Nearly 200 chimpanzees are part of the Ngogo community in western Uganda, but Garbo stands out. With deep wrinkles around her eyes and a hefty stature, the mother chimpanzee has the energy of a strong-willed heroine like those played by her namesake, Hollywood actress Greta Garbo.
She’s also around 67 years old, living many years past her biologic ability to have children — a startling observation only previously seen naturally in people and a handful of toothed whale species.
In a landmark discovery, researchers found that female chimpanzees at the Ngogo conservation site spend around 20 percent of their adult years in a state of menopause, surviving well past the end of their ability to reproduce. The findings complicate the mystery of why select species, including humans, experience menopause from practical and evolutionary perspectives.
“Nobody would have assumed that [menopause] was something that would ever be observed with chimps,” said Brian Wood, an author of the study. “Now we have a sense of the kinds of ecological conditions and social conditions that are necessary for it to emerge.”
Chimpanzees in captivity have previously shown signs of menopause, but researchers have struggled to document a similar behavior in wild populations. Part of the reason, the authors said, may be that most female chimpanzees don’t live long enough to reach a biological stage where they no longer reproduce.
Most chimp populations don’t reach the age of 50 “due to the recent negative impacts of humans,” said Kevin Langergraber, a study author and an anthropologist at Arizona State University. For instance, he said, people can spread diseases — even as simple as the common cold — that can wipe out chimp populations.
Enter Ngogo, a chimp haven.
Relatively undisturbed by people, Ngogo is one of two research sites that the Ugandan government maintains in Kibale National Park. Fig trees provide an abundant and stable supply of fruit for the chimps. Natural predators like leopards are also scarce. Here, these chimpanzees can live long lives. Adventures of the Ngogo primates, the largest group of chimpanzees in the world, were documented in the Netflix series “Chimp Empire” released this year.
Garbo, featured in the Netflix series, is the oldest living chimpanzee in the community now, but two females have reached the age of 69 in the past. Ma Rainey, who is around the age of 63, lives in the community with her son, Wes. Another female named Sutherland, 61, also is around.
These older females tend to become less social and spend more time alone, Langergraber said, probably because they have physically deteriorated by this point. They lose muscle mass, get skinnier, and become grayer, balder, more wrinkly and spotted. Their teeth also get worn down.
“Ngogo is different than other chimp groups in that we get a lot of females who live past 50,” said Langergraber, co-director of the Ngogo Chimpanzee Project. “For a long time, we noticed a lot of old females running around the forest who hadn’t reproduced for a long time.”
Using two decades worth of observations, the researchers collected demographic and hormonal data on female members in the community. They examined mortality and fertility rates of 185 chimpanzees and found, on average, that the females spent about one-fifth of their lives in a post-reproductive state.
They also examined urine samples, collecting them from leaves or catching them on plastic sheets, from 66 females at different reproductive stages. They found that older female chimps experienced changes in hormones such as gonadotropins, estrogens and progestins, similar to female humans entering menopause. The timing of these hormone changes was also similar to humans, gradually declining after 30 until shutting off around 50.
“The patterns of fertility in Ngogo and in humans actually have this similar shape,” Langergraber said. “It’s the same physiological mechanism that has underlying reproductive cessation in chimps as it is in humans.”
But explaining why menopause occurs in chimps — and humans — is a much harder question. Menopause has long a been a puzzle for scientists: If the genetic priority is for animals is to reproduce, then why may some live well beyond that ability? Answers seem to differ between chimps and humans.
In humans, one leading theory is that older females stop having children so they can help raise their children’s offspring, known as the “grandmother hypothesis.” But that’s not the case for chimps because daughters move away from their mothers and essentially never see one another again. The mother could help her son raise offspring, but previous studies showed that isn’t the case.
Even so, the uncanny similarity could shed light on how menopause evolved in humans. Given that chimpanzees are our closest relatives, these results suggest that postmenopausal survival could be a trait passed down from a common ancestor millions of years ago. But perhaps the trait has only since appeared under certain circumstances, such as those that allow for long life spans.
“The capacity for a prolonged life after reproduction could have existed widely in our ape ancestors,” said behavioral ecologist Phyllis Lee, who was not involved in the research. “This potential for an extension might be at the basis of the universal trait of menopause that we see really only now in humans.”
Lee, a researcher at University of Stirling, applauded the study and said it was “extremely well done” and has required huge investment in observation and hormone collection.
Questions still remain on how widespread menopause is across chimp populations. For instance, the team wants to know whether the survival rates at Ngogo are an exception, which have led menopause to occur. Or are the comparatively low survival rates at other wild chimp sites an aberration compared to past chimp evolutionary history?
“We had a very human specific story about [menopause], and for good reason, because that’s the species we know best,” Wood said. “I think this study is going to lead to a broadening and a wider appreciation for multiple different factors that could have influenced its evolution in our species.”