Chimpanzees also experience the menopause, reveals new research.
Before the study, menopause had only been seen among mammals in a few species of toothed whales, and, among primates, only in humans.
American scientists found female chimps at Ngogo in Uganda’s Kibale National Park experienced a menopausal transition similar to women.
Fertility among the chimpanzees studied declined after age 30, and no births were observed after age 50.
Researchers say their findings, published in the journal Science, could help to better understand why menopause and post-fertile survival occur in nature and how it evolved in humans.
Study first author Dr. Brian Wood, a UCLA Associate Professor of anthropology, said: “In societies around the world, women past their childbearing years play important roles, both economically and as wise advisors and caregivers.
“How this life history evolved in humans is a fascinating yet challenging puzzle.
“The results show that under certain ecological conditions, menopause and post-fertile survival can emerge within a social system that’s quite unlike our own and includes no grandparental support.”
Older female chimps don’t usually live near their daughters or provide care for grandchildren, yet females at Ngogo often live past their childbearing years.
Dr. Wood explained that while substantial post-reproductive life spans have not previously been observed in other long-term studies of wild chimps, they have sometimes been seen in chimpanzees and other primates in captivity, who receive good nutrition and medical care.
The researchers say that raises the possibility that the post-reproductive life spans of female Ngogo chimps may be a temporary response to unusually favorable ecological conditions, as the troop enjoys a “stable and abundant” food supply and low levels of losses to predators.
They said another possibility is that post-reproductive lifespans are actually an evolved, “species-typical” trait in chimpanzees but have not been observed in other chimp populations because of the recent negative impacts of humans.
Professor Kevin Langergraber has studied the Ngogo group of chimps for over 20 years.
Prof. Langergraber, of 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.”
The team examined the fertility and death rates of 185 female chimpanzees from demographic data collected from 1995 to 2016.
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 ages ranging from 14- to 67-years-old.
Thousands of hours of fieldwork at Ngogo were needed to collect the observations and samples required for the study.
Team member Dr. Jacob Negrey, of University of Arizona, said: “This study is the result of an extraordinary amount of effort.
“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 measured hormone levels associated with human menopause, which include increasing levels of follicle-stimulating hormone and luteinizing hormone, as well as decreasing levels of ovarian steroid hormones including estrogens and progestins.
Fertility in the chimps studied declined after age 30, with no births observed after age 50.
The hormone data showed that the Ngogo females experienced a menopausal transition similar to that of humans, beginning around age 50.
Also like humans, it was not unusual for the female chimps to live past 50.
A female who reached adulthood at age 14 was post-reproductive for around a fifth of her adult life, about half as long as a human hunter-gatherer.
Dr. Wood said: “We now know that menopause and post-fertile survival arise across a broader range of species and socio-ecological conditions than formerly appreciated, providing a solid basis for considering the roles that improved diets and lowered risks of predation would have played in human life history evolution.”
The researchers say that it will also be critical to track the behavior of older chimpanzees and observe how they interact with and influence other group members.
Dr. Wood added: “To allow such work, it is essential to support the long-term study of primates in the wild.”
Produced in association with SWNS Talker