Females in a community of wild chimpanzees in Uganda spend a fifth of their adult life in the post-reproductive stage, that is, living beyond their fertility. The specimens studied also present hormonal changes similar to those of human menopause. The finding, presented this Thursday in the journal Science, sheds new light on a phenomenon that could be common among captive mammals, but that has barely been observed outside of laboratories and zoos.
An international group of scientists has followed the Ngogo chimpanzee community, which inhabits Kibale National Park, in Uganda, for 22 years. Analysis of all observations and urine samples collected during this time has revealed that the females in the group have a fertility profile similar to that of other communities: they enter reproductive age at 14 years old, their fertility begins to decline at 30, and ends up disappearing shortly before 50, with hormonal changes similar to humans.
So far no surprises. However, the number of individuals who live beyond the age of 50 is much greater than that of the rest of the groups studied so far. That is to say, despite having a normal fertile life, Ugandan chimpanzees regularly reach menopause because their life expectancy is very long.
“The results of the study show that under certain ecological conditions, menopause and post-fertile survival can emerge in a social system quite different from ours,” said Brian Wood, an anthropologist at the University of California – Los Angeles, in a press release ( UCLA) and leader of the study. In the case of the Ngogo community, those exceptional conditions are the lack of predators in the area and abundant food.
A more pessimistic hypothesis is that survival beyond the fertile lifespan is actually a typical evolutionary characteristic of chimpanzees, and that the fact that it has not been observed until now is due to the fact that proximity to humans has shortened the lives of chimpanzees. cause, for example, of diseases.
The idea that life expectancy is key to surviving beyond the fertile years agrees with the data available from chimpanzees and other mammals in captivity. In protected environments, good nutrition and availability of medical care mean that animals can live longer and, consequently, transitions to menopause are repeatedly observed, as illustrated in a comment published also today in Cell .
Menopause is a poorly understood characteristic from an evolutionary point of view, and the fact that it is so rare in the natural world makes it difficult to study. At first glance, the fact that an individual’s fertile period ends when they still have years of healthy life ahead of them does not seem to provide great benefits if we think about the survival of a species.
In recent years, the scientific community has embraced (with some suspicion) the grandmother hypothesis to explain this post-fertile life. According to this theory, an adult woman contributes more to the survival of the species by advising and caring for her daughter and granddaughter, respectively, than by actively contributing to reproduction.
The explanation, also valid to describe the usefulness of menopause among whales, does not fit, however, in chimpanzee societies, in which daughters usually leave the group to reproduce. The researchers propose, as an alternative, that among these primates the end of reproduction has evolved to avoid conflict with younger females in a context of limited mating possibilities. An explanation that, however, does not fit with the ancestors of human beings.
Be that as it may, the fact that chimpanzees, the closest living relatives of humans along with bonobos, are capable of reaching menopause under natural conditions, suggests that this trait may not have evolved from scratch in hominins (the upright primates of which Homo Sapiens is today the only representative). It is possible that the characteristic is based on genetic variations from our last common ancestor with chimpanzees.
The Science and Cell investigations reveal new data on the prevalence of menopause in the animal world, but keep the scientific debate open about its evolutionary origin.
