Female chimps experience menopause and then survive for years, even decades

Female chimps in an East African forest go through menopause and then live for years, if not decades, after being biologically incapable of reproducing. This is a significant finding as previously, the only known species in which females live long after they stop being able to reproduce are humans and some whales.

A new article published on Thursday (Oct 26) in the journal Science argues that chimps should be added to the list too. The paper titled, "Demographic and hormonal evidence for menopause in wild chimpanzees", provided insights into the evolutionary imperatives underlying menopause in women.

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A senior author Kevin Langergraber of Arizona State University told AFP: "Chimpanzees have been studied in the wild for a long time, and you might think there's nothing left to learn about them. I think this research shows us that's not true."

The study mentioned that the vast majority of mammal females produce offspring till the end of their lives, but around the age of 50, humans experience a reduction in reproductive hormones and the permanent loss of ovarian function.

Notably, females of five species of toothed whale, including orcas and narwhals, similarly survive well beyond fertile age. It isn't obvious why natural selection would favour this trait, and only among a handful of species.

The authors said that data from our closest primate relatives is extremely useful for reconstructing and modelling human life history evolution. The researchers have combined demographic and hormonal data to calculate a metric called the post-reproductive representation (PrR), which is the average proportion of the adult life span that is spent in a post-reproductive state.

To carry out the study, the authors said they examined the mortality and fertility rates of 185 female chimpanzees in the Ngogo community of wild chimpanzees in Kibale National Park, Uganda, from 21 years of observation (1995–2016).

As in other chimpanzee populations and humans, fertility declined after age 30, and no births were observed after age 50. It was not unusual for Ngogo females to live past the age of 50 (N = 16 females).

The authors concluded that menopause ends reproduction around the age of 50 in both humans and wild chimpanzees.

Some scientists have put forward the "grandmother hypothesis" as a possible explanation: the idea that older females enter a post-reproductive state to consume fewer resources and focus on improving their grandchildren's odds of survival.

However, previous research has not discovered a significant proportion of wild female chimps in a post-reproductive stage. Some researchers gave one explanation which is that the excellent conditions at Ngogo allow more females to live until they are older.

Similar explanations have made to explain why many species, including chimps, go through menopause in captivity.

However, the researchers believe there is another possibility, that could be a long period of life after reproduction has been common throughout chimp evolution, but recent human activity at other sites, including Ngogo, has hampered the survival of wild populations, including older females, due to disease and habitat loss.

(With inputs from agencies)

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