Discovery that chimpanzees go through menopause challenges ‘grandmother hypothesis’

Chimpanzees — they’re just like us.

Female chimps, it turns out, go through menopause, and go on to live long (and, one hopes, fulfilling) lives afterward. A new paper published today in Science describes the discovery of the onset of menopause in a community of wild chimpanzees around the age of 50, with an overall decline in fertility starting at age 30.

Menopause as it occurs in humans is a phenomenon that previously had only been observed in a few species of whales, including orcas and belugas. In all other species, females tend to die shortly after the end of their reproductive stage of life. Not so for the Ngogo community of wild chimpanzees in the Kibale National Park in Uganda, who live about a fifth of their total lifespan (up to 67 years) after the end of their reproductive years.

The question for chimpanzees, whales, and humans is the same: Why? This new chimpanzee research challenges one of the most commonly accepted theories for why menopause occurs, known as the “grandmother hypothesis,” which posits that older females continue to contribute to the species survival by supporting their own children’s offspring. But Ngogo chimpanzee females, unlike many menopausal humans and whales, spend pretty much no time grandmothering.

The study, led by Brian Wood, a professor of anthropology at the University of California, Los Angeles, examined mortality and fertility rates in 185 female Ngogo chimpanzees. The chimps were observed from 1995 to 2016 to identify the length of their typical lifespans after the end of fertility.

To see whether chimpanzee menopause was comparable to human menopause, the researchers measured urine samples from 66 females in various reproductive stages, assessing the values of follicle-stimulating hormone (FSH), luteinizing hormone (LH), and ovarian steroid hormones (estrogens and progestins). As with humans, menopause in Ngogo chimpanzees leads to higher FSH and LH hormones, and lower levels of estrogens and progestins. Whether this finding is due to the longer lifespan of Ngogo chimps in particular or can be expanded to other chimpanzee groups requires further study.

The researchers note that there are limitations in the study, primarily the sample size — large for a primatology study, but smaller than it would have been in humans — and the fact that the chimpanzee age is estimated, leaving scope for error.

One key difference is that older Ngogo chimpanzee females are not involved in their grandchildren’s lives; they typically live apart from their daughters, who leave their original group as they enter adulthood.

So why would menopause occur in this case? According to Wood, there are at least two possible explanations. One is that the chimpanzees may simply live longer lives because they find themselves in an especially favorable living environment — in particular, one free of severe disease epidemics — and wouldn’t survive long after menopause elsewhere.

The other is the “reproductive conflict hypothesis,” which posits that as female chimpanzees age and have offspring, their pool of potential mates shrinks because they’re related to more male members of the group and are left to compete with younger females for the remaining options.

Menopause would remove older females from this competition.

“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” on whether that’s sufficient evidence for the kin competition theory, Susan Mattern, a professor of history at the University of Georgia and the author of The Slow Moon Climbs: The Science, History, and Meaning of Menopause, said via email.

Mattern, who was not involved in the study, noted that natural and reproductive lifespans develop separately, so it makes sense that the limited number of known species that outlive their reproductive life stage experience signs of menopause.

The research authors, who did not respond to a request for comment, note that the grandmother and reproductive conflict theory are not mutually exclusive. Both might be needed to understand menopause in humans, for whom the post-reproductive lifespan is typically longer than in other species experiencing menopause.

There are other possibilities, too, to be investigated through further research. In his commentary to the research, also published in Science, Michael Cant, a professor of evolutionary biology at the University of Exeter, suggests an additional explanation about how  female chimpanzees of post-reproductive age could contribute to their group’s survival thanks to their experience. “In killer whales, post-reproductive females lead the group in search of food, particularly when prey is scarce. Could older female chimpanzees confer similar benefits on others in their group?” he wrote.

Mattern said that it’s likely that chimpanzees may indeed participate in systems that show kinship after menopause. “In humans and orcas menopause is part of a highly cooperative social system, and if chimpanzees also have it, as the article explains, we’ll likely find that kind of behavior too somewhere,” she said.