Menopause is an extremely rare phenomenon in the animal kingdom. Research has so far shown that only humans and some whale species live for many years after losing their ability to reproduce. But now a new study shows that female chimpanzees in a population in Uganda also enter menopause and can survive around 14 years after the end of their reproductive ability. The discovery provides further clues about the evolution of menopause and its biological purpose in humans and other mammalian species.
The vast majority of mammals remain fertile until the end of their lives. However, we humans and some species of toothed whales are an exception: we go through menopause. For us humans, menopause usually occurs between the ages of 45 and 55. During this time, reproductive hormones decrease and the ovaries permanently stop functioning because they can no longer produce eggs. However, how and why menopause evolved in only a few species is still unclear. The evolutionary benefits are not obvious. According to a common hypothesis, survival after menopause evolved in humans so that grandmothers could promote the reproduction of their daughters or help raise their grandchildren. However, this theory has not been proven.
Evidence for menopause in chimpanzees in Uganda
A research team led by Brian Wood from the University of California has now found evidence that female chimpanzees in Uganda are also entering menopause. The scientists analyzed the behavior and demographics of 185 females from a Ngogo community of wild chimpanzees in Kibale National Park over a period of 21 years. From this, Wood and his colleagues calculated a metric called post-reproductive representation (PrR). This number indicates the average period of life during which adults are no longer able to reproduce. They also examined which hormones appeared in the urine of 66 female chimpanzees between the ages of 14 and 67.
Most mammals, including other chimpanzee populations, have a PrR value close to zero. This means that they do not reach menopause, but are fertile until death. However, the Ngogo chimpanzees had a PrR value of around 0.2, Wood and his colleagues discovered. This means that females spend an average of 20 percent of their adult years living in a state in which they can no longer reproduce, the so-called post-reproductive state. In the study, this lasted on average around 14 years. We humans and some whales live up to twice as long after menopause - even without the help of modern medicine. In the chimpanzees observed, fertility decreased, as in humans, from around the age of 30, and births stopped from around the age of 50. 34 of the 185 females were older than 40 years, 16 chimpanzees even older than 50 years.
The urine samples also revealed different amounts of reproductive hormones such as gonadotropins, estrogens and gestagens depending on the age of the female chimpanzees. During the transition to the post-reproductive state, their composition changed: the gonadotropins FSH and LH increased sharply, the estrogens estradiol and estrone as well as the progestogen pregnanediol decreased. Similar hormonal changes also occur in humans during menopause: some sex hormones decrease in women during menopause (estrogens and gestagens), others increase (FSH, LH). “Our results are the first documentation of a wild non-human primate population that exhibits substantial and statistically significant post-reproductive representation,” concludes Woods.
“Grandmother Hypothesis” refuted?
Unlike humans, however, the older female chimpanzees in Uganda did not take part in raising children when they were no longer able to reproduce themselves. The scientists conclude that the “grandmother hypothesis” does not apply to these great apes. However, unlike males, adult female chimpanzees generally no longer live in the same population as their mothers. This means that older females cannot directly influence the reproduction of their daughters. Studies also suggest that they have no influence on the reproduction of their sons. Instead, the older females are more likely to be in competition with other, younger females. Wood and his colleagues therefore consider it possible that the loss of reproductive ability could also serve to resolve this competitive situation.
The evolutionary development of menopause could also have other reasons. Since chimpanzees are among our closest relatives, survival after menopause could also serve another, as yet unknown purpose in humans in addition to the grandmother effect. However, it remains unclear whether this is the same purpose as in other primates and whether all chimpanzee females actually continue to live after menopause. If this is the case, it has not been observed anywhere else - possibly due to negative human influences such as disease epidemics, the researchers speculate.
However, it is also conceivable that the Ngogo chimpanzees only temporarily acquired the ability to survive after menopause, Wood and his colleagues point out. “One explanation for this discrepancy is that significant PrR could be a temporary response to unusually favorable ecological conditions in Ngogo, including low predation, high food availability and successful intergroup competition,” explains Woods.
“The study sheds light on the development of menopause and at the same time raises questions about it,” writes evolutionary biologist Michael Cant from the University of Exeter in a comment on the study. “It also underscores the importance of difficult, long-term field studies in transforming the fundamental understanding of human biology and behavior.”
Source: Brian Wood (University of California) et al., Science, doi: 10.1126/science.add5473