Growth patterns of theoretical bite force and jaw musculature in southern sea otters (Enhydra lutris nereis).

Abstract

The transition from milk to solid food requires drastic changes in the morphology of the feeding apparatus and its performance. As durophagous mammals, southern sea otters exhibit significant ontogenetic changes in cranial and mandibular morphology to presumably enable them to feed on a variety of hard-shelled invertebrate prey. Juvenile sea otters begin feeding independently by 6-8 months of age, but how quickly they reach sufficient maturity in biting performance remains unknown. Here, I found that the theoretical bite force of southern sea otters does not reach full maturation until during the adult stage at 3.6 and 5.0 years of age in females and males, respectively. The slow maturation of biting performance can be directly attributed to the slow growth and development of the cranium and the primary jaw adductor muscle (i.e., the temporalis) and may ultimately impact the survival of newly weaned juveniles by limiting their ability to process certain hard-shelled prey. Alternative foraging behaviors such as tool use, however, may mitigate the disadvantages of delayed maturation of biting performance. In analyses of sexual dimorphism, I found that female otters reached bite force maturation earlier, whereas male otters exhibit initial rapid growth in bite force-to quickly reach sufficient biting performance needed to process prey early in life-followed by a slower growth phase toward bite force maturation that coincides with sexual maturity. This biphasic growth in bite force suggests that male-to-male competition for resources and mates exhibits strong selection in the growth and development of skull form and function in male otters. Overall, this study demonstrates how the analysis of anatomical data can provide insight on the foraging ecologies and life histories of sea otters across ontogeny.