New York: A team of researchers have for the first time gathered ‘hormonal’ evidence that female wild chimpanzees experience menopause and post-reproductive survival.
Previously, the traits had only been found among mammals in a few species of toothed whales, and among primates — only in humans.
The study conducted among the Ngogo community of wild chimpanzees in western Uganda’s Kibale National Park showed that female chimpanzees experienced a menopausal transition similar to women. Fertility among chimpanzees studied declined after age 30, and no births were observed after age 50.
These new demographic and physiological data can help researchers better understand why menopause and post-fertile survival occur in nature, and how it evolved in the human species.
“In societies around the world, women past their childbearing years play important roles, both economically and as wise advisors and caregivers,” said Brian Wood, UCLA associate professor of anthropology. “How this life history evolved in humans is a fascinating yet challenging puzzle.”
“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,” Wood said.
The team of researchers examined mortality and fertility 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 varying reproductive statuses and ages, ranging from 14 to 67 years.
Thousands of hours of fieldwork at Ngogo were needed to collect the observations and samples needed for this study. Hormone samples were analysed by Tobias Deschner and Melissa Emery Thompson.
The researchers measured hormone levels associated with human menopause, which include increasing levels of follicle-stimulating hormone and luteinising hormone, as well as decreasing levels of ovarian steroid hormones including estrogens and progestins.
Like humans, it was not unusual for these female chimpanzees to live past 50. A female who reached adulthood at age 14 was post-reproductive for about one-fifth of her adult life, about half as long as a human hunter-gatherer.
“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.