Men opting for younger women may be the reason behind menopause.
A new study published in PLoS Computational Biology on June 13 shows that natural selection for a youthful mate has lowered the rate of reproduction for older women, rendering fertility a useless function.
"In a sense it is like aging, but it is different because it is an all-or-nothing process that has been accelerated because of preferential mating," author Dr. Rama Singh, a researcher with the department of Biology at McMaster University in Ontario, Canada, said in a press release. "Menopause is believed to be unique to humans, but no one had yet been able to offer a satisfactory explanation for why it occurs."
Menopause is the end of menstruation and fertility, according to the Mayo Clinic. It officially occurs 12 months from your last period. The average age is 51 in the U.S.
Women can still say healthy and have a sex life, but there are some symptoms including sleep disturbance, hot flashes, lower energy, anxiety and feelings of sadness or loss that may accompany menopause. Sometimes sex can be painful due to thinner, more fragile vaginal tissue, a condition known as dyspareunia, butto treat it.
Menopause can also lead to an increased risk of illness and death due to the hormonal changes. The loss of estrogen was shown to have negative effects on the brain and body metabolism, according to research at Tulane University in New Orleans.
Previous theories have stated that menopause was a natural safeguard to stop women from reproducing when they are too old to see a child grow to maturity. The "grandmother effect" would also allow older women to care for the younger generation's offspring, helping future generations survive.
To see the reason behind menopause, researchers created computational models based on computer simulations to see how men's preference could select for mutations that would end female reproductive abilities.
They discovered it would be highly unlikely that the "grandmother effect" was true because natural selection chooses the most optimal traits, and being able to reproduce is beneficial.
"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.
Instead it was more likely that menopause evolved in women because they no longer needed that ability over a certain age. This also means, menopause could eventually be reversed.
"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," he stated.
Dr. Maxwell Burton-Chellew, an evolutionary biologist from Oxford University who was not involved in the study, told the Guardian that he doesn't agree with the study because sterile worker bees, which are female, show that evolution can select for infertility.
"Having offspring is not the only way to pass on your genes -- you can also pass them on by helping your relatives, which is what good grandmothers do," Burton-Chellew said. "The authors argue that the menopause exists in humans because males have a strong preference for younger females. However, this is probably the wrong way round -- the human male preference for younger females is likely to be because older females are less fertile. The authors' paper offers no reason for why males prefer younger females -- they just take it as a given, which is surprising."
Lynnette Leidy Sievert, a biological anthropologist and a professor at the University of Massachusetts, Amherst, told HealthDay she also found some of the new research dubious. The hypothetical model in the study showed that by the age of 50 or 60, half of the population was still alive, which is not true.
"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," she pointed out.
Sievert also wasn't sure how the study explained why menopause came about.
"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."