Pity the poor chimpanzee. She lives scarcely 40 years in the wild, bearing young the entire time. She is fertile throughout her lifespan, growing old and gray while birthing baby chimps to the end of her days.
That’s just the way things are in the natural world. Only a couple kinds of whale—and human women—live beyond their years of fertility. This is because the evolutionary purpose of a species is to procreate, according to natural selection. If you’re not furiously making babies, you’re hogging precious resources, and if that state of affairs persists, you just may be relegated to the dusty Darwinian basement of interesting but extinct species.
Yet, human women can expect decades of life after fertility. And you can bet that evolutionary anthropologists are having a heyday with this nugget. “Human menopause is an unsolved evolutionary puzzle,” write the authors of “Mate Choice and the Origin of Menopause” in the June 2013 issue of Computational Biology.
Long age, scientists thought that women experience menopause simply because they run out of viable eggs. The ovaries are stocked with a finite number of eggs, as opposed to sperm, which is continually regenerating. Women’s reproductive systems last 30 to 40 years and then the ovaries fail and the eggs run out. The explanation for continued survival beyond menopause was a mystery.
A more recent view suggests that the difficulty and danger inherent in birthing human babies (large neonatal head size relative to the space in our upright-walking pelvis) along with the many years our helpless spawn require before they are able to hunt and gather on their own (not counting the cost of hockey gear and college tuition) are partly responsible for menopause and an infertile older age.
According to this view, it makes some evolutionary sense to limit the years of fertility so a human mother could focus on rearing the children she has instead of taking on the risk of having more children that she might not live long enough to see into adulthood. In other words, quality trumps quantity.
“There may be little advantage for an older mother in running the increased risk of a further pregnancy when existing offspring depend critically on her survival,” according to “The Evolution of Human Menopause,” a report by a pair of researchers at the University of Newcastle.
Then came the Grandmother Hypothesis. This theory emerged from the work of anthropologist Kristen Hawkes, at the University of Utah. In her study of the Hadza, an indigenous tribe in Tanzania, she observed that the tribe’s ace-in-the-hole with regard to survival was the grandmothers—the older, infertile women. These industrious gals spent their days foraging for food, which they distributed among the mothers and children. The grandmothers were a resource that assured not only survival, but also robust health for the Hadza’s most vulnerable members.
The Grandmother Hypothesis suggests that, given the rigors of rearing children, older, infertile women play a critical role in helping assure the survival of their children’s offspring. Those of us who have spent a month—or more—helping out after the birth of a grandchild know there might be something to this. An experienced caregiver in the household who can cook and clean and who just happens to love that new little bundle to pieces makes a huge difference. Plus, she’s free.
These theories may provide parts of the answer to the reason for menopause, but recently a team of researchers from McMaster University in Hamilton, Ontario, published the findings of yet another hypothesis in Computational Biology.
Ready for this?
Men are the cause of menopause, and specifically, their preference for younger women. After generations of being chucked for the spring chicken, so the theory goes, older women developed genetic mutations that selected against fertility but not against longevity. Thus, men remain fertile throughout their lifespan, while women go through menopause. Because apparently, fertility is wasted on us older hens.
But those rabble-rousing researchers didn’t stop there. Next, they tweaked various parameters of the mating preference paradigm with varying results. When the model allowed men and women to mate without regard to age, both genders remained fertile throughout their lifespan.
But when the computer models were adjusted to account for male preference for younger women, Voilà! Menopause. Older women gradually became infertile.
“If women were reproducing all along, and there were no preference against older women, women would be reproducing like men are for their whole lives,” says Rama Singh, an evolutionary geneticist and co-author of the study in this article in Science Daily.
You know, ladies, I think we owe those cradle-robbing men a debt of gratitude. Annoying as it may be to inevitably become the losers in the marketplace of youth and beauty, can you imagine having children in your 80s? From that perspective, menopause never looked better.
And never fear. Natural selection evens the score—on the computer models, at least. When our intrepid scientists adjusted their computers to create a female preference for younger men, then the old geezers lost their fertility, too, experiencing a male menopause just like ours.
Poetic justice, perhaps?