Theories abound as to why women go into menopause, but the latest hypothesis being put forward suggests it may be men – or specifically their preference for younger mates – that has led to women's loss of fertility at a certain age.
Researchers at McMaster University believe that over tens of thousands of years, a lack of reproduction among older women has given rise to menopause as an unintended result of evolutionary natural selection.
Using computer modelling, the researchers found that over time, competition among men of all ages for younger mates left older females with much less chance of reproducing.
"We are saying somewhere along the line, men began to change their preference in mating," said evolutionary biologist Rama Singh, co-author of the study published in this week's issue of the journal PLOS Computational Biology.
"What we're saying is that menopause will occur if there is a preferential mating with younger women, and older women are not reproducing," he said.
Singh said women would have had children when they were younger – from 15 to 30, based on the computer model.
Any genetic mutations that caused an end to fertility later in life would be passed down from generation to generation, he said.
And over time, an accumulation of such mutations harmful to female fertility ended up producing a menopausal period that became part of the overall human female genome, the hypothesis suggests.
"It's a very simple theory. What it does is it demystifies menopause.… It becomes a simple age-related disease, if you can call it that," said Singh.
"That's just like all the mutations that affect our aging – white hair, weak muscles, this and that. These are mutations which affect fertility."
The researchers' hypothesis runs counter to prevailing theories about menopause, including the widely held "grandmother theory."
That theory suggests that women evolved to become infertile after a certain age to allow them to assist with rearing grandchildren, thus helping to promote survival of their children's offspring.
Kristen Hawkes, an anthropologist at the University of Utah in Salt Lake City, is among the leading proponents of the grandmother theory.
Other primates, such as chimpanzees, do not live as long as humans, but females of the species have a loss of fertility, or parturition, at similar ages to their human counterparts, she said. Female chimps rarely live beyond their child-bearing years, usually dying in their 30s or sometimes their 40s.
"The preference men have for young partners is a striking contrast with other primates," Hawkes said by e-mail, noting that it's been well-documented that male chimpanzees prefer older females as mates.
"My guess about that has been it's a consequence of our life history – something selection would favour after – not before – our grandmothering life history evolved … when women continue to be healthy and competent beyond their fertility."
Singh, however, doesn't believe the grandmother theory holds up from an evolutionary perspective.
"How do you evolve infertility? It is contrary to the whole notion of natural selection. Natural selection selects for fertility, for reproduction – not for stopping it," he said.
"This theory says 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."
Besides ending reproduction, menopause also affects a woman's health as hormonal changes increase the risk for such conditions as cardiovascular disease and osteoporosis, said Singh.
He hopes the new theory might spawn research into ways to prevent menopause, which would lengthen a woman's reproductive life while also helping to maintain better health as she ages.
"Until now, it has been something that you could not do anything about except [deal with] the symptoms."