Marilyn He was 69 years old when he died. 64 was What a rainy day After death. Sutherland, aged 61, was still alive (October 2023) when scientists confirmed that all three had gone through or were going through menopause. They are three chimpanzees from a group living deep in the jungle of Uganda (Africa), where dozens of older females stopped ovulating years ago. This discovery was just announced in SciencesDeconstructs the idea that humans are the only primates that live past their reproductive lifespan. These findings cast doubt on hypotheses regarding the evolutionary function of menopause, but if confirmed to be exceptional, they could demonstrate the enormous damage humans have done to other great apes.
Within Kibale National Park, in Uganda, is one of the largest parks, with the least contact with humans and the best preserved populations ever. Pan cavesIts scientific name. Since the mid-nineties, scientists have begun to… Njojo Chimpanzee Project They follow a group of dozens of individuals. They knew almost everything about them: age, gender, number of offspring, who was with them, and even the genetic data of the entire group. Of the 185 females who were part of the community, the equivalent of 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 been spending long periods of time in Kibale since 2001 studying Njogo chimpanzees, so much so that he has been able to name them all, “except several babies,” admits this biologist from Arizona State University (USA). He co-authored the discovery detailed in Sciences, highlights: “Previous work with other populations of free-roaming chimpanzees that used demographic data as we did (dating births and deaths) showed no significant post-reproductive life expectancy.” To check this, they recorded the time that had passed since the last pregnancy or the last swelling of the reproductive organs (a sign of ovulation) and counted the years each female had been without offspring and, if possible, when she had died. They called this period survival rate (PrR). When survival and fecundity go in parallel, as they do for females of the vast majority of species, their PR is 0 or close to it. In women from traditional hunter-gatherer societies (they excluded comparison with women in modern societies because of the distortion caused by their long life expectancy), this ratio rises to 0.44. In Njogo’s chimpanzee, it reached 0.19. That is, they spend a fifth of their adult life after giving birth.
To confirm this, they analyzed urine samples for the endocrine pattern of menopause. As the end of the egg reserve approaches, a parallel process occurs in humans and chimpanzees: while the production of estrogen and progesterone decreases, the production of two other hormones, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), increases because they will run out of eggs to work with. In postmenopausal women, LH concentration rises five times, while FSH concentration rises 15 times. In the case of Njojo females, the rate of increase is very similar.
Co-director of the Njojo Chimpanzee Project, an evolutionary anthropologist Melissa Emery Thompsonhighlights the finding: “Although there are many reasons why older females may have reproductive problems (for example, poor health or infertility), this study is the first to conclusively prove, using the same hormonal markers used “To diagnose perimenopause and menopause in humans who have stopped having children due to menopause.”
Evolutionary meaning
Menopause has been a headache for scientists. According to natural selection, the fact that nature favors genes that extend life beyond reproduction is biological nonsense. In principle, genes that prolong reproductive options, thus perpetuating the species, should be favoured. This is the case in almost all vertebrate species: of more than 50,000 species of fish, amphibians, reptiles, birds and mammals, it was thought for centuries that only human species stop ovulating long before biological aging. So far in this century, the discovery that killer whales and other odontocetes (pilot whales, narwhals, belugas, and black killer whales) also undergo menopause has robbed the human species of its exceptionalism. To explain why the disconnect between reproductive life span and life itself occurs in only six species, several theories have been proposed. It was the grandmother hypothesis that generated the most consensus: in its simplest form it states that humans who have reached permanent menopause could, in their evolution, help their daughters raise their offspring, increasing the group’s prospects for moving forward. Now the chimpanzee Ngogo has complicated everything.
Like any great discovery, it raises more questions than it answers. How is it possible that, as the species closest to humans (along with bonobos) and one of the most studied, its females have not previously been discovered to also suffer from menopause? At the beginning of this century, ambitious work with several chimpanzee communities led by Thompson confirmed previous research: No evidence has been found that amenorrhea is a defining feature In the life history of these apes.
So what makes Ngogo special? One possibility suggested by the study’s authors is that this community lives in a kind of paradise: Since the last tiger was killed in the 1960s, they haven’t had any predators in the area. So far in this century, they have increased their land by 22%, which was already very rich in food. Moreover, it shows 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 group members. Like females of other species, chimpanzees have a predetermined number of eggs that are exhausted at approximately 47 years of age; By extending their lifespan, they enter the post-reproductive stage, like humans and some cetaceans.
Biologist Daniel Franks, a researcher at the University of York (UK), has been studying menopause for years, but not in chimpanzees, but in killer whales. Franks agrees with the authors that this discovery may be a ruse resulting from circumstances that are as exceptional as they may be temporary. In captivity, without predators, diseases or good nutrition, it has already been documented Some cases of menopause in chimpanzees.
Novelty hypothesis
“A very suggestive alternative, already pointed out by the authors, is that postmenopausal survival was actually very common among chimpanzee populations, which means it could be evolutionarily advantageous,” Franks says. “It would be huge if it were true.” The reason is that scientists have not seen this in other groups of chimpanzees, because, unlike Njogo’s chimpanzees, “these other groups live in degraded environments due to the negative impact of humans, and also suffer from a very high mortality rate from human diseases.” So It will not be the case that the chimpanzees living at Kibale live longer and therefore suffer from menopause, but rather that others live shorter and do not. This idea should be confirmed by research in other groups and other great apes, especially in their sister species , bonobo.
If menopause has been present in the genetics of chimpanzees for a long time, far from clarifying the function of this biological mechanism, it complicates it. To date, the novelty hypothesis has been confirmed in humans and in some toothed cetaceans, and it explains well the evolutionary function of the post-reproductive stage of life. Menopausal mothers will devote the time they do not devote to their potential offspring to caring for their grandchildren. But this does not suit chimpanzees. In this species (also in Ngogo), females leave the community in which they were born when they reach reproductive stage and give birth to their children in another group, so their mothers cannot help in raising them. Moreover, the aggressive relationship between communities and even within the community itself is well known, so a mother’s help to her daughter becomes more complicated.
Evolutionary biologist Michael Kant of the University of Exeter sheds some light on the puzzle: “Classical theory based on Darwin’s theory of natural selection predicts that any gene that extends life beyond the end of reproduction will not be selected; it will remain selected.” “It would be invisible to natural selection because it would confer no reproductive advantage.” However, there may be exceptions that would provide an advantage: “Post-reproductive survival can evolve if it confers benefits to genetic relatives, that is, if older post-reproductive females (or males) can provide a sufficient survival boost.” And reproduction.” Of their offspring.” This is what will happen to Ngogo females. What matters for some species with the greatest brain and social complexity is ensuring their genetic transmission, regardless of whether directly or indirectly. The result, among humans, is that the average Life expectancy has increased without increasing reproductive lifespan, something we have not allowed chimpanzees to do, except for those at Njogo.
“The ovary is the guardian of the body’s aging, and it is 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 switches to human menopause two years ago. He is not a primatologist nor, as he recalls, his field in the field of menopausal evolution, but he maintains that theories such as the novelty theory or other forms of cooperation would explain this mechanism. In their view, this may be relevant for species like ours or some cetaceans in which “individuals live for many years, establish social environments, and in which offspring have long periods of dependency in which they require maternal care.” All these conditions are now also met for Njogo’s chimpanzee. “The ovary is the body’s aging guardian, the first organ to age,” Roig recalls. For thousands of years this has not been a handicap, but rather an evolutionary advantage. But now, with longer life expectancy and delayed motherhood, menopause is beginning to be an evolutionary problem.
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