Total Dental Occlusal Area as a Feeding Constraint Feature in Extant Walruses ( Odobenus rosmarus ), and Implications for the Evolution of Molluscivory in Odobenidae

Mammalian dental formulae often are highly conserved, at least at a generic level. In walruses ( Odobenus rosmarus ), the constraints of dentition in light of documented high variability in tooth counts among walrus are examined. We propose that walruses do not have a constrained dental formula in terms of tooth position or tooth count. Instead, while walrus tooth counts vary, total occlusal area (TOA) is constrained relative to body mass, independent of tooth position or count. Nearly three-fourths of the 70 individuals sampled here had dentitions deviating from the previously reported dental formula of 1/1, 1/0, 3/3, 0/0, but there is a strong relationship between body size and total occlusal area. While the positive correlation between body size and TOA is consistent between sexes, the slope of the relationship differs significantly, suggesting an important sexual dimorphism in more than just walrus body size or tusk morphology. It is unclear if walrus teeth are involved at all in feeding, so TOA may impact positioning of prey during feeding or be completely independent of prey acquisition or feeding. Limited field observations support the hypothesis that dentition may play some role in prey positioning, with males feeding on both larger species and larger individuals of bivalves than females. Differences in food availability to individuals of differing body size have implications for conservation of walruses in the face of climate change and interactions with human fisheries, as well as diagnosing fossil taxa frequently defined in part by dental count. Extant walruses also may present an example of linking increased trait variability with the relaxation of functional constraints in response to shifting ecological roles.