Menopause and the Evolutionary Puzzle of Fertility

Menopause is the permanent end of menstrual cycles, diagnosed after twelve consecutive months without a period. In Canada and many other countries it typically occurs around the early fifties, though timing varies widely between individuals.

From a biological standpoint, menopause is unusual. Most mammals remain fertile for most of their lives, so researchers have long asked why natural selection has not removed a trait that ends reproduction in humans.

What happens in the body

Menopause follows a decline in ovarian activity and in the hormones oestrogen and progesterone. The years leading up to it, called perimenopause, may bring irregular cycles and symptoms such as hot flashes, sleep disruption and changes in mood or sexual comfort. Experiences differ, and some people have few symptoms.

The evolutionary puzzle

Evolutionary theory expects traits that cut off reproduction to be disadvantageous. Yet menopause appears across human populations. Only a small number of other species, including certain whales, are known to show a prolonged post-reproductive life stage, which makes the human case a frequent topic of comparison.

Main hypotheses

The grandmother hypothesis proposes that older women improve the survival of their grandchildren by providing food and care, which could offset the loss of their own fertility. The mother hypothesis suggests that stopping reproduction lowers the risks of late pregnancy and birth, allowing mothers to raise existing children.

A third family of models focuses on mate choice. If males in a population tend to prefer younger females, genetic variants that reduce fertility later in life face little selective pressure and could build up across generations. Computer simulations, including work by researchers at McMaster University, have tested this idea. The authors themselves described such factors as possibly insufficient on their own to explain menopause.

Where the debate stands

These hypotheses are not mutually exclusive, and many scientists see menopause as shaped by several pressures at once. Models depend on assumptions about population structure and behaviour, which are hard to confirm for ancient societies. The question remains open, and research draws on anthropology, genetics and demography.