Many women are bewildered by menopause. Even scientists struggle to explain its very existence in human females. You see, women (along with short-finned and killer whales) are among the small handful of species unable to reproduce until the end of life. It wasn't always that way -- menopause evolved over history -- and there are few good hypotheses to explain why.
Some biologists propose the 'grandmother effect,' a theory suggesting that natural selection led to infertility among aging women. It is hypothesized that if women were able to reproduce until their own death, they wouldn't be available to care for children of the next generation. Additionally, late life pregnancy meant mothers risked dying during childbirth or while young offspring were still dependent on them. Even if older women could deliver children, they might not be fit enough to provide for them. Menopause evolved, theorists believe, in order to better serve the survival our species.
More recent research points to a totally different hypothesis, one that dismisses the 'grandmother effect.' Geneticists from Canada's McMaster University believe it doesn't make sense that fertility was extinguished among older women through natural selection. Evolution, they say, just doesn't work that way. Only disadvantageous traits disappear and reproductive ability does not fall into that category. Instead, they propose the "male preference" theory, which states that men's interest in younger female partners led to the gradual extinction of fertility among older women. As females aged, they could no longer find mates and reproductive ability became unnecessary for them.
Up until this study, it was believed that men's desire for younger women was a response to --not the cause of -- menopausal symptoms. But these Canadian scientists say that their study proves it's the other way around. Professor Rama Singh, who led the research, hypothesizes that as males lost interest in older women, it resulted in a lack of purpose for continued fertility. His group examined human development over hundreds of thousands of years and concluded "Somewhere along the line in our evolutionary history, males did not mate randomly but preferred young women because they are more attractive." Using a mathematical model, Singh's group showed evidence that menopause evolved as a logical biological solution. "If there were no preference against older women, women would be reproducing like men are for their whole lives," he stated.
There are many who refute this theory, including Dr Maxwell Burton-Chellew, an evolutionary biologist at the University of Oxford. "I think it makes more sense to see the human male preference for younger females largely as an evolved response to menopause, and to assume that ancestral males would have been wise to mate with any females that could produce offspring." He adds, "Older females faced an interesting 'choice': have a child that may not reach adulthood before your own death, or stop reproducing and instead focus on helping your younger relatives reproduce."
Another biological anthropologist at the University of Massachusetts, Lynnette Leidy Sievert, questions the 'male preference' theory because of the sample population used for the study. It was based on people who lived until age 60, which just didn't happen years ago. "By the age of 50, the skeletal evidence shows that only 10 percent of Neanderthals lived beyond 50. Our own homo sapiens (humans) had about 17 percent living past the age of 40," says Sievert, pointing to a basic inaccuracy in the simulated mathematical model.
In another study, Johnstone and Cant looked at the possible commonalities between human females, short-finned and killer whales to explain why they undergo menopause. A similarity was found in the mating scenarios of these three species, one that favors younger breeders and older childrearing helpers. In humans, the female leaves home to join her husband's family, widening her connection to kin. In most other species, after mating occurs, the male leaves and the female remains behind. It's this behavioral pattern that they believe led to menopause in aging females.
Thomas Kirkwood, a biologist at Newcastle University in the United Kingdom isn't convinced that conclusions about the evolution of menopause can be drawn from mathematical models like those used by Singh or Johnstone, especially ones that predicts relatedness within a species. "It doesn't show how the presence or absence of the menopause affects Darwinian fitness, which is the all-important evolutionary yardstick."
While menopausal theories and killer whale comparisons may fascinate academics, it doesn't change the fact that most women don't really care. Bottom line, all women face hormonal changes as they age and have to deal with them. But unlike years ago, menopause no longer means hitting 50 with only a few years left to live. Today -- with life expectancy around age 80 -- some women are just beginning to bear children in their late 40s and many will have decades of fruitful years ahead.
The 'grandmother effect' and 'male preference' may have once been keys to female evolution, but in today's world, these trends have little impact on how we see ourselves. Perhaps over the next 100 years, greater longevity, extended fertility and increased productivity will alter our understanding of menopause. For now, this stage should be viewed more as a transition than an end, with potential for many years of continued vitality and life.
A version of this article was first posted at Poise.com.
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Vivian Diller, Ph.D. is a psychologist in private practice in New York City. She serves as a media expert on various psychological topics and as a consultant to companies promoting health, beauty and cosmetic products. Her book, "Face It: What Women Really Feel As Their Looks Change" (2010), edited by Michele Willens, is a psychological guide to help women deal with the emotions brought on by their changing appearances.
For more information, please visit my website at www.VivianDiller.com; and continue the conversation on Twitter @ DrVDiller.
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