The care women give to to their grandchildren was so important that it accounts for the existence of the menopause
It’s one of life’s mysteries: why do female humans stop reproducing relatively early on in their life spans unlike most other female animals, and unlike their male counterparts? A new study suggests that the evolutionary explanation of the menopause may lie in the relationship between women and their daughters-in-law.
The key to the situation is to be found, they say, in the survival rate of the women’s offspring. The study, published in the journal Ecology Letters, found that when a mother-in-law and daughter-in-law gave birth within a short time of each other, the babies were 50 percent less likely to survive into adulthood.
The scientists studied data covering 200 years from church registers in Finland. The information included birth and death rates from 1700 to 1900, a time before the industrialisation of Europe and before modern healthcare and contraception. This data, collected by Dr Virpir Lummaa of the University of Sheffield, and her student Mirkka Lahdenpera of Turku University, Finland, supplied the scientists with vital information on the lives of several generations of women, including who they lived with and their reproductive success.
While children born to a mother-in-law and daughter-in-law around the same time were twice as likely to die before they reached the age of 15, this didn’t apply to mothers and daughters who gave birth simultaneously. This is where genetics enters the picture: “The most obvious explanation here is that mother and daughter are related to each other, so they have much more of a vested interest in each others’ success,” says Dr Andy Russell, of the University of Exeter, co-author of the report.
“On the other hand, mothers-in-law and daughters-in-law are completely unrelated to each other, and so, in a sense, they don’t care about each other at all. However, the mother-in-law cares more about her daughter-in-law, than the daughter-in-law does about her. That’s because the mother-in-law is related to the offspring produced by her daughter-in-law. However the daughter-in-law really doesn’t want her mother-in-law to breed. She gains nothing from that.” However, having a mother-in-law available to help care for, protect and provide food for your baby would clearly improve their chances of survival.
“Conflict over reproduction is very common in animals. What is surprising in this situation is why the conflict should be won by the daughter-in-law, and not the mother-in-law, ie why you end up with the menopause, rather than women not breeding until they’re about 30. The reason that menopause starts is because the mother-in-law benefits from the daughter-in-law breeding. But it doesn’t apply the other way around, ” says Dr Russell.
Biologically, there is a clear benefit to women and daughters cooperating. They share 50 percent of the same genes, so it doesn’t make much sense for them to compete for food and other resources. However, a mother-in-law and daughter-in-law aren’t genetically related. Each would want the best for their own child, so they could successfully pass their genes on.
“The two key things that we show in this paper, is the competition between mother-in-law and daughter-in-law, and that fact that the offspring have terrible survival rates of those two are breeding at the same time”, says Dr Russell. “The other important point is the beneficial effect of being a grandmother. If you aren’t breeding, you can help to raise your grandchildren.”
“The research adds weight to the argument that menopause evolved because of the vital role that grandmothers played in rearing grandchildren in traditional societies,” explained Dr Virpi Lummaa. “Although family roles have changed, many grandmothers still play a vital role in caring for their grandchildren and in western society a large number provide daycare. It is interesting that even today, mothers rarely choose to have children at the same time as their offspring: even if they have not yet been through the menopause.”
Sign up to our fortnightly health newsletter for more fascinating features delivered straight to your inbox