Ontogenetic Scaling of Theoretical Bite Force in Southern Sea Otters (Enhydra lutris nereis)

Abstract

Sexual dimorphism attributed to niche divergence is often linked to differentiation between the sexes in both dietary resources and characters related to feeding and resource procurement. Although recent studies have indicated that southern sea otters (Enhydra lutris nereis) exhibit differences in dietary preferences as well as sexual dimorphism in skull size and shape, whether these intersexual differences translate to differentiation in feeding performances between the sexes remains to be investigated. To test the hypothesis that scaling patterns of bite force, a metric of feeding performance, differ between the sexes, we calculated theoretical bite forces for 55 naturally deceased male and female southern sea otters spanning the size ranges encountered over ontogeny. We then used standardized major axis regressions to simultaneously determine the scaling patterns of theoretical bite forces and skull components across ontogeny and assess whether these scaling patterns differed between the sexes. We found that positive allometric increases in theoretical bite force resulted from positive allometric increases in physiological cross-sectional area for the major jaw adductor muscle and mechanical advantage. Closer examination revealed that allometric increases in temporalis muscle mass and relative allometric decreases in out-lever lengths are driving these patterns. In our analysis of sexual dimorphism, we found that scaling patterns of theoretical bite force and morphological traits do not differ between the sexes. However, adult sea otters differed in their absolute bite forces, revealing that adult males exhibited greater bite forces as a result of their larger sizes. We found intersexual differences in biting ability that provide some support for the niche divergence hypothesis. Continued work in this field may link intersexual differences in feeding functional morphology with foraging ecology to show how niche divergence has the potential to reinforce sexual dimorphism in southern sea otters.

Effects of diet on cranial morphology and biting ability in musteloid mammals

“…One of the more innovative feeding behaviours is tool use, which durophagous sea otters ( E. lutris ) perform to break open hard‐shelled marine invertebrates (Riedman & Estes, ; Fujii et al ., ). Despite several cranial adaptations to facilitate durophagy [short, blunt skulls, taller and wider mandibular rami, bunodont dentition, fracture‐resistant dental enamel, and positively allometric bite forces (Riley, ; Constantino et al ., ; Ziscovici et al ., ; Law et al ., , )], sea otters have relatively lower bite forces for their cranial size compared to what is predicted by the allometric slope of all musteloids (Fig. ).…”

“…An important caveat to our interpretation is that bite forces estimated using the dry skull method have yet to be validated with empirical bite force measurements from live musteloids. Bite forces are generated from many other components other than skull morphology including fibre length, muscles mass, orientation, pennation angle and activation pattern of the different jaw adductor muscles (Herrel et al ., ; Davis et al ., ; Santana et al ., ; Law et al ., ). Therefore, these empirical data are essential to determine the accuracy of our bite force estimations.…”

“…Much research has examined the craniomandibular morphologies and musculature that facilitate these disparate dietary ecologies within musteloid clades (e.g. Riley, ; Friscia et al ., ; Figueirido et al ., ; Law et al ., ; Davis & Williams, ). In a comparative study of 31 musteloid species, Dumont et al .…”

Bite Force and Masticatory Muscle Architecture Adaptations in the Dietarily Diverse Musteloidea (Carnivora)

“…Several earlier studies have sought to describe masticatory anatomy and performance within select extant (and extinct) species of Musteloid. Though much of this work has focused upon Mustelids [e.g., Martes americana, Lontra canadensis, and Enhydra lutris (Riley, 1985); Enhydra lutris (Law et al, 2016)], several other extant taxa have been analyzed from the perspective of masticatory performance, including Procyon lotor (Christiansen and Adolfssen, 2005;Christiansen and Wroe, 2007;Davis, 2014;Law et al, 2018), P. cancrivorus, Ailurus fulgens, Gulo gulo, Taxidea taxus, Nasua nasua, and others (Christiansen and Adolfssen, 2005;Christiansen and Wroe, 2007;Law et al, 2018). The feeding performance of several giant fossil mustelids (including Megalictis, Eomellivora, Ekorus, Plesiogulo, and Enhydritherium) has similarly been considered (Valenciano et al, 2016).…”

Drivers of in vivo bite performance in wild brown mouse lemurs and a comparison with the grey mouse lemur

“…Bite performance, for example, has been studied in a wide range of vertebrates through estimations based on cranial morphology and biomechanical models, or by direct in vivo measurements of bite force. These studies span a wide array of taxa including alligators (Erickson, Lappin & Vliet, ), turtles (Herrel, Petrochic & Draud, ), carnivores (Christiansen & Wroe, ; Sakamoto, Lloyd & Benton, ; Law, Young & Mehta, ), bats (Nogueira, Peracchi & Monteiro, ; Santana, Dumont & Davis, ), various other mammals (Thomason, ; Freeman & Lemen, ) and birds (van der Meij & Bout, ; Herrel et al ., ,b). The importance of bite performance in feeding is well‐illustrated by Darwin's finches of the Galapagos Islands, where individuals with large beaks, which have high bite forces (Herrel et al ., ), are selected during drought episodes during which only large seeds are available (Boag & Grant, ).…”