Humans Aren't the Only Mammals Who Go Through Menopause

But there aren’t many others. Studying the few known species of mammals that experience menopause might help solve an evolutionary puzzle.

By Erica Goode
Science Sleuth
Dec 30, 2023

WHICH ANIMAL IS MOST LIKE US? When it comes to reproductive life spans, only female humans, orcas (above) and a handful of other mammal species live many years beyond the age they can reproduce. Wild female orangutans (below) and African elephants (bottom) are believed to remain fertile throughout their lives.

SEQUIM, A KILLER WHALE BETTER KNOWN to the researchers who study her as K12, has reason to expect a long life, foraging for Chinook salmon and cavorting with others of her kind along the Pacific Northwest coast. Born around 1972, she’s the oldest member of her pod of southern resident orcas, the mother of three living offspring and grandmother of Tika, a male born in 2001.

Along with her female orca counterparts, Sequim is also helping scientists answer an evolutionary mystery about older human females: Why do they live so long beyond the age when they can reproduce?

Classic Darwinian theory would predict otherwise: Natural selection should favor mutations that increase reproduction and put a cap on survival after reproduction ceases. “It doesn’t make sense from an evolutionary perspective to live half your life and then lose the ability to reproduce but continue living for so many years,” says Deena Emera, an evolutionary biologist at the Buck Institute for Research on Aging in California and the author of a recent book, A Brief History of the Female Body.

In fact, almost all female mammals—from elephants to squirrels, badgers and meerkats—live only for relatively short periods after reproduction ceases. Only a handful of species are believed to escape this seeming inevitability. They include female orcas, three other toothed whale species—belugas, narwhals and short-finned pilot whales—and according to a recent study, chimpanzees from one population in western Uganda.

What do toothed whales have in common with humans and chimpanzees when it comes to post-reproductive life? Researchers at the United Kingdom’s University of Exeter and University of York and at the Center for Whale Research in Washington state have been searching for clues. “As soon as I heard about the fact that [female orcas] stopped reproduction early I just had to try and understand why. It is such a fascinating question,” says Darren Croft, a behavioral ecologist at Exeter and scientific advisor to the whale research center, which has been tracking resident orcas off the Pacific coast since the 1970s.

An image of a Sumatran orangutan with her baby.

Evolutionary benefits and costs

In a raft of studies, Croft and his colleagues are coming up with plausible answers to that question. Whales and humans, he says, share “a really rare set of social and ecological conditions” that resulted in an evolutionary benefit for female longevity and a cost for continued reproduction later in life.

In humans, that longevity benefit falls under what University of Utah anthropologist Kristen Hawkes in the 1990s termed the “grandmother hypothesis”: the theory that grandmothers, by helping to feed and care for grandchildren, enable mothers to bear more offspring that carry both women’s genes. The theory can explain why human females live so long after their reproductive years are behind them, Hawkes says.

Croft and his colleagues have documented a similar social system in resident orcas, where both males and females remain with their birth pods. Post-reproductive females lead matrilineal groups, guiding younger whales to sources of salmon and, as the researchers reported in a recent study, helping to protect their sons from injury.

But the grandmother hypothesis alone is not sufficient to explain why fertility does not persist into old age, Croft notes. What might explain it, he and his colleagues contend, is that over time, the social structure of human and whale groups encouraged older female members to give up reproduction, because keeping it was too costly.

In orcas, Croft says that producing progeny puts older females in competition for resources with their daughters, and his group has shown that offspring of older females overlapping with those of younger whales are less likely to survive. Among early humans living in extended family groups, the reproductive risks for older women may have created a similar dynamic. The family structures of both humans and orcas also mean that females share more and more genes with other members of their family group as they age, making it less urgent to pass on their genes directly by giving birth.

An image of three African bush elephants.

Must menopause have purpose?

“I think it’s really cool that [Croft’s group is] doing studies in another species,” says Emera. “Seeing a trait like this pop up in other species always suggests, ‘Oh, there must be some purpose to it.’”

But not everyone agrees, and some scientists have offered alternative explanations for menopause. Rama Shankar Singh, professor emeritus of biology at Canada’s McMaster University, argues that women’s loss of fertility was simply an evolutionary response to men favoring younger mates. “Longevity and menopause are unrelated,” he says. “People are unnecessarily linking the two.”

Emera believes the theories are not mutually exclusive, however. “They all could have contributed,” she says. “There’s not one single-bullet answer.”

To help settle the debate, might scientists still discover more mammals that belong to the menopause club? Very possible, Croft says. But, he adds, “I think the answer is it’s not going to be a very long list.”

A former reporter and editor for The New York Times, Erica Goode is a science writer living in New York City.