Marlene was 69 years old when she died. Ma Rainey was 64 years old when she died. Sutherland, 61, was still alive (October 2023) when scientists confirmed that all three had or were experiencing menopause. They are three chimpanzees from a group that lives deep in the jungles of Uganda, Africa, where dozens of older females stopped ovulating years ago. The discovery, recently announced in Science, refutes the idea that humans are the only primates to live beyond their reproductive lifespan. The finding challenges hypotheses about the evolutionary function of menopause, but if confirmed to be exceptional, it could reveal the enormous damage humans have inflicted on the rest of the great apes.
Uganda’s Kibale National Park is home to one of the largest, least human-contact, and best-preserved populations of pan-troglodytes, its scientific name. Since the mid-1990s, scientists at the Ngogo Chimpanzee Project have been tracking a group of dozens of individuals. They knew almost everything about them: age, gender, number of offspring and with whom, even genetic data of the entire group. Of the 185 females that were part of the community, 1,611 years of observations are available. Researchers have proven that these chimpanzees live up to 19.5 years after they stop having children.
Kevin Langergraber has spent a long time in Kibale since 2001 studying the Ngogo chimpanzees, so much so that he was able to name them all, “except for the many babies,” admits the biologist from Arizona State University (USA). . As a co-author of the discovery detailed in Science, he points out: “Previous work with other chimpanzee communities in the wild, using demographic data like us (dates of births and deaths), had shown that there is no significant post-reproductive life “Expectation.” To check this, they recorded the time that had passed since the last pregnancy or last genital swelling (a sign of ovulation), and counted the years each female did not have offspring and, if necessary, the time of her death. They called this period the survival rate (PrR). When survival and fecundity run parallel, as is the case for females of the vast majority of species, their PrR is at or close to 0. For women from traditional hunter-gatherer communities (they excluded a comparison with women from modern societies due to the bias of their longer life expectancy) this ratio increases to 0.44. In Ngogo chimpanzees it reached 0.19. This means they spend a fifth of their adult life post-reproducing.
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To confirm this, they analyzed their urine samples for a typical menopausal endocrine pattern. As the end of egg reserves approaches, a parallel process occurs in humans and chimpanzees: While the production of estrogen and progesterone decreases, that of two other hormones, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), increases. There will be no eggs left to react to. In menopausal people, the concentration of LH increases five-fold, while that of FSH increases up to 15-fold. The rate of increase is very similar for the Ngogo females.
Co-director of the Ngogo Chimpanzee Project, evolutionary anthropologist Melissa Emery Thompson, highlights the finding: “Although there are many reasons why older females might have reproductive problems (e.g. poor health or infertility), this study is the first , which clearly proves this, using the same hormonal markers used to diagnose perimenopause and menopause in humans, that they have stopped reproducing due to menopause.”
The evolutionary significance
Menopause gave scientists headaches. According to natural selection, the fact that nature favors genes that extend life beyond the reproductive phase is biological nonsense. In principle, preference should be given to genes that extend reproductive opportunities and thus preserve the species. And this is the case with almost all vertebrate species: of the more than 50,000 species of fish, amphibians, reptiles, birds and mammals, it was believed for centuries that only humans stopped ovulating long before biological senescence. So far, the discovery that killer whales, and later other toothed whales (pilot whales, narwhals, belugas and black killer whales) were also menopausal, has robbed the human species of its exceptionality this century. To explain why the decoupling between the length of reproductive life and life itself has occurred in only six species, several theories have been proposed. The grandmother hypothesis received the most consensus: in its simplest version, it states that people who have achieved permanent amenorrhea could, in their evolution, help their daughters raise their offspring, increasing the group’s chances of advancing. Now the Ngogo chimpanzees have made everything more complicated.
Like any great discovery, it raises more questions than it answers. How is it possible that bonobos are the closest species to humans (besides bonobos) and one of the most studied, and that it was not previously discovered that their females also go through menopause? ? Earlier this century, ambitious work with several chimpanzee communities led by Thompson confirmed previous research: They found no evidence that menopause is a distinctive feature in the life history of these monkeys.
