New research reveals that female chimpanzees go through menopause and live through their reproductive years, challenging previous views and providing new insights into the evolution of menopause in mammals. A team of researchers studied the Ngogo community of wild chimpanzees in Kibale National Park in Uganda for more than two decades. Their recent paper in the journal Science reveals this.
Before this study was completed, these features had been found in only a few toothed whales among mammals and humans among primates. These new demographic and physiological data could help researchers better understand why menopause and post-reproductive survival persist in nature and how it evolved in humans.
The role of postmenopausal women in society
"In societies around the world, women past childbearing years play important roles, both financially and as wise advisors and caregivers," said Brian Wood, associate professor of anthropology at UCLA. "How this life history evolved in humans is a fascinating and challenging mystery."
Wood is the paper's lead author, working closely with Kevin Langrab of Arizona State University, Jacob Negre of the University of Arizona, and John Mitani and David Watts, founders and co-directors of the Ngogo Chimpanzee Project.
"The (research) results suggest that, under certain ecological conditions, menopause and post-reproductive survival can occur in a social system that is completely different from ours and does not include grandparental support," Wood said, referring to the grandmother hypothesis.
This hypothesis has been used to explain the existence of human postmenopausal survival. It believes that women can pass on more genes during the post-childbearing period by helping to increase the birth rate of their own children or directly caring for their grandchildren, thereby improving their grandchildren's chances of survival.
In fact, some studies on human grandmothers have found these positive effects. But chimpanzees have very different living arrangements than humans. Older female chimpanzees typically do not live near their daughters or care for their grandchildren, but Ngogo's females tend to live beyond their reproductive years.
Chimpanzee lifespan and human impact
While post-reproductive longevity has not been previously observed in other long-term studies of wild chimpanzees, it has sometimes been seen in captive chimpanzees and other primates because the animals receive good nutrition and medical care. This raises the possibility that the postreproductive lifespan of Ngogo female chimpanzees may be a temporary response to unusually favorable ecological conditions, as this population enjoys a stable and abundant food supply and low levels of predation. However, another possibility is that post-reproductive lifespan is actually a species-typical trait that evolved in chimpanzees but has not been observed in other chimpanzee populations due to negative human influence in recent years.
"Chimpanzees are extremely vulnerable to succumbing to diseases of human origin, to which chimpanzees have little natural immunity," Langraber said. "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 chances of them occurring."
The team studied mortality and fertility rates in 185 female chimpanzees from demographic data collected from 1995 to 2016. They calculated the proportion of time all observed female chimpanzees spent in a post-reproductive state as adults, and measured hormone levels in urine samples from 66 female chimpanzees who varied in reproductive status and age, from 14 to 67 years old.
It took thousands of hours of fieldwork in Ngogo to collect the observations and samples needed for this study. The hormone samples were analyzed by Tobias Deschner and Melissa Emery Thompson.
"This study is the result of a lot of hard work," Negre said. "Our team spent decades monitoring these chimpanzees so we could be confident that some females survived long after they stopped reproducing. We also spent thousands of hours in the forest collecting urine samples from these chimpanzees to study the hormonal signals of menopause."
The researchers measured hormone levels associated with menopause in humans, including increases in follicle-stimulating hormone and luteinizing hormone levels and decreases in ovarian steroid hormones, including estrogen and progesterone.
As with other chimpanzee populations and humans, the chimpanzees studied experienced reduced fertility after age 30 and did not have children after age 50. Hormone data show that the menopausal transition in female chimpanzees is similar to that in humans, starting around age 50.
Like humans, it's not uncommon for these female chimpanzees to live past 50 years. A female chimpanzee who reaches adulthood by age 14 spends about one-fifth of her adult life in menopause, about half the time it takes for a human hunter-gatherer to go through menopause.
"We now know that a wider range of species and socioecological conditions survive menopause and reproduction than previously understood, providing a solid basis for considering the role that improved diet and reduced predation risk may have played in the evolution of human life history," Wood said.
It's also crucial to track the behavior of older chimpanzees and observe how they interact with and influence other group members, the researchers said. To carry out these efforts, long-term research on wild primates must be supported.
Reference "Brian M. Wood, Jacob D. Negrey, Janine L. Brown, Tobias Deschner, Melissa Emery Thompson, Sholly Gunter, John C. Mitani, David P. Watts, and Kevin E. Langergraber, "Demographic and hormonal evidence for menopause in wild chimpanzees," published in Science on October 27, 2023.
DOI:10.1126/science.add5473
Compiled source: ScitechDaily