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 equally drastic changes in the morphology of the feeding apparatus and biting 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. However, how quickly juvenile otters reach sufficient maturity in biting performances remained unknown. Here, I found that theoretical bite force of southern sea otters does not reach full maturity 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. Alterative 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 growth in male bite force enables them to quickly reach sufficient biting performances needed to process prey early in life. This is followed by a slower growth phase towards bite force maturation that coincides with sexual maturity, thus enabling them to compete with other males for resources and mates. 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.