Why do women experience menopause?
What's up with menopause? As a woman in her late 40s experiencing hot flashes and other signs of the looming cessation of menstruation, I'm wondering why women lose the ability to reproduce while men retain it, at least theoretically, until death. Not that I'm going to miss the pill, tampons, etc., but there's a lack of symmetry here that bothers my sense of aesthetics. Please advise.
I know it doesn't seem fair, ma'am, but look at the bright side. Would you rather be part of the primary target demographic for Viagra?
There used to be a comforting theory about menopause. In a famous 1957 paper, evolutionary biologist George C. Williams argued that menopause differed from other consequences of old age, such as failing eyesight and wrinkles, in that (a) it happened relatively early in life, usually around the half-century mark, and (b) it was inevitable regardless of how much attention a woman paid to her health. What's more, he said, it seemed to be unique to humans. Other female mammals, most notably other female primates, were capable of bearing children till the end.
Williams explained this apparent oddity by claiming that menopause, far from being a sign of decrepitude, was actually a positive evolutionary adaptation. A postmenopausal woman could see her progeny through to maturity without fear that she'd die in childbirth after a late pregnancy. Others elaborated on this idea, noting that a postmenopausal woman could assist in raising her grandchildren and serve as a repository of tribal knowledge besides. This "granny effect," as it came to be known, was seen as one of the crucial factors that had enabled humanity to pull itself out of the swamps, right up there with the harnessing of fire.
Charming though it was, the granny hypothesis suffered from certain defects. The most obvious was that until the 20th century menopause wasn't just the time when a woman ceased ovulating; it was more or less the time when she died. Average age at menopause today is 51; average life expectancy in 1900 had only reached 47. In other words, during most of evolutionary history, the average woman had enough eggs to last a lifetime. Anthropological research turned up little support for the granny effect, and theoretical models suggested that, notwithstanding the social benefits of nonchildbearing grandmothers, more offspring would survive if women just kept having babies.
To many eyes, the final blow for the granny effect was a 1998 paper by University of Minnesota ecologist Craig Packer, who refuted the idea that only human females go through menopause. Citing earlier research, he noted that menopause occurs in a number of other mammalian species, including dogs, rabbits, elephants, whales, etc., and may well be a trait of all mammalian females.
What's more, the timing of menopause in other species suggests why it occurs when it does in humans, despite increased life expectancy. Female baboons experience a decline in fertility after age 21; few live past 26 or 27. But since a baboon takes only six years to reach puberty, that's OK — the young will be grown before the mother dies. Similarly, female lions become markedly less fertile after age 13 and are usually dead by 18 — again OK, from the standpoint of the species, because a lion cub reaches puberty at three. Turning to our own kind, women experience reproductive decline after 40 and until modern times typically died in their 50s, but their kids reach puberty at around 11 … you get the picture. Female mammals remain fertile long enough to be able to raise their young before they die, and no longer.
Packer and his associates went on to examine survival rates among lions with living grandmothers versus those without. They found no granny effect. On the contrary, grandma lions enhanced the survival of their pride's offspring only if they hadn't yet gone through menopause and could help suckle their daughters' cubs while raising more of their own.
Granny effect proponents haven't given up. Anthropologist Kristen Hawkes has studied an African tribe called the Hadza whose survival depends on grandmothers who forage for food while mothers breast-feed their infants. So the argument isn't over. But for now it appears menopause doesn't mark your graduation to some higher stage of nurturing; it just means you've gotten old.