Researchers say they have found the first proof of menopause in wild chimpanzees.
The team studied the Ngogo community of wild chimpanzees in western Uganda’s Kibale National Park for two decades, and found that in females fertility declined after the age of 30, and no births were observed after 50.
The findings can help researchers better understand why menopause, and why survival after this, occurs in nature and how it evolved in the human species.
Brian Wood, associate professor of anthropology at University of California – Los Angeles, USA, said: “In societies around the world, women past their childbearing years play important roles, both economically and as wise advisers and caregivers.
“How this life history evolved in humans is a fascinating yet challenging puzzle.”
Prior to the new research, the traits described in the study had only been found among mammals in a few species of toothed whales, and among primates – only in humans.
According to researchers, their findings raise the possibility that the post-reproductive life spans of female Ngogo chimpanzees may be a temporary response to unusually favourable ecological conditions.
This is because the group they observed has a stable and abundant food supply and low levels of predation.
Another possibility, however, is that post-reproductive life spans are actually an evolved, species-typical trait in chimpanzees but have not been observed in other chimpanzee populations because of the recent negative impacts of humans.
Kevin Langergraber, from Arizona State University, said: “Chimpanzees are extremely susceptible to dying from diseases that originate in humans and to which they have little natural immunity.
“Chimpanzee researchers, including us at Ngogo, have learned over the years how devastating these disease outbreaks can be to chimpanzee populations, and how to reduce their chances of happening.”
For the study, published in Science, the researchers examined death and fertility rates of 185 female chimpanzees.
They calculated the fraction of adult life spent in a post-reproductive state for all the observed females and measured hormone levels in urine samples from 66 females of varying reproductive statuses and ages, ranging from 14 to 67 years.
The study found that like humans, it was not unusual for female chimpanzees to live past 50.
A female who reached adulthood at age 14 was post-reproductive for about one fifth of her adult life, about half as long as a human hunter-gatherer.
Jacob Negrey, of University of Arizona, said: “It’s only because our team has spent decades monitoring these chimpanzees that we can be confident some females live long after they’ve stopped reproducing.
“We also spent thousands of hours in the forest to collect urine samples from these chimpanzees with which to study hormonal signals of menopause.”
The researchers say it will also be critical to track the behaviour of older chimpanzees and observe how they interact with and influence other group members.