Chimpanzees experience menopause: Study reveals surprising findings

Chimpanzees can also go through menopause. It has long been thought that only humans, orcas and pilot whales experience menopause, but in Uganda it appears that female chimpanzees can sometimes live for years after their fertile phase, it says. a study that Friday Science popped up.

A group of more than two hundred chimpanzees have been observed in the Kibale National Park in Uganda since 1995. It is the Ngogo community, named after the area where they live. It was known that chimpanzees can live quite old, but it was not known whether females have a post-reproductive phase of life like humans. Field workers therefore looked at hormones from females in different life stages in addition to longevity and reproduction and compared them with hormones from women before and after menopause.

The fertility of Ngogo females is no different than other chimpanzees. After age 30, the chance of becoming pregnant decreases to approximately zero as they approach age 50. But while females in other groups rarely live longer than fifty, Ngogo females live an average of fourteen years after their last birth. After their reproductive life, they still have about 20 percent bonus time. A few females even lived to be 69 years old.

Menopause is diagnosed retrospectively, one year after the last menstrual period. This has never been properly studied in chimpanzees in captivity. And in this population in the wild, it was even harder to tell when they last ovulated.

What was possible was to measure hormones in the urine that are related to fertility. Just like in humans, estrogens and progestins appeared to decrease. The level of other substances (FSH and LH), which influence ovulation, actually increased. Although the researchers had to make do with relatively small numbers (560 samples from 66 females), what they found was so similar to the hormone change in women that they dare to conclude that the end of the fertile period of these chimpanzees is caused by the menopause.

Grandmother hypothesis

The only question is: why or why? The fact that the Ngogo females can live so long without producing offspring may be a result of the favorable living conditions in the nature park. They are not hunted, there are no more leopards and there is plenty of food.

Another possible explanation the researchers give for longevity after reproduction is evolutionary. Due to human influences, such as infectious diseases, chimpanzees usually die before the end of their fertile phase. But they may have evolved to survive after menopause because this benefits the species.

This is where the grandmother hypothesis comes into play. It assumes that women continue to live after their childbearing years because they care for their grandchildren and thus increase their chances of survival. But that theory doesn’t hold true for chimpanzees, because females leave the group when they reach adulthood. And unlike humans, grandmothers don’t travel long distances to babysit grandchildren or support their sons – something bonobos do. A complicating factor is that the mothers do not know from whom their promiscuous sons have offspring.

The study does not report whether chimpanzees also have menopausal symptoms

Another explanation is that older females stop reproducing to prevent competition with young females from becoming too great and resulting in too many relatives in the group (inbreeding). This happens with orcas, where wise old women show young people the way to nutrient-rich places. After menopause, around the age of forty, orcas can pass on their knowledge for another fifty years.

The two theories are not necessarily mutually exclusive. And perhaps, the researchers write, humans and chimpanzees inherited menopause from a common ancestor, going back further than when humans and chimps diverged evolutionarily about 6 million years ago.

Jorg Massen, who studies chimpanzees as a behavioral biologist at Utrecht University, finds the American study convincing and interesting. “We knew from zoos and a few cases in the wild that chimpanzees live long beyond the reproductive phase. But does it have evolutionary significance? The combination of demographic data and hormonal changes shown in this study seems to indicate this.”

Like the authors, Massen hopes that research will continue in bonobos. If these close relatives of humans and chimpanzees undergo the same change, it supports the theory that menopause has an evolutionary advantage for them.

The study does not report whether chimpanzees also have menopausal symptoms, just like humans. Massen: “The body has to get used to hormone changes. I have no idea, but I wouldn’t be surprised if chimpanzees get hot flashes too.”

2023-10-26 10:09:46
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