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

Abstract

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 subsidized 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 the average male fertility rate below the female average. As R. A. Fisher explained, Mendelian inheritance requires equal contributions from both sexes to descendant gene pools. Higher fertility in one sex makes tendencies to overproduce it pass to more grandchildren, equalising averages in descendant generations. Yet lower average number of offspring per year in men persists, their fertility lasting decades longer than womens. Here, we present a simple mathematical model to investigate this phenomenon. We show that a male-biased sex ratio in the fertile ages is consistent with an offspring sex ratio of 1:1. 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 Fishers equilibrium offspring sex ratio under human life history conditions. Significance StatementWhy are offspring sex ratios usually near even in humans when the fertile careers of women and men differ so much? Fishers principle explains why parents are usually expected to produce equal offspring sex ratios. Since every sexually produced grandchild has both a mother and father, the average number of grandchildren expected through offspring of either sex depends on the sex ratio of the fertile pool. If that sex ratio is biased, the average must be higher for the rarer sex. Then any tendency to overproduce the sex with higher average fertility will spread, reducing its rarity until expected averages through sons and daughters are equal. Human life history complicates this simple logic. Female fertility ends in mid-life whereas male fertility does not, creating a male-biased fertile-age population in which average number of offspring per year is lower in males than in females. We show that sex differences in fertility duration can maintain Fishers equilibrium offspring sex ratio by altering Reproductive Value in overlapping generations, providing a foundation for persistent patriarchy.