Why are American mink sexually dimorphic? A role for niche separation

American mink are highly sexually dimorphic, with males being up to twice the size of females. Sexual dimorphism may arise for several reasons, including intra- or inter-sexual selection, inter-sexual competition, or divergent reproductive roles. Whether or not dimorphism arises from competition, a degree of niche separation is expected in dimorphic species. Sexual divergence in feeding niche has been reported for many species, including mink. This is likely to be manifested in a greater degree of dimorphism in those structures, such as teeth, that are used for the acquisition of prey. We tested the hypothesis that teeth and other trophic structures of male mink would be significantly larger than those of females, after controlling for underlying skeletal size differences. Canine and carnassial teeth, and several skull dimensions, were larger than predicted in males. There is good evidence that sexual dimorphism in mink trophic apparati is greater than predicted from allometry. We examined the development of dimorphism in various features with age and found that it was not consistent. Several trophic features were dimorphic amongst juveniles, and the degree of dimorphism remained relatively constant with age. Dimorphism in canines, and in relative body mass, was less apparent amongst juveniles and increased with increasing age. We discuss our results in the light of contemporary theories on the evolution and maintenance of sexual size dimorphism and argue that niche separation as a result of dimorphism in trophic features, while probably not the driving force behind sexual size dimorphism, may play a role in its maintenance.