Half as high for twice as long: male bias in the fertile-age sex ratio

Humans are distinguished from our closest living relatives, other great apes, by our extended postmenopausal longevity, later first births, and shorter birth intervals. Those features likely evolved as ancestral grandmothers’ foraging subsidised dependants in habitats lacking foods that youngsters could manage for themselves. If so, as female post-fertile years increased, older years increased in males too. Those still-fertile old males in the paternity competition pushed average male reproductive success below the female average. As R.A. Fisher explained, Mendelian inheritance requires equal contributions from both sexes to descendant gene pools. Higher average reproductive success in one sex makes tendencies to overproduce it pass to more grandchildren, equalising averages in descendant generations. Yet, men’s lower average reproductive success persists, their fertility lasting decades longer than women’s. Here, we present a simple mathematical model to investigate this phenomenon. We show that a male-biased adult sex ratio is consistent with an offspring sex ratio of 1:1. Assuming a stable age distribution, we show that if male fertile careers are twice as long as those of females, the extended male fertility soon balances the higher female average, maintaining Fisher’s equilibrium under human life history conditions.