Chimpanzees also have menopause

Female chimpanzees living in a community in Kibale National Park, Uganda, also experience menopause, a biological process that until now had only been documented in humans and some whale species such as orcas.

Now, a team of scientists who observed this community of great apes between 1995 and 2016 have found signs in their females that show that they survive many years after ceasing to be fertile.. The conclusions of this research led by Brian Wood, from the Max Planck Institute for Evolutionary Anthropology and the University of California, are published this week in the journal Science.

According to its authors, these signs of menopause in wild chimpanzees will help to better understand the evolution in humans of this process that is very rare in nature.. And despite the fact that in general women stop being fertile between 45 and 55 years – the average age is between 51 and 52 years – and can live many decades without the ability to reproduce, the majority of females Other species of mammals can have offspring throughout their lives.

Menopause begins when the eggs in the ovaries run out.. This cessation of ovarian function in turn causes a marked decrease in the levels of estrogen and progesterone, which are hormones produced by the ovaries, and a decalcification of the bones, which in turn leads to the appearance of the symptoms that accompany to menopause.

The benefits of this biological process are not at all clear, so it is a challenge for scientists to explain why women acquired this evolutionary trait.. It also remains uncertain why menopause evolved in humans but apparently did not do so in any other long-lived primate.. In this work, this team presents demographic and hormonal evidence of menopause in wild chimpanzees after observing a total of 185 specimens in the Ngogo community for 21 years.. Chimpanzees (Pan troglodytes schweinfurthii) are, along with bonobos, the closest relatives to humans in the animal kingdom.

The results showed that, as in other chimpanzee and human populations, fertility in female Ngogo chimpanzees decreased after the age of 30, and no births were observed after the age of 50.. But unlike other chimpanzee communities, and just like humans, it was not strange for Ngogo females to live for many years past their 50s.

A chimpanzee in Uganda's Kibale National Park

Specifically, the researchers calculated a parameter called post-reproductive representation (PrR), which is the average proportion of the adult life spent in a post-reproductive state.. While most mammals, including other chimpanzee populations, have a PrR close to zero, the authors found that Ngogo chimpanzees had a PrR of 0.2, meaning that, on average, females live 20% of adulthood without having children. In women, the post-reproductive state is usually 30% of their life.

On the other hand, urine samples from 66 female chimpanzees of different ages (between 14 and 67 years) and therefore in different reproductive phases, showed that the transition to this post-reproductive state was marked by a series of changes in hormones such as gonadotropins, estrogens and progestins. According to the authors, these hormonal variations are similar to those that occur during human menopause.

However, unlike people, the chimpanzees of the Ngogo population in a post-reproductive stage did not participate in raising their children's children, that is, their grandchildren, which according to this scientific team, suggests that The popular grandmother hypothesis, which has been used to explain adaptive evolution in women, would not serve as an explanation.. This theory proposed in the 1950s by evolutionary biologist George Christopher Williams points out that upon reaching menopause, older females can dedicate themselves to caring for their offspring instead of continuing to reproduce, increasing the chances that they will survive.. But what is common in chimpanzees is that older females live away from their daughters, since they usually leave the group in which they were born when they reach adulthood.

Another theory to explain menopause is the reproductive conflict hypothesis, which states that there is competition for resources between females that can have offspring and that they must compete to ensure successful reproduction.. Older females could stop reproducing to avoid these conflicts, which would favor their survival.. According to these scientists, the grandmother hypothesis and the reproductive conflict hypothesis are not mutually exclusive, and both may be necessary to explain why all human societies have a higher PrR than that documented here for chimpanzees.

As Michael Cant, from the University of Exeter, comments in an article in the same magazine, among killer whales, “females who have passed menopause lead the group when it comes to searching for food, especially when there is a shortage of prey,” for which considers that “older chimpanzees could also provide benefits to their group through their knowledge and experience”. From their point of view, this study “illuminates as well as raises questions about the evolution of menopause.”