First proof of female wild chimps experiencing menopause found

Similar to humans and other chimpanzee populations, fertility in western Uganda's Ngogo community of wild chimpanzees declined after age 30, with no births observed after age 50, a team including researchers from the University of California Los Angeles (UCLA), US, found.

New Delhi: First proof of female wild chimpanzees experiencing menopause and having a post-fertile life was found in a new study.

Similar to humans and other chimpanzee populations, fertility in the western Uganda's Ngogo community of wild chimpanzees declined after age 30, with no births observed after age 50, a team including researchers from the University of California Los Angeles (UCLA), US, found.

Examining mortality and fertility rates of 185 female chimpanzees at the Kibale National Park from demographic data collected from 1995 to 2016, the research team calculated the fraction of adult life spent in a post-reproductive state for all the observed females.

The team also measured hormone levels in urine samples from 66 female chimps associated with human menopause, which include increasing levels of follicle-stimulating and ovulating hormones, as well as decreasing levels of ovarian steroid hormones including estrogens and progestins. The chimps were of varying reproductive statuses and ages, ranging from 14 to 67 years.

The hormone data showed that the Ngogo females experienced a menopausal transition similar to that of humans, beginning around age 50, the researchers said in their study published in the journal Science.

'The 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 Brian Wood, UCLA associate professor of anthropology, referring to the grandmother hypothesis.

The hypothesis proposes that females in their post-reproductive years may be able to pass on more of their genes by helping to raise the birth rates of their own children or by caring directly for grandchildren, thereby increasing grandchildren's odds of survival. It has been used to explain the existence of human postmenopausal survival.

Prior to this study, these traits had only been found among mammals in a few species of toothed whales, and among primates in captivity who receive good nutrition and medical care - only in humans, the researchers said.

They further found that 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, they said.

"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.

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, the researchers said.