Supersize me: heavy eastern grey kangaroo mothers have more sons

Abstract

The Trivers-Willard hypothesis predicts that, in polygynous and sexually dimorphic mammals, mothers able to provide a large amount of care should produce more sons. Tests of this prediction, however, have generated equivocal results, possibly because multiple factors, including environmental conditions, simultaneously influence progeny sex ratio. We tested the influence of maternal mass, condition, size, previous reproduction and age class on offspring sex ratio in two populations of eastern grey kangaroos (Macropus giganteus). In one population, heavier and taller mothers produced more sons than did lighter mothers, but maternal condition had no effect on progeny sex ratio. Primiparous females, however, produced an even offspring sex ratio despite being smaller than multiparous females. A strong year effect on progeny sex ratio suggested a likely effect of environmental conditions. In the second population, none of the variables tested influenced progeny sex ratio. Different environmental conditions between the two populations could partly explain these results. Because maternal size and mass likely correlate with reproductive potential, we suggest that our results support the Trivers-Willard hypothesis, but other variables likely also influence progeny sex ratio.