Marlene He was 69 years old when he died. 64 had Ma Rainey upon death. sutherland, aged 61, was still alive (October 2023) when scientists confirmed that all three had or are experiencing menopause. They are three chimpanzees from a group that lives deep in the jungle of Uganda (Africa), where dozens of older females stopped ovulating years ago. The discovery, just announced in Science, dismantles the idea that humans are the only primates that live beyond their reproductive lifespan. The finding questions hypotheses about the evolutionary function of menopause but, if confirmed to be exceptional, it could show the tremendous damage that humans have done to the rest of the great apes.
Within the Kibale National Park, in Uganda, there is one of the largest, with the least contact with humans and best preserved populations of Pan troglodytes, its scientific name. Since the mid-1990s, scientists from the Ngogo Chimpanzee Project They follow a group of dozens of individuals. They knew almost everything about them: age, sex, number of offspring and with whom, even genetic data of the entire group. Of the 185 females that have been 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 in Kibale since 2001 studying the Ngogo chimpanzees, so much so that he could name them all, “except for the many babies,” acknowledges this biologist from Arizona State University (United States). Co-author of the discovery detailed in Science, highlights: “Previous work with other communities of free-ranging chimpanzees that used demographic data as we have done (dating of births and deaths) had shown the absence of a substantial post-reproductive life expectancy.” To verify this, they recorded the time that passed since the last pregnancy or the last genital swelling (a sign of ovulation) and counted the years that each female continued without having offspring and, in her case, when she died. They called this period the survival ratio (PrR). When survival and fertility go in parallel, as is the case with females of the vast majority of species, their PrR is 0 or close to it. In women from traditional hunter-gatherer communities (they ruled out comparison with those from modern societies due to the distortion introduced by their extended life expectancy), this ratio rises to 0.44. In Ngogo’s chimpanzees it reached 0.19. That is, they spend a fifth of their adult life post-reproductive.
To confirm this, they analyzed their urine samples looking for a typically 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 increases, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), because They are going to run out of eggs to act on. In menopausal humans, the concentration of LH rises up to five times, while that of FSH rises up to 15 times. In the case of Ngogo females, the increase ratio is very similar.
The co-director of the Ngogo Chimpanzee Project, the evolutionary anthropologist Melissa Emery Thompsonhighlights the finding: “While there are many reasons why older females could have reproductive problems (for example, poor health or sterility), this study is the first to definitively demonstrate, using the same hormonal markers used for diagnose perimenopause and menopause in humans, who have ceased reproduction due to menopause.”
The evolutionary meaning
Menopause has been a headache for scientists. According to natural selection, the fact that nature favors genes that extend life beyond the reproductive phase is biological nonsense. In principle, genes that prolong the options to reproduce and thus perpetuate the species should be favored. And this is the case in almost all vertebrate species: of the more than 50,000 species of fish, amphibians, reptiles, birds and mammals, for centuries it was believed that only human species stopped ovulating long before biological senescence. So far this century, the discovery that killer whales and later other odontocete cetaceans (pilot pilot whales, narwhals, belugas and black killer whales) also had menopause robbed the human species of its exceptionality. To explain why the decoupling between the duration of reproductive life and life itself has occurred in only six species, several theories had been proposed. The one that aroused the most consensus was the grandmother hypothesis: in its simplest version it states that, in their evolution, humans who reached permanent amenorrhea could help their daughters raise their offspring, increasing the possibilities of the group of get ahead. Now the Ngogo chimpanzees have complicated everything.
Like any great discovery, it raises more questions than it answers. How is it possible that, being the species closest to humans (along with bonobos) and one of the most studied, it was not discovered before that its females also have menopause? At the beginning of this century, ambitious work with several chimpanzee communities led by Thompson confirmed previous research: found no evidence that menopause is a characteristic feature in the life history of these apes.
