Variation, Sexual Dimorphism, and Allometry in Molar Size of the Black Bear

The authors investigated molar-crown–size variation, sexual dimorphism, and allometry in the black bear (Ursus americanus), using hunter-shot specimens (n =429) from the island of Newfoundland, and museum specimens from elsewhere in Canada and the continental United States (n =502). The authors predicted higher variation in and weaker correlations among molar size in this omnivorous species than in other species of Carnivora with dentition more specialized for carnivory, because of relatively weak normalizing selection on food-processing mechanisms in U. americanus. Molar-size variation in Newfoundland bears (mean coefficient of variation ~5.6%) was intermediate between species of Carnivora with simpler (e.g., pinnipeds) and more complex (e.g., canids) postcanine dentition. There was negligible size variation within the molar teeth, unlike some mammals. Bilateral symmetry was strong, especially in mandibular length (r ~1.0 between left and right sides; r ~0.95 for other mandibular and maxillary variables and molar size); symmetry in molar size was higher than in phocid seals. Size was positively correlated across molars, especially between adjacent (but not occluding) molars; patterns were similar between sexes and geographic regions, and correlation levels did not differ from other species of Carnivora. The authors also predicted (and found) smaller sexual differences (=100 [(male/female) 1]) in molar size than in body size, because definitive molar size is attained early in life: differences in molar size averaged 5.5% in Newfoundland (higher in continental subsamples), which is less than differences in cranial size (7–9%) or body mass 1/3 (21%). Sexes did not differ in relative molar size. Molar size was mainly isometric or positively allometric to adult body size (using mandibular and maxillary size as proxies).