Why does women's fertility end in mid-life? Grandmothering and age at last birth

TL;DR

This paper presents an agent-based model explaining why women typically cease fertility before age 50, highlighting the role of grandmother effects in evolving human-like longevity and reproductive patterns.

Contribution

It introduces a model that coevolves female lifespan and fertility end age, addressing why menopause occurs earlier than maximum lifespan in humans.

Findings

Grandmother effects drive evolution of human-like longevity.

The model explains why women's fertility ends before age 50.

Fertility termination is maintained by grandmother effects.

Abstract

Great apes, the other living members of our hominid family, become decrepit before the age of forty and rarely outlive their fertile years. In contrast, women - even in high mortality hunter-gatherer populations - usually remain healthy and productive well beyond menopause. The grandmother hypothesis aims to account for the evolution of this distinctive feature of human life history. Our previous mathematical simulations of that hypothesis fixed the end of female fertility at the age of 45, based on the similarities among living hominids, and then modeled the evolution of human-like longevity from an ancestral state, like that of the great apes, due only to grandmother effects. A major modification here allows the age female fertility ends to vary as well, directly addressing a version of the question, influentially posed by GC Williams six decades ago: Why isn't menopause later in humans? Our model is an agent-based model (ABM) that accounts for the coevolution of both expected adult lifespan and end of female fertility as selection maximizes reproductive value. We find that grandmother effects not only drive the population from an equilibrium representing a great ape-like longevity to a new human-like longevity, they also maintain the observed termination of women's fertility before the age of 50.