So what’s so special about Ngogo’s? One possibility suggested by the study’s authors is that this community lives in a kind of paradise: since the last leopard was killed in the 1960s, they have had no predators in the region. So far this century, they have increased their territory by 22%, which was already very rich in food. Furthermore, they show the highest vegetative growth rate known in this species in the wild and there have been no major cataclysms caused by humans (persecution, pathogens…). The consequence has been an increase in the life expectancy of the members of the group. And like females of other species, chimpanzees have a predetermined number of eggs that are exhausted around the age of 47; By extending their life, they enter a post-reproductive phase, like humans and some cetaceans.
Biologist Daniel Franks, a researcher at the University of York (United Kingdom), has been studying menopause for years, but not in chimpanzees, but in orcas. Franks agrees with the authors that the discovery could be an artifice caused by conditions that are as exceptional as they are possibly temporary. In captivity, without predators, or diseases and good nutrition, it had already been documented some case of menopausal chimpanzee.
The grandmother hypothesis
“There is a very suggestive alternative, already pointed out by the authors, that postmenopause survival has actually been quite common among chimpanzee groups, which implies that it could be evolutionarily beneficial. “It would be a huge thing if it were true,” says Franks. The reasoning is that scientists have not seen it in other groups of chimpanzees because, unlike the Ngogo chimpanzees, “these other groups live in deteriorated habitats due to the negative impact of humans, also suffering extremely high mortality from human diseases.” . So it would not be so much that Kibale’s chimpanzees live longer and that is why they have menopause, but rather that the others live less and do not have it. This idea would have to be confirmed by research in other groups and other great apes, in particular, in their sister species, the bonobos.
If menopause has been present in the genetics of chimpanzees for a long time, far from clarifying the function of this vital mechanism, it complicates it. Until now, confirmed in humans and in some cetaceans with teeth, the grandmother hypothesis explains very well the evolutionary functionality of a post-reproductive phase of life. Menopausal mothers would dedicate the time they do not dedicate to possible offspring of their own to caring for their grandchildren. But this doesn’t fit 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 their mothers cannot lend a hand in raising them. Furthermore, the aggressive relationship between communities and even within the same community is well known, so the help from mother to daughter becomes even more complicated.
Evolutionary biologist Michael Cant of the University of Exeter sheds some light on the mystery: “Classical theory based on Darwin’s theory of natural selection predicts that any gene that extended life beyond the end of reproduction would not be selected; “It would be invisible to natural selection because it would not confer any reproductive advantage.” However, there would be exceptions that would provide an advantage: “Post-reproductive survival could evolve if it conferred benefits to genetic relatives, that is, if older post-reproductive females (or males) could provide a sufficient boost to the survival and reproduction of their descendants.” This is what would happen to the Ngogo females. What is relevant among some of the species with the greatest brain and social complexity is to ensure their own genetic transmission, no matter whether it is direct or indirect. The consequence, among humans, is that life expectancy has increased without increasing reproductive life, something that we have not allowed chimpanzees to do, except for those from Ngogo.
“The ovary is the sentinel of the body’s aging, the first organ that ages”
Professor Ignasi Roig, head of the team at the Institute of Biotechnology and Biomedicine of the Autonomous University of Barcelona, was part of the group that identified the genetic keys to human menopause two years ago. He is not a primatologist nor, as he recalls, is his area of expertise the evolution of menopause, but he maintains that theories such as the grandmother’s theory or other forms of collaboration would explain this mechanism. From his perspective, it would be relevant in species like ours or some cetaceans in which “individuals live many years, establish social environments and the offspring have long periods of dependency where they require maternal care.” All of these conditions are now also met by Ngogo’s chimpanzees. Roig remembers that “the ovary is the sentinel of the body’s aging, it is the first organ that ages.” For millennia this has not been an obstacle, rather an evolutionary advantage. But now, with such a long life expectancy and such late motherhood, menopause is beginning to be an evolutionary problem.
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