We Aren't the Only Primates to Experience Menopause
A other mammals do, but we thought Humans were the only primates
Hiya!
Menopause is a well-known Human experience anyone with a uterus will go through. As you know, menopause signals the end of fertility. Once someone goes through it, they cease to have menstrual cycles and become infertile. However, you might not know that menopause makes us an outlier among mammals — besides us, five whale species also experience it, but that’s all as far as we know.
Generally, most mammals die relatively soon after they cease to have offspring. Yet, human females will go through menopause and live for years, sometimes decades after. And since menopause occurs around the same age for everyone who can experience it, regardless of location or culture, it suggests life beyond reproduction evolved within our species. But now researchers have identified another mammal that also experiences menopause — Chimpanzees.
The Research
Researchers belonging to the Ngogo Chimpanzee Project have observed a grouping of wild chimpanzees called the Ngogo community in Kibale National Park, Uganda, since 1993. If the name sounds familiar, it’s likely because this group of chimpanzees was featured recently in the Netflix docu-series Chimp Empire.
In addition to observing the chimps, the researchers collected urine samples using a non-invasive technique. Over the years, and while analyzing the demographics of the Ngogo community, the team identified 185 females, 16 of whom lived past the age of 50. Fifty years old is the age at which menopause typically begins for humans too.
The urine samples the researchers collected showed hormonal changes in the female chimps around the 50-year mark. More specifically, the older female chimps experienced the same endocrinological changes as a human female of around the same age — follicle-stimulating and luteinizing hormone levels increase while progestins and estrogen levels decrease.
In other words, the results confirmed the chimps were in menopause. The team recently published their research in the journal Science. They found that in ideal circumstances, female chimps can spend about a fifth of their lives in this post-reproductive stage, which is impressive. But humans can spend at least 40 percent to almost half of their lives post-menopause.
First of all, I had no idea menopause is so seemingly rare among mammals, and while I’m thrilled to know about this research, it’s a bit bewildering that experts are just now learning this. I mean, chimpanzees have long been touted as our closest primate relative, so you’d think the fact that they, too, experience menopause would have been discovered long ago… right?
How did we not know this?
Well, it turns out that while studying menopause in other animals seems simple enough, the truth is that it’s anything but easy. After all, it’s already challenging to understand the inner workings of other humans. But trying to study the internal experience of large and strong wild animals with long lifespans without harming them is significantly more so.
Chimpanzee longevity, especially in captivity, is a big obstacle for researchers, too, because there just isn’t any previous research studying hormonal fluctuations in the same population of chimps for that long, let alone in primate groups across Central and West Africa.
This is a big reason the two decades’ worth of research and data collected by the Ngogo Chimpanzee Project is so valuable. There have been other studies, but this is the longest-running study of its kind.
Cautions
All that said, the Ngogo population could be unique within the species since they reside in basically a chimpanzee oasis. The Kibale National Park offers plenty of protection from predators and lots of resources and food. Not to mention, they prefer to live in the middle of the park, far away from humans and the pathogens we can spread to them, which are known to devastate chimp communities.
Many other primates live a good life in the wild, and their populations will thrive. And yet, a primatologist at Arizona State University and senior author of the study, Kevin Langergraber, said that despite this, “they don’t show substantial post-reproductive life spans.”
Of course, it could also be that other animals are capable of experiencing menopause and live long afterward, but few regularly live long enough in the wild to make it to that life stage.
What about the Grandmother hypothesis?
Regarding humans, the Grandmother hypothesis is the generally accepted explanation behind why humans continue to live well beyond their reproductive years.
If you’re unfamiliar, the thought is that human females evolved to live long past menopause to become grandmothers. The idea is that grandmothers help raise their grandchildren so the parents can focus on having more kids — or whatever community duties they may have had. And it’s not just us. The grandmother effect has also been observed in orca whales and Asian elephant communities.
But chimpanzee communities are vastly different from our own. For starters, chimps are promiscuous in their mating habits, and long-term bonds between pairs aren’t common. Mothers are the sole carers of their offspring, and when the female youth reach maturity, they leave in search of new communities — while the males remain in the society they’re born into. This means grandmother chimpanzees likely have no relationship with their grandchildren.
However, an evolutionary anthropologist at the University of California, Los Angeles, and the study’s leader, Brian Wood, points out:
“It doesn’t mean that all these older females aren’t doing things that are consequential.”
While she wasn’t part of the Ngogo research, work by Catherine Hobaiter, a primatologist at the University of St. Andrews in Scotland, who studies chimpanzees in Uganda’s Budongo Central Forest Reserve, supports Wood’s assessment.
Hobaiter has observed older female chimpanzees at Budongo withdraw from regular daily competitions that are standard parts of chimpanzee lifestyles. The older females also appear to lead, command respect, and make decisions for the group. Hobaiter tells National Geographic about a particular elder female chimp named Nambi, who has resided in Bundongo for around 60 years.
“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.”
Researchers are eager to learn more about the post-reproductive stages of female lives in primates, but there’s no way to speed up time, and we’ll just have to wait a while to learn more.
Still, the work by the Ngogo Chimpanzee Project is a great start and has already inspired new questions to research in the future. For instance, now that we know chimps experience menopause and live for years afterward, it likely means such a feat didn’t evolve uniquely in our human ancestors.
Instead, the researchers state, the ability could have “built on existing genetic variation in the last common ancestor we shared with chimpanzees.”
Perspective Shift
This is a prime example of why I love doing what I do. I think people, myself included, often assume we know more than we do.
I assumed we’d studied every molecule of chimpanzees, considering their close relation to ourselves. Yet, the fact that both of our species experience and live beyond menopause is just being discovered. So wild. Then again, considering the severe lack of research into female biology might have something to do with it.
Still, it’s exciting to know we’re not the only primate to live beyond reproductive years. I feel like there are probably many, many other mammals with the ability, but like wild primates, perhaps they don’t live long enough to reach that stage of life.
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