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A dapper older gentleman spurns his mate of a certain age to take a fresh-faced young lover. You've seen that movie before, right?

Well, this choice of youth may turn out to be more than a Hollywood trope. Researchers say decisions like that one may have been the evolutionary source of menopause.

While conventional wisdom says that men prefer younger women because they're fertile, a recent report in PLOS Computational Biology suggests that it's precisely this preference for younger mates that caused older women to become infertile in the first place.

Computational biologist Jonathon Stone of McMaster University in Ontario, Canada, spoke with Shots about the mathematical model he and some colleagues used to come up with that hypothesis.

First, for the sake of computation, they assumed that women have infinite capacity for both fertility and survival. Then, they slowly introduced common genetic mutations into the math mix and observed how those changes led women to become infertile and eventually die.

They were able to tweak the model to account for age preferences between mates. When they changed the program to allow men and women of all ages to mate, the data consistently showed that women's survival and fertility plummeted simultaneously.

It was only after they used mathematical trickery to adjust their program to skew toward a preference of men for younger women that the data showed a 20-year loss of fertility before death in older women.

The phenomenon of having several decades without fertility at the end of a woman's life is unique to humans. This period of life begins at menopause, and its origins have been confusing to researchers for years.

Until recently, evolutionary biologists had widely accepted the grandmother hypothesis, the idea that menopause evolved as a way for women to focus their energies on the youngest in the clan. "This idea," Stone says, "frees up daughters to have more offspring, an evolutionary advantage, since this increases the number of offspring twofold."

University of Utah anthropologist Kristen Hawkes, who wasn't involved in the mathematical research, says she has her money on the grandmother hypothesis. Hawkes, whose work deals with the evolutionary origins of menopause, points out that great apes, with whom we share a great deal of evolutionary history, exhibit a similar pattern of losing fertility in their mid-40s.

The difference is that while the apes' fertility rates seem to be in sync with those of humans, their longevity rates aren't. "They usually die before they get to those post-fertile years," she says. "They get to be old ladies, gray and frail, while they're still cycling."

But Hawkes acknowledges the human male's apparent difference in taste. "I agree the preference men have for young partners is a striking contrast with other primates — especially since it is well-documented that chimpanzee males prefer older females," she says.

Stone says that the model also shows that the outcome could be reversed. "If we begin by assuming that all women are choosing to mate with younger men, male menopause will be the eventual evolutionary outcome." In fact, members of his team are currently testing this prediction in fruit flies, which reproduce rapidly enough to track evolutionary changes in next to no time.

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