Scientists have rarely observed menopause in the animal kingdom—beyond humans, they’ve only recorded the phenomenon in a few species of toothed whales.
But in a new study, published Friday in the journal Science, researchers suggest that some female chimpanzees experience menopause after reaching the age of 50. These chimps, which lived past their reproductive years, had changes in hormone levels similar to those of humans undergoing menopause.
“If you had told me about this result before I read the paper, I would have been skeptical,” Susan Alberts, a primatologist at Duke University who did not contribute to the findings, tells Scientific American’s Joanna Thompson. But the results were too robust to argue with, Alberts says to the publication.
“Nobody would have assumed that [menopause] was something that would ever be observed with chimps,” Brian Wood, a co-author of the study and evolutionary anthropologist at the University of California, Los Angeles, tells the Washington Post’s Kasha Patel. “Now we have a sense of the kinds of ecological conditions and social conditions that are necessary for it to emerge.”
Menopause, the natural end of menstrual cycles, occurs at around the age of 50 for humans. Among wild vertebrates, however, most don’t live for long after they can no longer reproduce, according to the study. But the researchers chose to examine the long-lived chimps in the Ngogo community in Uganda’s Kibale National Park to look for signs of menopause.
“Ngogo is different than other chimp groups in that we get a lot of females who live past 50,” Kevin Langergraber, a co-author of the study and anthropologist at Arizona State University, tells the Washington Post. “For a long time, we noticed a lot of old females running around the forest who hadn’t reproduced for a long time.”
For 21 years—from 1995 to 2016—the researchers tracked mortality and fertility in 185 female chimpanzees in the Ngogo community. They found that fertility declined after the animals turned 30, and none of them gave birth after turning 50.
Some of the chimps, however, continued to live into older age. Sixteen survived past the age of 50, and the chimps on average lived about 20 percent of adulthood in a post-reproductive state.
The researchers also measured the levels of five hormones in urine samples from 66 female chimps between the ages of 14 and 67. In a similar way to humans experiencing menopause, levels of follicle-stimulating hormone and luteinizing hormone increased in chimps, while levels of estrogens and progestins decreased.
The findings indicate that menopause ends reproduction in chimps at around the age of 50, the study authors write.
“What’s surprising [in the new study] is so many females living so long after menopause,” Kristen Hawkes, an anthropologist at the University of Utah who wasn’t involved in the research, tells Science News’ Bruce Bower.
Menopause might be specific to the Ngogo chimps, which live longer lives due to a lack of predators and a lot of available food—and maybe that could explain why the phenomenon hadn’t been observed in other wild chimpanzees, the study authors write. The Ngogo chimps have also been studied more extensively than other groups.
Alternatively, menopause may have been more common in chimps before human impacts, like logging and introduced diseases, began affecting the animals’ mortality.
The findings are a point against the theory that the evolutionary reason why some animals live past their reproductive years is to help raise the offspring of their offspring—an idea known as the “grandmother hypothesis.”
“That’s not really possible for chimpanzees, because they don’t live with their daughters,” Peter Ellison, an evolutionary anthropologist at Harvard University who did not contribute to the findings, tells Scientific American.
Instead, older females may stop reproducing to prevent them from competing with younger females for chances to breed, per the study.
“If we think chimpanzee natural lifespan is really closer to 60 than 45 and that they have significant post-reproductive lifespan, it lends some support to the ‘kin competition’ hypothesis for the origin of menopause. But the jury is still out,” Susan Mattern, a historian at the University of Georgia who wasn’t involved in the study and has researched the history of menopause, tells Stat News’ Annalisa Merelli.
The research suggests the last common ancestor of humans and chimps may have gone through menopause. To get a better idea of how menopause evolved, scientists could study how common it is across different chimp communities, as well as whether bonobos—a species that, along with chimps, are humans’ closest living relatives—also live long after they stop reproducing, the study authors write.