Absence of Sexual Dimorphism in Molar Morphology of Muskrats

The muskrat (Ondatra zibethicus) lives in both modern and prehistoric wetlands. Sexual dimorphism is a characteristic of many species that is relevant to both modern behavior, such as the partitioning of resources, and the accurate assessment of fossil populations. If sexual dimorphism is present in muskrats, what is known about their interaction with the environment may be inaccurate, as could be the interpretation of the species’ evolutionary history because the study of fossil muskrats relies primarily on lower first molar morphology. Prior research on muskrat dimorphism has produced conflicting results and never directly evaluated the extent of variation in the lower first molar. We therefore tested the length and width of 204 modern lower first molars for sexual dimorphism. Molars of male and female muskrats consisting of specimens from 11 different subspecies were not statistically different in size or shape. Individual subspecies and populations within subspecies also were generally not dimorphic. Sexual dimorphism, therefore, is not a source of variation in the molars of fossil muskrats. The molars are unsuitable for determining sex in modern muskrats, and the similarity of the molar between males and females suggests that food resource partitioning between the sexes probably does not occur. JOURNAL OF WILDLIFE MANAGEMENT 66(4):1189–1196