Sexual dimorphism and synchrony of breeding: variation in polygyny potential among populations in the common brushtail possum, Trichosurus vulpecula

Male-biased sexual dimorphism in both size and body mass is a common characteristic of many polygynous mammals and is often attributed to sexual selection favoring large males. The degree of dimorphism is thought to be related to the potential for some males to monopolize access to estrous females, which is in turn related to the distribution of receptive females in space and time. In the present study, we investigated the relationship between the temporal distribution of breeding females and the degree of mass dimorphism among 11 populations of the common brushtail possum from northern Australia. Breeding patterns varied from complete aseasonality in some populations to a seasonal concentration of births within 2 or 3 months in others. We predicted that in populations in which mating opportunities were distributed throughout the year, dominant males would be able to monopolize access to larger numbers of estrous females than in populations in which matings were more seasonal, and in such populations, large body size in males would be favored. We found that dimorphism was related to seasonality of breeding, being greatest in populations with a more aseasonal pattern. Mean body mass of male possums also decreased with increasing population density. Population density may influence the degree of breeding synchrony within populations, particularly in locations with a more seasonal climate. The present study is the first to demonstrate plasticity in mass dimorphism in response to local variation in the synchrony of breeding in a mammal species.