In a groundbreaking discovery, a group of wild chimpanzees in Uganda’s Kibale National Park has become the first non-human primates to be documented experiencing menopause. This revelation, published in the journal Science, adds a new layer of complexity to the question of why a few mammal species, including humans and toothed whales, have evolved extended female lifespans beyond their reproductive years.
Tobias Deschner, a primatologist at Osnabrück University in Germany, expresses the puzzle’s essence, noting that most species continue reproducing until they can no longer do so. The enigma lies in understanding why some creatures deviate from this pattern, ceasing reproduction long before the end of their lives.
A group of researchers closely followed 185 female members of the Ngogo community of chimpanzees in Kibale National Park from 1995 to 2016. Their data unveiled a pattern mirroring that observed in humans and other chimpanzees: the number of births dwindled after the age of 30 and ceased entirely around the age of 50.
However, several female chimpanzees continued to live post-reproductive lives, sometimes extending into their 60s. This post-reproductive period comprises around one-fifth of their adult lives, roughly half as long as that of human hunter-gatherer women. The extent of this post-reproductive life took the research team by surprise.
Menopausal or hormonal transition of Chimpanzees
The hormonal transition experienced by these female chimpanzees closely resembles that seen in human menopause. Researchers collected urine samples from post-reproductive Ngogo females, revealing a decline in estrogen and progestin levels, accompanied by elevated levels of follicle-stimulating hormone and luteinizing hormone, which are hormones controlling ovulation and the renewal of the uterine lining after menstruation.
The research underscores the value of long-term observational studies, emphasizing the importance of maintaining such projects to gain valuable insights over time.
However, the question remains: are the Ngogo chimpanzees an exceptional case, or is this phenomenon more widespread among wild chimpanzee populations? Another study led by Angela Goncalves, a cancer biologist at the German Cancer Research Center in Heidelberg, reveals that the cessation of ovulation is common among animals in captivity. It is still unclear where this process begins, whether it originates in the ovaries or the hormone-producing pituitary gland.
Goncalves suggests that the Ngogo chimpanzees might be living in protected circumstances that differ from the natural conditions in which they evolved, just like captive animals. According to the Ngogo research team, this group is relatively isolated and less affected by human activities and diseases compared to other chimpanzee populations. In many populations, female chimps typically pass away shortly after their reproductive years.
The question of whether extended post-reproductive life is the norm or an anomaly in chimpanzees in the wild remains open. Researchers will require more studies conducted at different research sites to draw a definitive conclusion.
So far, a long post-reproductive life in the wild has only been recorded in five other mammals: orcas, short-finned pilot whales, narwhals, beluga whales, and false killer whales.
Various hypotheses attempt to explain menopause’s evolution in different species, but the one-size-fits-all “grandmother hypothesis” may not apply to chimpanzees, as young females typically leave their family groups to mate, separating them from their mothers. An alternative explanation is the reproductive-conflict hypothesis, suggesting that older females stop reproducing to prevent competition with younger females who are more likely to be their granddaughters or close relatives over time.
The debate continues over whether menopause is an evolutionary adaptation or a byproduct of other processes. Some scientists argue that it is essential for preserving high-quality mitochondria in egg cells, particularly crucial for brain cells in large-brained mammals like humans. Mitochondria are inherited only from the mother, and as females age, they run out of eggs with good mitochondria.
The heritability of menopause timing in humans suggests that it is an adaptive trait that has been selected for over evolutionary time, rather than a random occurrence. This ongoing discussion underscores the complexity of menopause’s evolution and its role in different species.
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Reference: https://www.nature.com/articles/d41586-023-03308-8
