This discovery promises intense debate among primatologists, anthropologists and doctors alike. On Friday, October 27, American scientists announced, in the journal Science, that they had for the first time demonstrated the presence of menopausal females in a group of wild chimpanzees.

The observation was made by Kevin Langergraber's team from the University of California, Los Angeles (UCLA), using data collected over a period of 21 years on the Ngogo community, a group of over 200 chimpanzees based in Uganda's Kibale National Park. According to the researchers' calculations, females spend an average of 20% of their adult lives in a "post-reproductive" state.

In the wild, almost no vertebrate lives beyond its reproductive period, with the exception of a few species of toothed whales (including orcas) and humans. The laws of evolution seem clear: what's the point of a species burdening itself with individuals unable to procreate? As for humans, there are two opposing theories. The cooperative theory, often summarized as the "grandmother hypothesis," holds that post-menopausal females help their daughters with maternal care, thereby boosting their grandchildren's chances of survival, and therefore the transmission of their genes. The other, known as the competition theory, considers that menopause helps to avoid a struggle for resources to raise a calf within the same family, between mothers of two different generations – mother and daughter or mother-in-law and daughter-in-law – from which the older ones could come out on the losing side. In orcas, the key role played by matriarchs argues in favor of the cooperation theory.

What about non-human primates? In zoos, several studies have found that females spend an average of 20% to 25% of their lives without reproducing. But captivity upsets all life parameters, especially longevity. So it's out of the question to draw any rules from such observations. Conversely, menopause has never been observed in any wild population.

It's now a fact, said the UCLA researchers. They base their claim on 21 years of data collected between 1995 and 2016 at Ngogo, in the center of Kibale National Park, in southwestern Uganda. A considerable amount of work according to program coordinator Kevin Langergraber. "This means 20 years in the forest, almost every day, to record births, deaths, emigrations and female immigrations between the different groups," he explained. They collected demographic data on 185 females.

They show that these animals began to give birth from the age of 13 or 14, with a peak between 15 and 19, and that a clear decline in fertility was observed from the age of 30 onwards, with a total halt "near age 50, as in humans," the article stressed.

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