The theory that mothers become infertile so that they can help care for their grandchildren is under attack, raising questions about female identity
Sitting at my desk today is a benefit made possible by my mother-in-law. She is taking care of my son, leaving me free to do other work and ideally, in biological terms, have more babies. That, in short, is the leading explanation for why she and other women of her age have evolved to stop having babies of their own and live long post-menopausal lives. It’s known as the grandmother hypothesis.
However, this idea, and its comforting portrait of family cooperation, is being challenged. It has been half a century since scientists began to explore why human females were one of only a couple of species to became infertile so early in their lives. The American evolutionary biologist George Williams wrote in 1957 that the menopause may have emerged to protect older women from the risks linked to childbirth, keeping them alive long enough to make sure their children grew up to have grandchildren.
Since then, the scientific debate has heated up. As the study of menopause has grown, with more female researchers joining the ranks, it has become tinged with gender politics. Indeed, some scientists have even been the target of abusive mail from the public. The reason behind the menopause is no longer just a biological conundrum; it’s a question of female identity.
On one side of the divide are those who insist that older women have proven themselves so useful they have evolved to survive beyond their reproductive years. On the other are those who claim that the menopause is little more than a by-product of increased longevity or, more controversially, that infertility arose simply because men don’t fancy older women.
Answers to the problem may be coloured by gender bias, suggests Dr Rebecca Sear, an evolutionary demographer at the London School of Hygiene and Tropical Medicine. “A lot of menopause work is done by women,” she says. In contrast, “a lot of work on sexual selection by men is done by men”.
At the heart of the menopause puzzle are two biological facts: nature is efficient, and the purpose of life is to reproduce. Chimps in the wild, for instance, rarely survive beyond their 40s. Elephants live longer but carry on having babies into their 60s. A long post-menopausal life is so rare a phenomenon in nature that humans are believed to share it with only a couple of species of whale.
Humans are unusual in other ways, too. Our infants depend on us for far longer than those of other species do; and we co-operate. All this implies that the contribution of grandmothers may be vital.
In the 1980s the grandmother hypothesis got a boost thanks to the fieldwork of Professor Kristen Hawkes, an anthropologist at the University of Utah. Observing the Hadza, a partly hunter-gatherer tribe in Tanzania who she says live a life as close to our early ancestors as anyone is likely to find today, she came to a surprising revelation. “There they were right in front of us. These old ladies who were just dynamos,” she says. There was a division of labour in the tribe that not only included older women, but depended on them.
According to Sear, whose work has also shown that grandmothers improve child mortality in some societies, this should hardly have been surprising. It is only our assumptions, she says, which are built on decades of research that put men at the heart of evolution, that make it appear odd.
“It is estimated that women do 80% of the work in African subsistence agricultural societies, even into old age,” says Sear. “The man-the-hunter model of human evolution really does drive me nuts. Men and women in hunter-gatherer societies tend to bring back equal amounts of food. The idea that women are new to the workplace is outdated.”
Hawkes found that the Hadza grandmothers helped their daughters raise more and healthier children. They were vital to reproduction even if they weren’t themselves having babies. She claims mathematical models prove that this contribution – small though it may seem – could be important enough to account for why women who can no longer give birth have evolved to live for so long.
Last year, however, evolutionary biologist Dr Rama Singh, at McMaster University in Canada, came up with a completely different explanation. He and two colleagues also at McMaster, Richard Morton and Jonathan Stone, published a controversial paper that claimed that men were the reason for female infertility.
“Let’s assume mating is not random. We know that men, young and old, prefer younger women. So in the presence of younger women, older women will not be mating as much,” says Singh. If they aren’t having sex, his argument goes, they don’t need to be able to reproduce.
Hawkes and Sear are among his critics. “It’s a stupid argument and it was trashed when it came out. It’s a circular explanation. The reason men don’t prefer post-menopausal women is that they’re post-menopausal and they can’t get pregnant, not the other way round,” says Sear.
Singh, meanwhile, insists that his argument is obviously correct. “Whether you believe it or not, just look around society today. The science is cut and dry. The problem with evolutionary biologists is that they like a story. The truth is, nature doesn’t care about sympathy or feeling,” he says. His lab, which focuses on male sexual selection, is now working to find out when in history the menopause evolved.
But this dogged focus on male sexual selection is out of date, suggests Hawkes. “So much of the focus of human evolution was on what men were doing,” she says. The grandmother hypothesis has changed that.
That is not to say that grandmothers are necessarily heartwarmingly selfless babysitters. A supplement to the theory proposes that inter-generational conflict is what forces women into caring for their grandchildren rather than having babies of their own. Dr Virpi Lummaa, an evolutionary biologist at the University of Sheffield, studied church parish record data in Finland, finding that if resources are limited, infant survival was drastically reduced if daughters-in-law and mothers-in-law had babies at the same time.
“If a mother-in-law cares for her grandchildren, she still benefits because she is genetically related to them. There is no such benefit the other way round for the daughters-in-law,” says Lummaa.
Less harmonious though this family picture may be, it still affirms that the menopause does not mark the beginning of the end as far as nature is concerned, but the start of another equally productive phase in a woman’s life.
The menopause question is a particularly tough one for scientists because there are almost no species to compare us with. The notable exception is the female killer whale, which stops reproducing in her 30s or 40s but can survive into her 90s.
In this case, there seems to be less of a “grandmother effect”, than a “mother effect”, according to Dr Darren Croft, a behavioural ecologist at the University of Exeter, who is studying whale menopause. “Our research shows that female killer whales act as lifelong carers for their own offspring, particularly their adult sons,” he says.
However, with evolutionary research of this kind, which looks at behaviour and culture, theories are so difficult to prove that almost anything can be true, says Sear. “The problem with the evolution of a trait is you can never know for certain. Basically, you can make a model tell you anything.”
Among the explanations out there, Sear, like many researchers in the field, does at least find the grandmother hypothesis plausible. “I do also like hypotheses that give women some agency in evolution,” she says. “It is encouraging that, hypothetically, there might be a use for me after menopause.”
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