Circle of life: Chimpanzees go through menopause too

For biologists, menopause is baffling. If natural selection favours genes that produce more descendants, why don’t women remain fertile their entire lives? What’s the evolutionary benefit of living for so many years without having more babies?

The mystery has only deepened as scientists have looked for menopause in mammals in the wild and found clear evidence of it only in a few species of whales. “It is very, very rare,” said Kevin Langergraber, a primatologist at Arizona State University. That rarity has led some researchers to argue that menopause played a crucial part in the evolution of humans. Perhaps, they proposed, it was a crucial ingredient in raising children whose big brains need lots of time — and parental support — to fully develop.

But a study published on Thursday by Dr. Langergraber and his colleagues challenges this view. After decades of observations in a rainforest in Uganda, they discovered that some chimpanzees go through menopause, too.

Susan Alberts, a biologist at Duke University who was not involved in the research, said that she would have been skeptical of such a claim in the past. She and her colleagues have performed some of the key studies showing that other primates do not go through menopause.

But she said the data from the new study, which includes observations of older female chimpanzees as well as measurements of hormones in their urine, had persuaded her. “The data are beautiful,” she said. “It’s just clear in their analysis that they have dotted every i and crossed every t.” In 1966, the British evolutionary biologist William Hamilton speculated that women’s long post-reproductive life must have been important in the course of human evolution. Other scientists later turned Hamilton’s musings into detailed theories, including the famous Grandmother Hypothesis.

Over the course of human evolution, that theory goes, our species gained much larger brains than other apes. As children’s brains slowly develop, they are relatively helpless, depending on adults for food and protection for many years.

At the same time, as women get older, giving birth and raising children gets more dangerous both for themselves and for their offspring. Instead of taking that risk, their later years could be focused on helping raise their grandchildren. The Grandmother Hypothesis has been bolstered by some studies of women who live in farming villages or in bands of hunter-gatherers. In those groups, children who get extra food and care from their grandmothers are more likely to survive than children who do not.

“We make transfers in a big way to the next generation, and the generation after that,” Dr. Alberts said. In recent years, however, Dr. Langergraber and his colleagues have questioned the theory. Since 1995, they and others have been watching the so-called Ngogo community of chimpanzees in Uganda. They noticed a number of healthy, old female chimps that were not having more babies. A chimp named Garbo, for example — one of the stars of the “Chimp Empire” series on Netflix — is now 67 years old. She had her last known pregnancy at age 38.

Carl Zimmer writes the “Origins” column