Menopause? Blame the Grandchildren
The age-old battle between wives and their mothers-in-law is the reason women go through the menopause, researchers have concluded.
Competition for food meant that in ancient communities there was often only enough to feed one woman’s offspring – and the daughters-in-law won.
The younger women were more single-minded and selfish than their mothers-in-law and the behaviour led to the older women losing the ability to breed eventually.
The evolution of the menopause is thought to have started about 50,000 to 300,000 years ago and took place because it was the female of the species who left home to find a breeding partner.
The daughter-in-law had no blood relatives in the family she moved in with. The only genetic investment she had was with her own children. By contrast, mothers-in-law had to choose between having more children or helping to raise their grandchildren. With food often difficult to find, the mothers-in-law tended to avoid competition with their daughters-in-law and to help with the grandchildren instead.
The evolution of such behaviour led eventually, researchers suggest, to the older women losing the ability to breed. The menopause developed as a means to avoid competition between women.
“When more than one female breeds, every mouth you feed is one less for your own,” said Michael Cant, of the University of Exeter, who carried out the study with Rufus Johnstone, of the University of Cambridge. “One of our unique characteristics is we share food among family members but having another female producing a baby means the offspring are competing for food and helpers for many years.
“If it comes down to a choice between breeding and helping with other children, the younger woman has nothing to gain by helping because she’s not related to anyone in the group. But the older female can help to rear her grandchildren. It gives the young female the advantage. She’s going to breed no matter what.”
Humans are the only primate to have the menopause and it has long been a puzzle to scientists as to why it developed when so many other social animals, such as chimpanzees, meerkats and wild dogs, continue breeding into old age. Even long-lived creatures such as elephants breed into their sixties and baleen whales have been known to give birth in their nineties.
The “grandmother theory” was proposed 50 years ago to argue that women lived well beyond the age of fertility because they were programmed to help with their children’s offspring. The theory has won widespread support but has troubled some researchers who argued that the genetic benefits were too limited when measured against the cost of losing the ability to have children.
The researchers argued in their paper, published in the journal Proceedings of the National Academy of Sciences, that there is surprisingly little overlap in the ages of motherhood and grandmotherhood.
Women across the world, irrespective of access to medicine, generally have children between the ages of 19 and 38, stopping shortly after they first become a grandparent.
Such rigidity suggests, said the researchers, that the “fertility schedule” is hard-wired into the genes.
Dr Johnstone said of the study: “It should open up new avenues for research on menopause and fertility in humans and provide new insights into the evolution of menopause in the two other species in which it occurs under natural conditions – killer whales and pilot whales.”
Source: Times Online, UK
http://www.timesonline.co.uk/tol/life_and_style/health/article3656046.ece
