Senescence of reproduction may explain adaptive menopause in humans: a test of the "mother" hypothesis.

The "mother" hypothesis is one of the main adaptive explanations of human menopause. It postulates that reproductive cessation constitutes a strategy that has been selected for during human evolution because mothers at older ages might maximize their fitness by investing resources in the survival and reproduction of their living children rather than by continuing to reproduce. This study provides a test of this hypothesis. Fertility functions that maximize fitness are built into a model incorporating the fact that the survival of females during the rearing period is a major determinant of their children's survival. Results are given according to different scenarios of increase with mothers' age of maternal mortality risk and risk of stillbirth and birth defects (on the assumption that these females do not experience menopause). Different estimates of the effect of a mother's death on her child's survival were also incorporated. Finally, a population genetics framework allows us to estimate selection on these optimal fertility functions. To determine whether or not these fertility functions show a menopause, three criteria are discussed: the rapidity of fertility decline, if any; the magnitude of selection on menopause compared with a nonmenopausal strategy; and the selection on survival during post-reproductive life. Our results show that menopause and subsequent post-reproductive life are significantly advantageous when two conditions are satisfied: a marked increase in stillbirth and risk of birth defects as well as in maternal mortality with mother's age.