Can women blame men for menopause?
They may have a case, according to new research that suggests it was men's interest in mating with younger females that gave evolutionary rise to menopause by sidelining older women from reproduction.
Menopause -- when a woman stops getting menstrual periods and can't become pregnant -- is unique to humans and its cause is still unknown, explained study author and evolutionary biologist Rama Singh. "We accept as a given the idea that older women tend to be unable to reproduce," but Singh said this is actually an "evolutionary puzzle."
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It has long been thought that menopause is what causes women, primarily in their early 50s, to stop being able to get pregnant, but the researchers found evidence that things could actually have occurred the other way around. In other words, infertility may have been the cause, not the effect, of menopause in early humans.
There are at least 10 theories of why menopause occurs, according to the researchers, including ideas based on the fact that women are living longer and depleting the number of eggs in their ovaries, to what is called the "grandmother hypothesis." That idea holds that menopause allows older women to provide childcare that contributes to the survival of their grandchildren, making them more fit or valuable to the human tribe.
But Singh's research, published online June 13 in the journal PLOS Computational Biology, suggests something altogether new.
"This paper is saying that men have played the major or dominant part in choosing mates," said Singh, who is a professor of population genetics and evolution at McMaster University, in Canada. "Somewhere along the line in our evolutionary history, males did not mate randomly but preferred young women because they are more attractive."
Going way back in human history, people reproduced all their lives, explained Singh.
While it's possible that some women may have experienced menopause 30,000 years ago, now 100 percent of women experience it. "Menopause is an evolutionary phenomenon," he said.
The scientists found that the development of menopause seems to have done nothing to improve the chances of human survival over time, but rather occurred because women of a certain age weren't finding mates, and thus reproductive ability was unnecessary for them.
Yet Singh pointed out that if women long ago had been the ones choosing younger mates, older men would have been the ones losing their fertility, not women.
The process of natural selection favors the most fit, so women who are most likely to reproduce are protected, explained Singh. Natural selection is the gradual, non-random process through which biological traits become either more or less common, due to the way reproduction occurs, Singh explained.
The researchers used computational models and computer simulations to show how male mating preference for younger females could increase the number of mutations that stopped women's reproductive ability, creating menopause.
Singh said his research suggests that it might be possible for women who delay childbearing to also postpone menopause, allowing them to have a longer window in which to conceive.
"We might be able to extend the time period in which you can have children, rather than rush it," he said.
Lynnette Leidy Sievert, a biological anthropologist and a professor at the University of Massachusetts, Amherst, raised questions about the study.
"The study showed that by the age of 50 or 60, 50 percent of the population was still living, but that just doesn't match what we know about human evolution," she said. "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."
Sievert, a member of the board of trustees of the North American Menopause Society, also questioned whether the concept of men mating with younger women fully explains menopause.
"Because it's a human and mammalian pattern for men to die younger [than women], you have a younger female with an older male who is going to die," she explained. "I get mixed up about how that pulls a woman's lifespan across menopause."
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Singh said he's planning to do more simulations based on a Canadian long-term study of aging that is following 50,000 men and women. He is interested in learning more about the relationship between menopause, reproduction and genetic markers. "I really want to see if you can do something to delay menopause," he explained.