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Published on Friday, 14 June 2013 20:04


MD CZJ 630px 13 06 14Men who behave like Michael Douglas, Rod Stewart and Ronnie Wood are to blame for women ending their reproductive life early, according to a new theory.

All three celebrities are famous for wooing much younger partners. And it is the male preference for young mates that has led to the menopause, researchers have said.

Through the forces of natural selection, men have unwittingly stacked the Darwinian deck of cards against older women remaining fertile, it is claimed.

"In a sense it is like ageing, but it is different because it is an all-or-nothing process that has been accelerated because of preferential mating," evolutionary biologist Professor Rama Singh, from McMaster University in Canada, said.

The average woman hits the menopause at 51, but for some the "change" can come in their 40s. But quite why human women become infertile in middle age is an unsolved mystery. Only two other species, pilot and killer whales, are known to experience a menopause in a similar way to humans. Female chimpanzees, our closest animal cousins, only stop being fertile near the end of their lives, typically around the age of 45.

The new theory turns the conventional view that the menopause prevents older women from continuing to reproduce on its head. Instead, it holds that lack of reproduction has given rise to the menopause.

Another idea called the "grandmother theory" suggests that women evolved to become infertile after a certain age to free them up to assist with rearing grandchildren. This in turn improves the survival of kin, and so is an example of positive selection.

Evolutionary biologist Professor Rama Singh, whose theory is published in the online journal Public Library of Science Computational Biology, argues that this makes no evolutionary sense. "How do you evolve infertility?" he said. "It is contrary to the whole notion of natural selection.

Natural selection selects for fertility, for reproduction – not for stopping it. 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 said argues that the menopause did not emerge to benefit the species, but simply because fertility served no purpose beyond a certain age. Natural selection, which favours the survival of the fittest, protected fertility in women while they were most likely to reproduce.

Inherited genetic mutations that cause infertility at younger ages are weeded out, because young women carrying them cannot have babies. But the same reproductive check is not there to quell the accumulation of mutations interfering with fertility in middle age. Over many generations this has led to the menopause, the theory states.

If women had a history of choosing younger "toy boy" mates, the situation would be reversed, with men losing their fertility in their 50s, Dr Singh argues.

He and two colleagues developed computer simulations showing natural selection at work to back their theory.

But British expert Dr Maxwell Burton-Chellew, an evolutionary biologist from Oxford University, strongly rejects the hypothesis. He pointed to the evolution of sterile worker bees – which are all female – as proof that natural selection can favour 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," Dr 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."


Last Updated on Friday, 14 June 2013 20:06