Paternity in eastern grey kangaroos: moderate skew despite strong sexual dimorphism

Understanding sexual selection requires adequate measures of reproductive success. In wild mammals, reliable data on variation in male reproductive success are available for very few species. We assessed the distribution of paternities and quantified skew in male reproductive success in 2 populations of a marsupial with strong sexual dimorphism, the eastern grey kangaroos (Macropus giganteus) over 5 years. We assigned fathers to 356 juveniles, or 79% of those with known mother. We found a relatively weak mating skew and the most successful males did not monopolize a large fraction of paternities. Nearly half of the adult males we monitored fathered at least 1 young. The yearly opportunity for sexual selection (I s) for males ranged from 1.80 to 3.98, and Nonacs’ B index of mating skew was significant but low, ranging from 0.01 to 0.07. Considering the strong sexual dimorphism, long breeding season, and strong male dominance hierarchy, our results suggest an unexpectedly low reproductive skew. That is surprising given the wide range in male weights: the smallest fathers weighed 40% less than the heaviest ones. Skew in eastern grey kangaroos is weaker than that estimated for other species with lower sexual size dimorphism. We found substantial year-to-year variability in reproductive skew. Because male mating success varies according to the characteristics of competitors and the distribution of breeding opportunities, multiple years of monitoring are required to obtain reliable estimates. In the absence of data on paternity, strong sexual dimorphism cannot be assumed to imply high polygyny and strong sexual selection.