The Times featured an interesting article recently on how grandmothers may hold the key to menopause…
Chimpanzees don’t get hot flushes, orangutans rarely complain about their plummeting hormones, and bonobos don’t talk sotto voce about “the change”. In fact, of all the animals in the world, only whales and humans seem to have a menopause.
A study suggests that part of the answer could be because grandmothers help to ensure fewer deaths in childbirth.
Menopause has long posed a conundrum. Evolution works through the passing on of genes and a naive understanding of its mechanisms would lead you to believe that no animal would persist while unable to pass on its genes again. If such an animal did exist, you would expect it to evolve an increasingly later menopause, with natural selection favouring those members of the species who could reproduce longer and so have more offspring.
Yet in humans, in apparent defiance of Darwin, women can easily spend 30 years or more unable to reproduce.
Alison Gemmill, of the University of California, Berkeley, has found evidence why this might be so. By looking at 19th-century records from Denmark, England and Wales, France and Sweden, she found that when there were more older women, fewer younger women died. Her explanation is simple. She suspects that grandmothers, historically, helped to lessen the risk of the main killer of young women: childbirth.
“Older women’s knowledge of pregnancy and childbirth should benefit women who have yet to experience reproduction,” she said. Unshackled by their own young children, she argued that these older women “have more time available to invest in younger women’s fertility, including direct care during pregnancy and childbirth”.
Her research, published in Evolution, Medicine and Public Health, fits with a popular theory known as the “grandmother hypothesis” which argues that the menopause exists because there is more than one way to reproduce.
Animals can reproduce by passing on their genes directly. They can also reproduce indirectly by helping their relatives, who carry similar genes, to reproduce. In particular they can look after grandchildren. This may be especially important in humans, whose children require more care for longer than almost any other species.
In the West a grandmother’s help might simply mean free childcare and extra baking, but at some points in history having an extra forager could have been the difference between life and death. Studies in hunter-gatherer populations have shown that the presence of grandmothers ensures healthier grandchildren and lower infant mortality.
Grandmothers may also mean that mothers can get back to reproducing faster, said Ms Gemmill, who is studying for a PhD. “Grandmothers, as non-maternal helpers, provide both supplemental care of children and reduce the workload of breeders, usually by provisioning food or assisting with domestic labour,” she said. “These activities bolster infant and child survival and enable women to shorten birth intervals.”
While the grandmother hypothesis might explain the persistence of the menopause, it may not explain how it developed in the first place. Some researchers have argued that it exists because men evolved to live longer, and that as a consequence women did too, but their reproductive systems never caught up. Another suggestion is that it is the mirror image of our long childhoods, with women needing to stay alive long enough to ensure that their last child reaches adulthood.
Either way, plenty of mysteries remain — and that’s without even considering why it also happens in whales.
( with thanks to Tom Whipple, Science editor at The Times, & researcher Alison Gemmill )