This finding is significant as it highlights that humans are not the only primates to experience a long post-fertile life stage.
The research team found that female chimpanzees in the Ngogo community went through a menopausal transition comparable to that of human females. The fertility of the chimpanzees waned past the age of 30 and completely ceased after age 50.
This discovery adds a new layer to our understanding of menopause and post-fertile survival in nature, and how such traits evolved in our species.
Prior to this study, signs of menopause had only been identified in a few species of toothed whales and, among primates, exclusively in humans.
This revelation will now aid in a more profound understanding of why menopause occurs in nature and its evolutionary trajectory in humans.
“In societies around the world, women past their childbearing years play important roles, both economically and as wise advisors and caregivers,” said study first author Professor Brian Wood. “How this life history evolved in humans is a fascinating yet challenging puzzle.”
Wood’s team included Kevin Langergraber from Arizona State University, Jacob Negrey of the University of Arizona, and Ngogo Chimpanzee Project founders John Mitani and David Watts.
“The (study) 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,” Wood said, referring to the grandmother hypothesis.
This theory suggests that postmenopausal survival in humans evolved because older women could increase their genetic legacy by aiding in raising their grandchildren.
While this has been observed in humans, the Ngogo chimpanzees present a different picture. Older female chimpanzees typically neither live near their daughters nor care for their grandchildren, yet they often outlive their reproductive years.
Though other long-term studies of wild chimpanzees had not observed substantial post-reproductive life spans, such phases have been noted in captive primates provided with good nutrition and medical care.
One theory suggests the observed post-reproductive lifespan of the Ngogo chimpanzees could be attributed to favorable ecological conditions, including a bountiful food supply and minimal predation.
Another theory suggests this might be an inherent trait in chimpanzees, previously unobserved due to detrimental human impacts.
“Chimpanzees are extremely susceptible to dying from diseases that originate in humans and to which they have little natural immunity,” said 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 their chances of happening.”
An immense effort went into this research. The team examined data from 1995 to 2016, involving 185 female chimpanzees. They studied hormone levels in urine samples from 66 females ranging from 14 to 67 years.
The hormonal analysis mirrored the signs associated with human menopause, further confirming their findings.
“This study is the result of an extraordinary amount of effort,” said Negrey. “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.”
Fertility in the chimpanzees declined after age 30, and no females gave birth after 50 – the age when Ngogo females experienced a menopausal transition. The researchers noted that, like humans, it was not unusual for these female chimpanzees to live past 50.
“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,” said Wood.
According to the researchers, it will also be critical to track the behavior of older chimpanzees and observe how they interact with and influence other group members. “To allow such work, it is essential to support the long-term study of primates in the wild,” said Wood.
The research is published in the journal Science.
Image Credit: The Ngogo Chimpanzee Project
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