Evolution of Menopause and the Mamas Boy Hypothesis

Abstract

Post-reproductive lifespan is an evolutionary puzzle. In most mammals female fertility tracks survival, yet humans and a few toothed whales show survival after reproduction ends. Explaining when and why post-reproductive lifespan evolves is central to understanding the evolution of ageing, social structure, and intergenerational helping across species. Kinship-dynamics theory predicts that when males are philopatric, a females local relatedness--especially to male descendants--increases with age, potentially favoring late-life helping over continued reproduction. We develop an age-sex-structured kin-selection model to test whether a rare menopause-inducing modifier allele can invade an initially non-menopausal population through its direct effects on survival and fecundity and its indirect effects on relatives. We consider two evolutionary pathways: stop early, where reproduction ceases earlier with little change in lifespan, and live long, where lifespan extends beyond reproduction under disposable-soma trade-offs. Parameterized with demographic, dispersal, and helping-effect estimates from eight mammalian taxa, the model predicts empirically plausible ages of reproductive cessation and post-reproductive representation in humans and killer whales, but no invasion across plausible cessation ages in non-menopausal taxa. Global sensitivity analyses identify male dispersal and the effect of post-reproductive help on male survival as determinants of whether menopause evolves, motivating the "mamas boy hypothesis": menopause is most strongly favoured by selection when late-life care increases the survival and lifetime fitness of philopatric sons and grandsons.