Adjustment of offspring sex ratios in relation to availability of resources for philopatric offspring in the common brushtail possum

Abstract

The Local Resource Competition (LRC) hypothesis predicts that where philopatric offspring compete for resources with their mothers, offspring sex ratios will be biased in favour of the dispersing sex. This should produce variation in sex ratios among populations in relation to differences in the availability of resources for philopatric offspring. However, previous tests of LRC in mammals have used indirect measures of resource availability, and have focussed on sex ratio variation among species or individuals, rather than local populations. Here we show that the availability of den sites predicts the offspring sex ratio in populations of the common brushtail possum. Female possums defend access to dens, and daughters but not sons occupy dens within their mother's range. However, the abundance of possums on our study areas was determined principally by food availability. Consequently, in food-rich areas with high population density the per capita availability of dens was low, and the cost of having a daughter should have been high. This cost was positively correlated with male biases in the sex ratio at birth.