If we’re lucky enough to have grandmothers in our lives, they can play an integral role in our well-being. From being a trusted friend to being a connection to a family’s heritage and ancestry, and a source of wisdom, there's a lot to learn from grandmothers.
From an evolutionary perspective, they’re not just here to pass on old family traditions and recipes for fun. Many biologists and anthropologists believe that grandmothers are key to human survival. In a recently published study in the journal PLOS One, anthropologists suggested that women were hunters in hunter-gatherer communities and grandmothers were actually the most skilled ones. Yet, a transformative and complex hormonal process in a human female’s life, the one that enables them to be free to look after their own kids’ kids, remains a mystery to scientists: menopause.
Menopause is a normal part of human aging. Between the ages of 40 and 50, most women lose their ability to reproduce as their monthly menstrual cycles end. As a result, a woman’s ovaries atrophy. The hormones that stimulate the process decline. Yet, women still live decades into their postmenopausal lives. If natural selection favors genes that reproduce, why can’t women have children throughout their entire lifespans? It’s an important question that doesn’t quite have a direct answer.
“We do not know for sure, but I think the ‘grandmother hypothesis,’ is the best theory that we have right now to explain not only menopause, but the post-reproductive lifespan that women have,” Agnès Lacreuse, a professor in the department of psychological and brain sciences at the University of Massachusetts Amherst, told Salon.
The Grandmother Hypothesis first originated in a 1957 paper by the ecologist George C. Williams, and as Lacreuse said, is often used to explain the existence of menopause. It states that human females evolved to live after menopause for the purpose of investing in and helping their grandchildren, which would better improve their own fitness instead of reproducing until old age.
In the 1980s, scientists began to make observations of a group of hunter-gatherers in Tanzania called the Hadza. What they found was a correlation between how well children grew and their mother’s foraging work. But when the mother had another kid, the kid’s growth correlated with the grandmother’s foraging work. Their observations suggested that a grandmother’s support allows a mother to focus their energy and resources on having more children, leaving more copies of her genes in future generations, with the help of a grandmother. In other words, it supported the grandmother hypothesis.
Until recently, the only non-human species of mammals that were known to experience menopause were a select few specie, such as orcas — not primates. But in late October, a new study came out suggesting that our closest relative, chimpanzees, also experience menopause. The findings were based on observations made in the Ngogo chimpanzees in Uganda. Researchers stated in their study, published in the journal Science, that they "found clear evidence for menopause in females living past the age of 50.” But unlike what has been observed in killer whales, these females were “not involved in the raising of related offspring, suggesting that a different process is driving its development."
Scientists have observed that chimpanzees leave their groups and families before reproducing. The reasons are unknown. Some speculate that it could be a way to avoid the consequences of inbreeding. But if younger female chimpanzees are leaving their homes, while the older females stay and go through menopause, how does that change what we know about why human females get menopause through the lens of the grandmother hypothesis?
Kevin Langergraber, a professor and primatologist at Arizona State University and lead author of the study, told Salon he does believe that his latest research weakens the grandmother hypothesis, and it has caused other researchers to rethink it as well. But he elaborated that it weakens a “strict version of the grandmother hypothesis.”
“That's not to say that the grandmothering might have still been quite important in lengthening the post reproductive lifespan because it is still about twice as long in humans as it is in chimps,” he said. “But maybe it's some other reason that it sort of gets off the ground, and then grandmothering helps extend it and make it more extreme in humans.”
Notably, observations have been made in other mammal species that experience menopause, like the killer whale, that support the grandmother hypothesis. In one study published in 2019, grandmother orcas appeared to increase the chances of survival for their grand-offspring. They played an important role in leading the group to food when few salmon were around.
Langergraber said there has been a history of skeptics around the grandmother hypothesis, in part because of how difficult it is to test and observe.
“Clearly, grandmothers have these benefits, they have these fitness benefits on their descendants,” he said. “But people had questioned whether those benefits were sufficiently large enough to offset the costs of stopping reproduction.”
Lacreuse said she is hesitant to suggest that just because one population of chimpanzees appears to go through menopause, doesn’t mean that all do. She noted that this finding only applies to this specific population, which appears to live longer than other populations, and perhaps there’s more to mine about menopause within this context.
“I think there is something special about this particular population,”Lacreuse said. “They have plenty to eat, it seems some kind of Eden for chimpanzees and in these conditions, yes, it can live long enough to show menopausal state.”
From a scientific point of view, she said it would be helpful to understand what it is about these specific conditions that have led to menopause. For this reason, Lacreuse said she doesn’t believe that the most recent chimpanzee study discredits the grandmother hypothesis.
“There is another hypothesis that females would cease reproduction to avoid competition with younger females, and that seems to explain some of the data in chimpanzees,” Lacreuse said. “It’s obviously a valid hypothesis, and one hypothesis doesn't negate another one, both can be in place.”
In general, menopause remains an evolutionary mystery in part because it’s a rare phenomenon. As Langergraber said: “It's always hard to understand the evolution of traits that occur in only a few species.”
