Menopause in chimpanzees hints at why women live long after they can reproduce

How it works: Natural selection favors traits that are beneficial. In the biological world, reproduction is the benefit of interest, and most mammals die soon after they stop having offspring.

• But women live past menopause, which they experience at roughly the same time in life, despite differences in their societies and environments. That suggests the trait evolved.
• Aside from humans, only a few whale species are known to live long past their reproductive years.

Driving the news: Scientists now say there is evidence wild chimpanzees can be added to that short list.

• Studying the demographics of a chimpanzee community with 185 females in Uganda's Kibale National Park, researchers found 16 female chimpanzees lived past the age of 50, around when reproduction ends. The female chimpanzees spent about one-fifth of their adult life in this post-reproductive stage. (Women can spend at least 40% of their life in it.)
• They also measured the levels of hormones in the female chimpanzees' urine and found changes similar to those seen during menopause in humans, including a drop in the level of estrogens and progestins.
• The researchers report their findings today in the journal Science.

What's happening: The post-reproductive phase isn't seen in other wild chimpanzees but it has been documented in chimpanzees, gorillas, orangutans and macaques living in captivity, where there is ample food, no predators and medical care.

• Unusual conditions in Kibale Park could explain how chimpanzees there are living longer than their counterparts in other communities, says Kevin Langergraber, a primatologist at Arizona State University and co-author of the new paper. Leopards, a major predator, are absent from the park, and the chimpanzees have an abundant and stable food supply.
• Or, it could be that living beyond reproductive years is a trait that was common throughout the evolutionary history of chimpanzees.
• Perhaps it just hasn't been seen in other modern chimpanzee communities because recent negative impacts from humans, especially disease, have prevented them from living past 50 years in other places, Langergraber says.

Between the lines: Other primates live in good conditions in the wild, and their populations are thriving, "yet, they don't show substantial post-reproductive life spans," Langergraber says.

• "The fact that we only see them in chimps suggests that maybe the last common ancestor of chimps and humans was sort of predisposed for the evolution of these post-reproductive life spans."

Details: Scientists have come up with a few possible explanations for the evolution of the post-reproductive life span.

• One is the grandmother hypothesis, which centers on the idea that females evolved to live past their reproductive years in order to help their grand offspring survive, rather than risk having or raising their own offspring in older age. There is evidence that children with grandmothers in hunter-gatherer societies live longer.
• Whether those benefits outweigh the significant costs of no longer reproducing is highly debated. "No amount of grandmothering makes up for not reproducing yourself," Pat Monaghan, a professor of zoology at the University of Glasgow and who wasn't part of the new study, argues.
• And the grandmother hypothesis is unlikely to explain why chimpanzees live beyond their reproductive years because females tend to leave their family group once they become adults, the study authors say.

Another hypothesis centers on conflict and says that older females cede breeding opportunities to younger females in the group and then give up reproducing.

• Both hypotheses may be needed to explain the evolution of the much longer post-reproduction life span in humans — or it may require other explanations, Langergraber says.

The intrigue: To explain longer post-reproductive life spans, "we will need to consider just what old females can do," primatologist Sarah Hrdy says.

• Aging female primates have been found to have an increased willingness to invest more in each infant or protect the infants of others. "But such opportunities to serve as heroines may not occur all that often," she says, and not frequently enough to impact selection favoring longer life spans.
• Food sharing does though, she says, adding it is a part of daily life among hunter-gatherers that makes the difference for the survival of younger members of the group. "I am convinced that sharing food was huge."

Another key question is why males carry on reproducing through life but females don't.

• Monaghan and her colleagues suggest it is a consequence of physiology. Only females pass down mitochondria and the DNA they contain. Social animals with large brains, including humans, require strict quality control of the mitochondria in developing eggs and those with damaged mitochondria or mitochondria that aren't compatible with the nucleus, are culled. Eggs are stored over the course of life and their mitochondria deteriorate in long-lived species.
• "You can think of it as reproductive life curtailed as cost of having this big-brained offspring ... The cost of that is falling on females but the benefits [are] falling on males and females."

What's next: Langergraber and his colleagues plan to use high-resolution genetic data to test the hypothesis about reproductive conflict.

• Researchers say more data is needed about wild populations of bonobos and other chimpanzee communities, but there aren't yet decades of data available.

The bottom line: The researchers say post-reproductive life spans may not have evolved in human ancestors but instead "built on existing genetic variation in the last common ancestor we shared with chimpanzees."

Go deeper: The Ngogo community of chimps is documented in the Netflix series "Chimp Empire."