The Ngogo community of Kibale has both the highest vegetative growth rate (births minus deaths) and high life expectancy. Tracy Kivell/Ngogo Chimpanzee Project
What’s special about Ngogo’s? One possibility the study authors suggest is that this community lives in a kind of paradise: There have been no predators in the region since the last leopard was killed in the 1960s. So far this century they have increased their territory, which was already very rich in food, by 22%. In addition, they have the highest vegetative growth rate known for this species in the wild and there have been no major human-caused disasters (persecution, pathogens…). The result was an increase in the life expectancy of the group members. And like females of other species, chimpanzees have a predetermined number of eggs that are exhausted around age 47; By extending their lives, they enter a post-reproductive phase, like humans and some whales.
Biologist Daniel Franks, a researcher at the University of York (Great Britain), has been studying menopause for years, although not in chimpanzees, but in orcas. Franks agrees with the authors that the discovery may have been an artifice caused by conditions that were both extraordinary and possibly temporary. In captivity, without predators, disease and good nutrition, several cases of menopausal chimpanzees have been documented.
The Grandmother Hypothesis
“There is a very suggestive alternative, which the authors have already pointed out, which is that survival after menopause is actually quite common in chimpanzee groups, implying that it could be evolutionarily advantageous.” “It would be a big deal if it would be true,” says Franks. The reason for this is that scientists have not observed it in other groups of chimpanzees because, unlike the Ngogo chimpanzees, these other groups “live in habitats degraded by the negative impacts of humans and also suffer from extremely high mortality rates from human diseases.” So it’s not so much that the Kibale chimpanzees live longer and therefore have menopause, but rather that the others live less and don’t have it. This assumption would have to be confirmed by studies on other groups and other great apes, especially on their sister species, the bonobos.
If menopause has long been present in chimpanzee genetics, the function of this vital mechanism is far from clear and more complicated. So far confirmed in humans and some cetaceans with teeth, the grandmother hypothesis explains very well the evolutionary functionality of a post-reproductive phase of life. Menopausal mothers would devote the time not devoted to potential offspring to caring for their grandchildren. But that doesn’t suit chimpanzees. In this species (also in Ngogo), females leave the community in which they were born when they reach the reproductive phase and have their children in another group, so that their mothers cannot help with raising them. Furthermore, the aggressive relationship between communities and even within the same community is well known, making mother-to-daughter help even more complicated.
Evolutionary biologist Michael Cant from the University of Exeter sheds some light on the matter: “The classical theory, based on Darwin’s theory of natural selection, predicts that no gene will be selected to extend life beyond the end of reproduction; “It would be invisible to natural selection because it would provide no reproductive advantage.” However, there are exceptions that would offer an advantage: “Post-reproductive survival could evolve if it confers advantages on genetic relatives, that is, if older post-reproductive ones Females (or males) could sufficiently increase the survival and reproduction of their offspring.” This is what would happen to the Ngogo females. In some of the species with the greatest brain and social complexity, it is important to ensure their own genetic transmission, whether this occurs directly or indirectly. The consequence of this is that life expectancy in humans increases without increasing reproductive length, which we have not allowed chimpanzees to do except those from Ngogo.
“The ovary is the guardian of the aging of the body, the first organ to age”
Professor Ignasi Roig, head of the team at the Institute of Biotechnology and Biomedicine at the Autonomous University of Barcelona, was part of the group that identified the genetic keys to menopause in humans two years ago. He is neither a primatologist nor, as he recalls, his area of expertise is the development of menopause, but he claims that theories such as grandmother theory or other forms of cooperation would explain this mechanism. From their perspective, this would be relevant in species like ours or some whales, where “individuals live for many years, build social environments, and the offspring have long periods of dependence during which they require maternal care.” Ngogo’s chimpanzees now also meet all of these requirements . Roig remembers: “The ovary is the guardian of the aging of the body, it is the first organ to age.” For thousands of years, this has not been an obstacle, but an evolutionary advantage. But now, with such a long life expectancy and such late motherhood, menopause is starting to become an evolutionary problem.
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