the southern sea otter (E. l. nereis, a specialist). We found significant differences in cranium shape and size between subspecies and between male and female sea otters. These differences were subtle yet consistent with most subspecies classifications and known sexually dimorphic traits. Cranial morphological differences did not translate into differences in estimated bite force between subspecies or sexes, but dentary strength differed significantly between male and female sea otters. Sea otters have short, blunt crania with pronounced sagittal and lambdoidal crests, and strong mandibles. These traits combine to produce high bite forces for their size. We propose that high bite performance capacity in sea otters enables resource-use variation by widening the diversity of available food resources they can procure from their environment; this allows them to behave as either generalists or specialists within different habitats.