Chimpanzee Menopause Could Unravel Human Evolution Mystery

Menopause has been observed in a population of wild chimpanzees for the first time, challenging long-held beliefs about the evolution of post-reproductive survival in humans.

The discovery, published in the journal Science on October 26, represents two decades of research into the Ngogo community of wild chimpanzees in Kibale National Park, western Uganda, by scientists from the University of California, Arizona State University and the Ngogo Chimpanzee Project.

"Lots of species have menopause or some other physiological mechanism that results in reproductive cessation," co-author Kevin Langergraber, from Arizona State University, told Newsweek.

"But what is much more rare are species where it is typical for females to spend a substantial fraction of their adult years in a post-reproductive state.

"Previously this had only been documented, among populations living under natural conditions in the wild, in a few whale species and in humans."

In humans, menopause is marked by the end of monthly menstruation, when the ovaries stop releasing eggs for fertilization, and usually occurs between the ages of 45 and 55. But why is this process so unusual in the animal kingdom?

"Darwin's theory of natural selection predicts that any gene which extended lifespan past the end of reproduction … would be invisible to natural selection because it would not confer any reproductive advantage," Michael Cant, a professor of evolutionary biology at the University of Exeter in the U.K., told Newsweek.

Cant, who has written a related perspective article about the research, also in the journal Science, added: "There is no [evolutionary] fitness benefit from survival per se, unless you are reproducing—in fact, you might be using up food resources that could instead be used by your children."

What else might be at play here?

"There are several theories to explain the existence of substantial post-reproductive lifespans in those few species where they occur," Langergraber said.

"The most prominent is probably the grandmother hypothesis, which argues that the fitness benefits that old females gain by helping their offspring and grandoffspring survive and reproduce are greater than the fitness costs of ceasing their own reproduction."

Menopausal mammals also tend to have bigger brains, language and social learning. "These factors probably increase the benefits that older females can confer on their group and their offspring," Cant said.

This would make sense when you look at humans and at killer whales, which both have tight-knit family groups with grandmothers often playing important roles as auxiliary carers for their grandchildren.

However, chimps tend not to make great grandmas. "They present a challenge for the idea that menopause requires helpful grandmothers, because chimpanzee grandmothers don't appear to be hardworking helpers," Cant said.

Yet, by examining mortality and fertility rates along with urinary hormone levels in dozens of Ngogo chimpanzees, the researchers were able to demonstrate that these females can live for many years after they begin to show the hormonal signs of menopause and cease to reproduce.

This study is the first proof of chimpanzee menopause in the wild, but menopause has been previously documented in captive chimpanzees. Could the post-reproductive survival be down to their environment?

"Conditions at Ngogo are unusually good," Langergraber said. "We already know that lots of species, including chimpanzees, show substantial post-reproductive lifespans under the protective conditions of captivity, where they receive lots of medical care, lots of food and have no predators. So, maybe post-reproductive lifespans at Ngogo are like those in captivity, an artefact of unusual survivorship conditions which were rarely experienced throughout chimp evolutionary history."

There is an alternative, according to Langergraber. "But a second possibility is that chimps experienced more Ngogo-like conditions and survivorship patterns throughout their evolutionary history, and survivorship patterns at other wild chimp sites are artificially low due to recent negative human impacts, chiefly habitat degradation and especially disease."

More research will need to be done to establish the roles of improved diet and lower risk of predation on human evolution and the development of menopause in our species. However, these studies take time.

"It takes decades to collect this sort of really valuable data, so it is very rare indeed to have this life history data, together with the physiological data on hormones," Cant said. "These kind of hard-won data on life history help to stimulate new ideas and hypotheses and promising new directions for the future."