Wild Chimpanzees Can Experience Menopause Similar to Humans
Menopause occurs in just a few mammal species, but how this evolved in humans is a "challenging puzzle".
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Female chimpanzees in the wild can experience menopause and survival past their reproductive years in a similar way to humans, reports a new study in Science.
A benefit of the “Grandmother effect”?
Menopause is not unique to humans, though it is restricted to just a few mammal species. Toothed whale species – such as orcas – are the only other animals observed to experience menopause and humans were thought to be among the only primates.
“In societies around the world, women past their childbearing years play important roles, both economically and as wise advisors and caregivers,” said the study’s first author Brian Wood, an associate professor of anthropology at the University of California, Los Angeles. “How this life history evolved in humans is a fascinating yet challenging puzzle.”
One major theory behind menopause’s existence in some species is the so-called “Grandmother hypothesis”. This suggests that females who reach their post-reproductive years can support the survival of future generations by helping to care for their grandchildren, thereby ensuring their genes are successfully passed on. This seems to be the case for humans, but chimpanzees – one of our closest primate cousins – live in very different societies from us; it is not typical for older female chimpanzees to live near their daughters, or to help care for their grandchildren.
Researchers have been studying the Ngogo community of wild chimpanzees in the Kibale National Park in western Uganda, where female chimps often live beyond their childbearing years, for two decades. This population enjoys unusually favorable conditions, with a plentiful food supply and low predation – a possible explanation for their post-reproductive lifespan, as similar life expectancies have been reported among chimps in captivity, but not in long-term studies of wild chimps. It is possible that post-reproductive survival could also be an evolved, species-typical trait that has not been observed by researchers due to mankind’s detrimental effect on chimpanzee populations.
“Chimpanzees are extremely susceptible to dying from diseases that originate in humans and to which they have little natural immunity,” said Kevin Langergraber, an associate professor at Arizona State University and the senior author of the study. “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.”
New data from the current study may help researchers to understand why menopause and post-fertile survival occur in nature as well as its evolution in humans.
Ngogo chimps experience similar menopausal changes to humans
The researchers collected data on mortality and fertility among 185 female chimpanzees from 1995–2016, the culmination of thousands of hours of fieldwork.
Using hormone levels from the urine samples of 66 chimps aged 14–67, the researchers calculated the fraction of their adult lives spent in a post-reproductive state. Hormonal changes associated with human menopause – such as rising levels of follicle-stimulating hormone and luteinizing hormone as well as drops in estrogens and progestins – were also measured.
The analysis revealed that chimpanzee fertility began to steadily decline after age 30 – similar to other chimpanzee populations as well as humans – and no births occurred after age 50. It was not unusual for the Ngogo chimps to live past 50, around which time they also experienced a menopausal transition in a similar fashion to humans.
“This study is the result of an extraordinary amount of effort,” Negrey explained. “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.”
Female Ngogo chimps who reached adulthood at 14 years of age would be post-reproductive for approximately one-fifth of their adult lives, the findings revealed. To put it into context, this is about half as long as was experienced by human hunter-gatherers.
Understanding the evolution of the menopause
“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,” said Wood.
“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,” he added.
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 – an effort that will require long-term study of primates in the wild.
Reference: Wood BM, Negrey JD, Brown JL, et al. Demographic and hormonal evidence for menopause in wild chimpanzees. 2023. Science. doi: 10.1126/science.add5473
This article is a rework of a press release issued by the University of California, Los Angeles. Material has been edited for length and content.