Sexual selection and social rank in bighorn rams

For many ungulates, male reproductive success increases with social rank. Because rank is established through contests, it should be correlated with individual mass and select for high sexual dimorphism in body mass. It is difficult to weigh free-ranging ungulates, however, so empirical data on the relation between mass and social rank are scarce. We monitored individual mass and social rank of marked bighorn rams, Ovis canadensis, at Sheep River, Alberta, Canada over 5 years. Each year, rams were organized in a linear hierarchy. Social rank increased with age, and rank in one year was a good predictor of rank in the next year. The stability of dyadic relationships increased with the difference in age of individuals in the dyad but decreased as rams aged. Until about 6 years of age, the positive effects of age and individual mass on social rank were indistinguishable, because rams gained mass each year. The relation between body mass and social rank strengthened with age, probably because, after the heavier rams attain their lifetime asymptotic weight, they can challenge older conspecifics. In mature bighorn rams, social rank is a major determinant of reproductive success. By providing evidence that mass is an important determinant of rank, our study supports the contention that sexual selection leads to high sexual dimorphism in this species.