Adaptive sex allocation in relation to life-history in the common brushtail possum, Trichosurus vulpecula

1. Maternal control of offspring sex remains a contentious topic in the evolutionary and behavioural ecology of vertebrates. Two models of adaptive sex allocation - the Trivers-Willard (TW) and local resource competition (LRC) hypotheses - have been supported in some studies, but in many other cases the forces shaping offspring sex ratios are unclear. One reason that past studies may not have provided clear support for either model is that the two processes interact, producing complex patterns of sex allocation in relation to the life histories of individual mothers. 2. The TW hypothesis predicts that in dimorphic, polygynous mammals, females in good condition should be more likely to produce male offspring. The LRC hypothesis predicts an independent effect of age on the offspring sex ratio; females should be more likely to produce sons early in their reproductive lives and daughters later, in order to minimize the duration and reproductive cost of mother-daughter competition. We tested these two predictions in a study of sex allocation in the common brushtail possum, Trichosurus vulpecula. 3. Offspring sex was strongly dependent on maternal age: females breeding for the first time were likely to produce sons, and at subsequent breeding attempts the sex ratio of offspring was slightly biased towards females. After the first breeding season, sex of offspring was influenced more strongly by body condition than by age, and females in good condition were more likely to have sons. Individual females that showed a between-year gain in condition were also more likely to produce a male in the subsequent year. 4. Body mass of male offspring at 5 months was positively correlated with maternal condition. No relationship was found between maternal condition and mass of female offspring. 5. We conclude that patterns of change in sex allocation in relation to individual life histories of female common brushtail possums can be explained by the interaction of TW and LRC effects.