Until now, menopause was only known in humans and certain species of cetaceans. But according to a new study, chimpanzees need to be added to the list.
This research, published in the journal Science on Thursday, offers new insight into the evolution of menopause in women.
“Chimpanzees have been studied in the wild for a long time, and one might think that there is nothing more to learn from them. Kevin Langergraber, of Arizona State University and co-author of the study, told AFP. “I think this study shows us that that’s not true.”
The vast majority of female mammals have young until the end of their lives, but humans experience a decline in the production of reproductive hormones, until it stops completely, generally around age 50.
In orcas and narwhals, females also live well beyond the moment when they cease to be able to reproduce.
But it is not clear why natural selection favored this evolution — and moreover only in certain species.
According to some scientists, a possible explanation is that of the role of the “grandmother”: females who can no longer reproduce therefore have more time and energy to devote to the survival and success of their grandchildren.
Female chimpanzees postmenopause 20% of their life
For their study, the researchers examined the fertility and mortality rates of 185 female chimpanzees from the Ngogo community in Kibale National Park, Uganda, between 1995 and 2016.
They calculated an indicator to determine the average time of adult life spent after losing reproductive capacity.
Previous attempts had encountered statistical difficulties, while this indicator is more reliable, Brian Wood, of the University of California at Los Angeles and lead author of the study, told AFP.
Result: female chimpanzees — unlike chimpanzees from other populations — live on average 20% of their adult life after having ceased to be able to reproduce, they found, a little less than in humans.
To exclude the possibility that an illness had, for example, caused sterility in the entire generation of elderly female chimpanzees, the researchers also studied their hormonal status.
They took urine samples from 66 females, of different ages and reproductive abilities, and measured hormone levels (gonadotropin, estrogen, progesterone). Conclusion: The observed trends followed those of human women going through the menopausal transition.
Bad grandmothers
The researchers offer two interpretations.
The first is based on the fact that wild animals in captivity have long post-reproductive lives, when protected from predators and disease. It is possible that the chimpanzees in this population are evolving under abnormally favorable conditions, such as an absence of leopards, which have been hunted to extinction in this region.
The second hypothesizes that this is indeed a historical population, not influenced by changes caused by humans.
In this case, according to Brian Wood, scientists must revise their theory about menopause.
Indeed, among chimpanzees, females leave the community in which they are born, while the males who remain reproduce with numerous partners.
Maternal grandmothers are therefore absent, and paternal grandmothers do not know who their “grandchildren” are, their sons themselves not knowing which little ones are theirs. The grandmother hypothesis therefore does not apply.
According to Brian Wood, menopause could instead serve to reduce competition between older and younger females.
When a female chimpanzee joins a new group, her bonds with it are weak, but they grow over time as she reproduces and passes on her genes. Later, this imperative fades.
Dan Franks, a biologist at the University of York who has studied menopause in orcas, called the study “fascinating.”
“This research describes for the first time menopause in non-human primates in the wild“, he stressed to AFP, adding that the second interpretation put forward by the researchers was “exciting” in terms of its implications.
The authors of the study wish to examine the same question in the future in bonobos, which are, with chimpanzees, the species of the animal kingdom closest to humans.
