Adaptive biases in offspring sex ratios established before birth in a marsupial, the common brushtail possum Trichosurus vulpecula

Offspring sex ratios in the common brushtail possum are male biased in many populations, and there is evidence that inter-population differences in sex ratios represent adaptive responses to local conditions. However, how these biases are produced is not known. Using comparisons between populations with and without biased offspring sex ratios, we show that biases in this species are not produced by sex-differential mortality between birth and weaning or sex-selective termination of pregnancy. Rather, adjustment in the sex ratio of offspring are evidently due to shifts in the probability of conceiving male and female offspring.