A simple model for adaptive variation in the sex ratios of mammalian offspring

We present a simple mathematical model that describes how primary and secondary sex ratios of offspring may vary adaptively in order to maintain equal numbers of the sexes at the age of reproductive maturity. The model postulates that the sex of an offspring depends probabilistically on a weighted linear combination of maternal testosterone and male vulnerability. The model operates at population level, and is based on three physiological phenomena: first that maternal testosterone in follicular fluid is normally distributed, with levels above the mean more likely to be associated with the conception of males; secondly, that males are more vulnerable than females from conception onwards; and thirdly that under conditions of chronic stress, increased secretion of female testosterone coincides with increased male vulnerability. Thus during times of chronic stress, more males are conceived, but their number of live births is moderated by increased male loss. Variations in secondary sex ratios should therefore be related not only to the stressfulness of environmental conditions, but also to the timing of changes in stressfulness.