Law, Vikram Venkatram, Rita S. Mehta; Sexual dimorphism in craniomandibular morphology of southern sea otters Enhydra lutris nereisJournal of MammalogyVolume 97, Issue 6, 5 DecemberPages —, https: 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. Here, we tested the Sexual dimorphism in marine mammals that male and female southern sea otters exhibit Sexual dimorphism in marine mammals 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.
The divergence of biological traits between the sexes sexual dimorphism is a widespread phenomenon Fairbairn Darwin was the first to hypothesize that sexual selection is the primary mechanism for the evolution and maintenance of sexual dimorphism in many endotherms. Under sexual selection, sexually dimorphic traits e. Darwin and others Hedrick and Temeles ; Shine also postulated that sexual dimorphism could arise through natural selection, Sexual dimorphism in marine mammals intersexual niche divergence.
Under this hypothesis, differences in phenotypic traits reflect ecological divergence between the sexes, possibly including reduction intersexual competition for resources and habitat use Smith ; Berwaerts et al. Empirical evidence for the intersexual niche divergence hypothesis, however, is often difficult to obtain, and few studies have demonstrated a direct link between niche divergence and sexual dimorphism Hedrick and Temeles ; Shine Thus, the majority of studies emphasize sexual selection, rather than niche divergence, as the primary mechanism that promotes and maintains sexual dimorphism.
The mammalian order Carnivora provides some of the most striking examples of sexual dimorphism Lindenfors et al. After body size, craniodental morphology is perhaps the 2nd most prominent feature of sexual size dimorphism in these polygynous carnivorans Gittleman and Valkenburgh ; Brunner et al.
Males have larger and more robust craniodental traits, including total cranium length, canines, rostra, masseteric fossa, mastoid breadth, zygomatic breadth, and sagittal crests Gittleman and Valkenburgh ; Brunner et al. Larger craniodental traits translate to increased surface area for jaw muscle attachment sites and larger teeth, thereby allowing for stronger biting ability and heavier heads for use in territorial conflicts with male competitors Radinsky ab.
Studies of sexual dimorphism in carnivorans are typically focused on species that mate on land.
In contrast, carnivorans such as most phocids that mate in the water or on ice are not restricted to small breeding sites Le Boeuf Therefore, males have fewer opportunities to defend and mate with groups of females Le Boeufand the maintenance of extreme sexual size dimorphism seen in terrestrially mating phocids does not occur Lindenfors et al.
Studies investigating intersexual differences in the craniomandibular morphology of these phocids are consistent with this pattern and have found minimal to moderate "Sexual dimorphism in marine mammals" dimorphism Amano et al. Like many phocid species, sea otters Enhydra lutris exhibit an aquatic polygynous mating system, suggesting that sexual dimorphism typically seen in terrestrially polygynous carnivorans should be reduced.
However, when compared to water- or ice-mating phocids Lindenfors et al. Male southern sea otters E. Like most carnivoran clades, male-biased size dimorphism is the ancestral state across mustelids Moors Sexual size dimorphism is generally hypothesized to evolve as a result of male—male competition for access to females Moors ; Ralls and Harveyand sexual dimorphism in sea otters is also assumed to have evolved from sexual selection Estes However, although male sea otters establish resource-based territories to attract females Loughlin ; Jameson ; Pearson and Davisaquatic territories are often difficult to defend from competing males Jameson ; Pearson et al.
Studies based on daily observations have found that nonterritorial and transient male sea otters use resources and interact with females within the territory of a territorial male and that body masses and body condition do not differ significantly between territorial and transient males Jameson ; Pearson et al. In addition, male otters in some subpopulations switch between defending territory and actively roaming in search of mating opportunities Jameson Sexual dimorphism in marine mammals lines of evidence suggest that larger male body sizes may not necessarily confer reproductive success in the aquatic mating system of sea otters, and therefore other selective forces aside from sexual selection may be contributing to the maintenance of sexual dimorphism in this species.
Niche divergence and the partitioning of resources provide an alternative mechanism that could maintain sexual dimorphism in sea otters. As one of the smallest marine mammals, sea otters exhibit the highest known mass-specific metabolic rates of marine mammals Morrison et al.
Thus, the morphological, behavioral, and functional traits associated successful feeding may be under strong selective pressures in sea otters, and intersexual niche divergence in diet could serve as a mechanism to reduce dietary competition between the sexes.
Along the central California coast, male and female southern sea otters use different habitat and dietary resources in response to increased population density: Female southern sea otters are therefore now seen as the primary drivers of intraspecific dietary specialization Elliott Smith et al.
As a result, females potentially exhibit increased feeding efficiency or Sexual dimorphism in marine mammals in consuming hard-shelled prey compared to male otters Tinker et al. Increased feeding efficiency in females may offset the energetic demands associated with raising pups, which in turn may further facilitate dietary partitioning between the sexes. Although additional work is needed to investigate the extent of resource partitioning i. If intersexual dietary divergence occurs, we expect to find differences in the feeding apparatus between the sexes.
Previous studies have noted that skulls of male southern sea otters are larger Scheffer ; Roest ; Wilson et al. A pronounced sagittal crest increases the attachment site for important masticatory muscles, suggesting intersexual differences in biting ability.
However, the extent of size and shape differences in craniomandibular traits or their influence on biting ability was not tested in these studies.