Study from Uganda’s Ngogo community finds first evidence of menopause in wild chimpanzees, showing that humans are not the only primates to live long post-fertility lives
Female chimpanzees from Uganda’s Ngogo community experience a menopausal transition similar to that of women. The fertility of the chimpanzees studied decreased after the age of 30, and no births were observed after the age of 50. These data can help researchers better understand why menopause and postfertility survival occur in nature and how they evolved in the human species.
A team of researchers who have been studying the Ngogo community of wild chimpanzees in Kibale National Park in western Uganda for two decades have published a report in Science showing that females in this population can experience menopause and survive post-reproductively.
Before the study, these features had only been observed in mammals, some species of toothed whales and, in the case of primates, only in humans. This new demographic and physiological data can help researchers better understand why menopause and post-fertility survival occur in nature and how they evolved in the human species.
Why survive if you are no longer fertile?
“In societies around the world, women of childbearing age play important roles, both economically and as wise advisors and caregivers,” says Brian Wood, associate professor of anthropology at UCLA. “How this life story evolved in humans is a fascinating but challenging mystery.”
Wood, lead author of the paper, worked closely with Kevin Langergraber of Arizona State University, Jacob Negrey of the University of Arizona, and the founders and co-directors of the Ngogo Chimpanzee Project, John Mitani and David Watts.
“The results show that under certain ecological conditions, menopause and post-fertility survival can emerge in a social system that is very different from ours and that does not include grandparental support,” Wood said, referring to the grandmother- Hypothesis.
This hypothesis, which has been used to explain postmenopausal survival in humans, proposes that women of postmenopausal age may be able to pass on more of their genes by helping to increase the birth rates of their own children or by directly caring for them grandchildren. , thereby increasing their chances of survival. And in fact, several studies on human grandmothers have confirmed these positive effects. But chimpanzees live very differently than humans. Older women do not typically live near their daughters or care for their grandchildren, but Ngogo women often live past childbearing age.
Although significant increases in post-reproductive lifespan have not been observed in other long-term studies of wild chimpanzees, they have been observed in captive chimpanzees and other primates that received good nutrition and medical care. This raises the possibility that the post-reproductive lifespan of female Ngogo chimpanzees is a temporary response to unusually favorable ecological conditions, as this population has a stable and abundant food supply and low levels of predation. However, another possibility is that post-reproductive lifespan is actually an evolved and species-typical trait in chimpanzees, but has not been observed in other chimpanzee populations due to recent negative impacts by humans.
“Chimpanzees are extremely vulnerable to dying from human-caused diseases to which they have little natural immunity,” says Langergraber. “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 the likelihood of them occurring.”
An extraordinary effort
The research team examined the mortality and fertility rates of 185 female chimpanzees using demographic data collected between 1995 and 2016. They calculated the proportion of adult life that all observed females spent in a post-reproductive state and measured hormone levels in urine samples from 66 females of different reproductive states and ages, between 14 and 67 years old.
Thousands of hours of field work were necessary in Ngogo to collect the observations and samples necessary for this study. Hormone samples were analyzed by Tobias Deschner and Melissa Emery Thompson.
“This study is the result of an extraordinary effort,” says Negrey. “Just because our team has observed these chimpanzees for decades, we can be sure that some females live long after they stop reproducing. “We also spent thousands of hours in the jungle collecting urine samples from these chimpanzees to study hormonal signals of menopause.”
Researchers measured hormone levels associated with menopause, including increased levels of follicle-stimulating hormone and luteinizing hormone, and decreased levels of ovarian steroid hormones, including estrogens and progestins.
As with other chimpanzee populations and in humans, the fertility of the chimpanzees studied declined after age 30, and births were no longer observed after age 50. Hormonal data showed that Ngogo women experienced a menopausal transition similar to that of humans starting around age 50.
As with humans, it was not uncommon for women to live past 50 years. A female that reached adulthood at 14 spent a fifth of her adult life post-reproductive, about half as much as a human hunter-gatherer.
“We now know that menopause and postfertility survival occur in a broader range of species and socio-ecological conditions than previously thought, providing a solid basis for considering the role that improved nutrition and reducing predation risk in the evolution of human life history,” says Wood.
The researchers say it will also be important to track the behavior of older chimpanzees and observe how they interact with and influence other members of the group.
REFERENCE
Demographic and hormonal evidence of menopause in wild chimpanzees