Sensational discoveries in chimpanzees could solve mysteries of menopause

Chimpanzees are our closest living relatives in the animal kingdom. Now researchers have discovered that female chimpanzees experience menopause, which was previously only seen in humans and some whale species. The reasons for this are still unclear. But the finding could help finally solve the mystery of menopause.

Garbo, a mother chimpanzee from the Ngogo community, is very special. She is approximately 67 years old and has been living outside her biological reproductive capacity for years. This is an astonishing observation because, aside from humans, it has only been observed in a handful of species of toothed whales in the wild. And Garbo is not the only one in Ngogo with her old age. Other chimpanzee mothers are similar in age, some even older.

These older women tend to be less social and spend more time alone – probably because they are physically weaker. You lose muscle mass and become thinner. Their coat becomes grayer, thinner, wrinkled and patchy. Teeth also wear out.

Now researchers from Arizona State University (USA) have discovered that female chimpanzees in the Ngogo Reserve spend about 20 percent of their adult lives in menopause, living well past the end of their reproductive potential.

“Nobody thought menopause would occur in chimpanzees,” said Brian Wood, one of the study’s authors. “Now we know what ecological and social conditions are required for this.”

Captive chimpanzees have shown signs of menopause before: they can reach an age beyond their reproductive capacity. However, so far it has not been possible to document similar behavior in wild populations. The authors mention one possible reason: most female chimpanzees do not live long enough to reach a biological stage where they no longer reproduce.

“Most chimpanzee populations don’t even live to be 50 years old due to human influences,” said Kevin Langergraber, one of the study’s authors and an anthropologist at Arizona State University. In this way, humans would spread diseases that could wipe out chimpanzee populations. Even a simple cold can be enough.

This is where Ngogo comes into play, a sanctuary for chimpanzees. Ngogo is one of two research stations maintained by the Ugandan government in Kibale National Park and is relatively undisturbed by people. The fig trees provide the chimpanzees with a rich and stable food supply. There are also hardly any natural enemies such as leopards. The chimpanzees can live a long life here.

“Ngogo differs from other chimpanzee groups in that we have many females that live longer than 50 years,” said Langergraber, co-leader of the Ngogo chimpanzee project. “We have seen for a long time that there are many old females wandering through the forest who have not reproduced for a long time.”

Based on observations over twenty years, researchers collected demographic and hormonal data on the female members of the community. They examined the mortality and fertility rates of 185 chimpanzees and found that the females spend, on average, about a fifth of their lives in a post-reproductive state.

They also examined urine samples from 66 females at different reproductive stages that they collected on leaves or plastic sheets. They found that older female chimpanzees showed changes in hormones such as gonadotropins, estrogens and progestins, similar to menopausal humans. The timing of these hormonal changes was also similar to those in humans: after 30 years they gradually decreased until they stopped at age 50.

“The fertility patterns in Ngogo chimpanzees and humans are similar,” says Langergraber. “It is the same physiological mechanism underlying reproductive arrest in chimpanzees as in humans.”

But the question of why chimpanzees – and humans – go through menopause is much harder to answer. Menopause has long puzzled scientists: If animals’ genetic priority is reproduction, why do some live far beyond that capacity? The answers seem to be different between chimpanzees and humans.

In humans, one of the leading theories is that older women no longer want to have children so they can help raise their children’s offspring. However, this is not the case with chimpanzees, as the daughters leave their mothers and almost never see each other again. Mothers could help raise their sons. However, previous research has shown that this is not the case.

Still, the similarity could shed light on how menopause evolved in humans. Because chimpanzees are our closest relatives, these results suggest that survival after menopause may be a trait inherited from a common ancestor millions of years ago. But it is also possible that menopause only occurs under certain conditions that allow longevity.