Female chimpanzees residing in an East African forest experience menopause and go on to live for an extended period, if not decades, after their reproductive capabilities have ceased. This is a significant discovery as, prior to this, the only known species in which females continue to live long after their reproductive years are humans and some whale species.
A newly published article in the journal Science on October 26 argues that chimpanzees should now be included in this category. The paper, titled “Demographic and hormonal evidence for menopause in wild chimpanzees,” provides insights into the evolutionary significance of menopause in women.
Co-author Kevin Langergraber from Arizona State University stated, “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 notes that the vast majority of female mammals continue to produce offspring throughout their lives. However, humans, around the age of 50, experience a decline in reproductive hormones and the permanent loss of ovarian function. Interestingly, females of five species of toothed whales, such as orcas and narwhals, also survive well beyond their fertile years. The reasons why natural selection would favor this trait, and only in a few species, remain unclear.
The authors highlight that data from our closest primate relatives are highly valuable for understanding and modeling the evolution of human life history. The researchers employed demographic and hormonal data to calculate a metric known as post-reproductive representation (PrR), which measures the average proportion of the adult lifespan spent in a post-reproductive state.
To conduct the study, the authors examined the mortality and fertility rates of 185 female chimpanzees in the Ngogo community of wild chimpanzees in Kibale National Park, Uganda, based on 21 years of observations from 1995 to 2016.
Similar to other chimpanzee populations and humans, fertility declined after the age of 30, and no births were observed after the age of 50. Notably, it was not uncommon for Ngogo females to live past the age of 50, with 16 such females identified.
The study’s conclusion is that menopause ceases reproduction at around the age of 50 in both humans and wild chimpanzees.
Some scientists have put forward the “grandmother hypothesis” as a potential explanation, suggesting that older females enter a post-reproductive state to consume fewer resources and focus on improving their grandchildren’s chances of survival. However, previous research had not observed a significant proportion of wild female chimps in a post-reproductive stage. Some researchers explained this by suggesting that the favorable conditions at Ngogo allow more females to live to an older age.
Similar explanations have been offered to account for why many species, including chimps, experience menopause in captivity.
However, the researchers propose an alternative possibility: that a long period of life after reproduction has been a common feature throughout chimp evolution, but recent human activities at various sites, including Ngogo, have disrupted the survival of wild populations, particularly older females, due to disease and habitat loss.
Post Your Comments