Humans are no longer the only primates that go through menopause

Nana, Abuela, Grandma, Grams—whatever you call them, humans are one of the very few species in which females live long past their reproductive years to become grandmothers.

In fact, the grandmother club is so small, you can count the other species known to experience and outlive menopause on one hand. They include orcas, short-finned pilot whales, narwhals, beluga whales, and false killer whales. (Related: “Menopause is very rare among animals. Here’s why orcas go through it.”)

But a new, landmark study confirms that at least one population of chimpanzees can now be added to the elite list.

The discovery comes as the result of 21 years’ worth of observing the Ngogo community of wild chimpanzees in Kibale National Park, Uganda. Well, that, and collecting a lot of chimp urine from the treetops.

“What we do is we break off a little sapling that has a nice ‘Y’ at the end of it. Then you put a very thin plastic bag over that,” says Kevin Langergraber, a primatologist at Arizona State University and senior author of the study published today in the journal Science.

“You just hope you don’t get too much splatter,” he laughs. 

While the circumstances may sound silly, studying urine from 66 Ngogo females, aged 14 to 67, showed that their hormone levels changed after approaching 50, confirming they were in menopause. Interestingly, 50 is also the age when many people begin to experience menopause.

Langengraber and other primate researchers had long wondered why humans experience menopause while none of our closest evolutionary cousins appear to.

“It’s really cool to finally have that piece of the puzzle come into place,” says Catherine Hobaiter, a primatologist at the University of St. Andrews in Scotland who also studies chimpanzees, in Uganda’s Budongo Central Forest Reserve, but who was not part of the new study.

The right conditions for menopause

So why did menopause take so long to be discovered in chimps? The short answer is it’s extremely difficult to study the inner workings of large, wild animals without harming them. (Related: “Jane Goodall’s original tale of chimpanzees still astonishes today.”)

Studying chimpanzees presents several other challenges, namely that they are extremely long-lived—especially in captivity. One female, known as Little Mama, was thought to have been in her late 70s when she died at a Florida safari park in 2017. This means that scientists simply don’t have two decades’ worth of data for many chimpanzee groups across central and west Africa.

But the length of the Ngogo Chimpanzee Project, which began in 1993, and the non-invasive technique of collecting urine gave the scientists confidence in their finding.

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Specifically, the team found that elderly females undergo the same endocrinological changes as a woman in middle age; her estrogens and progestins levels fall, while levels of follicle-stimulating and luteinizing hormones rise.

However, Langergraber cautions the Ngogo population may be an outlier when it comes to the rest of the species.

That’s because the Ngogo community lives in a chimp Eden of sorts: the resource-rich, well-protected Kibale National Park that also lacks leopards, their main predator. And because the Ngogo community is found in the heart of the park, its only neighbors are other chimps—not humans who can expose chimpanzees to pathogens that have devastated other communities.

The flip side could be true: That all chimpanzee populations once lived in the relative prosperity that the Ngogo chimps now enjoy, but people have put so much pressure on the animals they no longer regularly live long enough to undergo menopause.

Of course, the answer could also lie somewhere in the middle, says Langergraber.

Do chimpanzee grandmothers matter?

Another intriguing question is whether chimpanzee grandmothers have any extra evolutionary value.

After all, researchers have shown in humans that the presence of a living grandmother can pass down benefits to grandchildren through things like providing extra food and childcare (something that Ninny and Grandma Pickles do in my own family). Scientists have also observed evidence for this grandmother effect in Asian elephants and orcas. (Related: “Why do orca grandmothers live so long? It's for their grandkids.”) 

The answer is unclear, particularly because chimpanzee societies are so different than those of humans, says study leader Brian Wood, an evolutionary anthropologist at the University of California, Los Angeles.

For instance, both male and female chimpanzees mate promiscuously, rather than forming long-term pair-bonds. Mothers care solely for their offspring. And when they reach maturity, females leave in search of new communities, while males stay in the area they were born. All of which means grandmother chimpanzees likely don’t know who their grandchildren are in the same way humans, or even orcas, do.

“It doesn’t mean that all these older females aren’t doing things that are consequential,” says Wood. “But that’s all future work to be done.” 

In her study population in Budongo, Hobaiter has witnessed older females withdraw from the day-to-day competitions that are part of chimpanzee life. (Related: “Chimpanzee moms are like us: They mourn, dote, and take 'me' time.”)

But they still seem to command prestige and respect. One elder, named Nambi, has lived in Budongo for probably 60 years or more, and Hobaiter has witnessed times where she appears to lead and make decisions for the group.

“What she has seen in that forest, the different seasons she’s known, the different areas of the forest, the interactions with the neighbors, that’s this incredible legacy of her knowledge.”