Sexual dimorphism in craniomandibular morphology of southern sea otters (Enhydra lutris nereis)

Abstract

The niche divergence hypothesis suggests that if a species exhibits intersexual differences in diet, selection should favor divergence in the feeding apparatus between the sexes. Recent work revealed that male and female southern sea otters ( Enhydra lutris nereis ) utilize different dietary resources in response to increased population density; females exhibit more specialized diets as a function of smaller home ranges, whereas males exhibit larger home ranges, potentially allowing them to expand their dietary breadths by feeding on prey items that are not found in female home ranges. These dietary differences suggest the potential for sexual dimorphism of the feeding apparatus (i.e., the skull). Here, we tested the hypothesis that male and female southern sea otters exhibit differences in craniomandibular traits directly related to biting ability. Univariate and multivariate analyses of 12 craniomandibular traits showed that size is the primary axis of skull variation, whereas only a handful of craniomandibular traits demonstrated significant shape differences between the sexes. Relative postorbital constriction breadth, masseter in-lever length, and cranial height differed significantly between the sexes. These 3 traits can increase the surface area of jaw muscle attachment sites and thus are directly linked to the mechanics of biting ability. Collectively, these morphological differences indicate that niche divergence may be an important mechanism maintaining sexual dimorphism in southern sea otters.