Weaning represents a major ontogenetic dietary shift in southern sea otters (Enhydra lutris nereis), as juveniles must transition from depending on mother’s milk to independently processing hard-shelled invertebrates. When the skulls of juveniles have reached sufficient maturity to transition to a durophagous diet remains to be investigated. Here, we conducted a comprehensive analysis of skull development and growth and sexual dimorphism using geometric morphometric approaches in 204 southern sea otter skulls. We found that southern sea otters of both sexes exhibit dramatic changes in cranial and mandibular shape and size over ontogeny. Although the majority of these changes occur in the pup stage, full development and growth of the skull does not occur until well after weaning. We hypothesize that the slower maturation of the crania of newly weaned juveniles serves as a handicap by constraining jaw adductor muscle size, biting ability and feeding on hard-shelled prey. In our analysis of sexual dimorphism, we found significant sexual shape and size dimorphism in adult craniomandibular morphology that arose through differences in developmental and growth rates and duration. We postulate that males are selected to attain mature crania faster to presumably reach adult biting ability sooner, gaining a competitive advantage in obtaining food and in male–male agonistic interactions.
