Chimpanzees also go through menopause

For biologists, menopause is intriguing. If natural selection favors genes that produce more offspring, why don’t women remain fertile throughout their lives? What is the evolutionary advantage of living for so many years without having more young?

The mystery only deepened when scientists looked for signs of menopause in wild mammals and found clear evidence in only a few species of whales. “It’s very, very rare,” says primatologist Kevin Langergraber of Arizona State University.

This rarity has led some researchers to assert that menopause played a crucial role in human evolution. They said it may have been an essential part of raising human children, whose big brains need a lot of time – and support from their parents – to fully develop.

But a study published Thursday by Professor Langergraber and colleagues challenges this view. After decades of observation in a rainforest in Uganda, they discovered that some female chimpanzees were also menopausal.

Susan Alberts, a biologist at Duke University who was not involved in the research, says such a claim would once have left her skeptical. She and her colleagues have done some of the key studies showing that other primates don’t have menopause.

But she says the data from the new study, which includes observations of older female chimpanzees as well as measurements of hormones in their urine, convinced her. “The data is great,” she says. “Their analysis clearly shows that they have dotted their i’s and crossed their t’s. »

Grandma’s Hypothesis

In 1966, British evolutionary biologist William Hamilton hypothesized that women’s long post-reproductive lives must have played an important role in human evolution. Other scientists subsequently built on Hamilton’s insights to develop detailed theories, including the famous grandmother hypothesis.

According to this theory, during human evolution, our species acquired a much larger brain than that of other primates. Because children’s brains develop slowly, they are relatively helpless and dependent on adults for food and protection for many years.

At the same time, as women age, giving birth and raising children becomes more dangerous, both for themselves and their offspring. Instead of taking this risk, they could devote their remaining years to raising their grandchildren.

The grandmother hypothesis has been supported by some studies of women living in agricultural villages or in hunter-gatherer groups. In these groups, children who receive extra food and care from their grandmothers are more likely to survive than those who do not.

“We are making significant transfers to the next generation and the one after that,” explains M.^me Alberts.

For several years, however, Mr. Langergraber and his colleagues have questioned this theory. Since 1995, they and others have observed the chimpanzee community known as Ngogo, in Uganda. They noticed that a number of older, healthy female chimpanzees were no longer giving birth to young. A female named Garbo, for example, one of the stars of the series Chimp Empire on Netflix, is now 67 years old. Her last known pregnancy was when she was 38.

Brian Wood, an evolutionary anthropologist at the University of California, Los Angeles, performed a statistical analysis of data collected from 185 ngogo females and found that a significant number of them lived long after their last known pregnancy.

Jacob Negrey, then a graduate student at Boston University, then collected urine from young and old chimpanzees. Sometimes he collected it by placing plastic sheets under the trees where they slept. In other cases, he collected it on leaves.

Later, Melissa Emery Thompson studied urine in her laboratory at the University of New Mexico, measuring estrogen and other hormones in the samples. The researchers found that hormone levels changed over the lifespan of female chimpanzees, similar to those in humans.

“I think it’s clear that these females live a long time after they stop breeding,” says evolutionary biologist Michael Cant of the University of Exeter, who was not involved in the new study.

Whales

The grandmother hypothesis does not explain how chimpanzees could have evolved into menopause. With relatively small brains, baby chimpanzees are not as dependent on their parents as human children. Additionally, Dr. Langergraber and his colleagues did not observe Garbo or other older females providing additional food to their grandchildren.

To find other evolutionary explanations for menopause in chimpanzees, Langergraber and his colleagues look to whales.

In many species of wild whales, females become less fertile with age. But until now, only five species of whales showed the hallmark signs of menopause, defined as an abrupt cessation of the reproductive years well before the end of life.

Studies of orcas have found that the offspring of older females are less likely to survive than those of younger females. “Older females lose out when they breed at the same time as younger females in the same group,” said Cant, who led part of the research. It appears that female whales come into conflict, perhaps over food.

Menopause among orcas could allow them to focus their efforts on helping their pod stay alive, rather than making more babies. Mr. Cant and his colleagues found that older females often led their peers on long trips to hunting grounds, perhaps relying on memories dating back decades.

Dr. Langergraber speculated that menopause may have evolved similarly in small-brained monkeys. Later, when our ancestors developed big brains and helpless little ones, the benefits of grandmothers’ help may have further promoted menopause. “It’s probably a story of multiple causes,” thinks Mr. Langergraber.

This article was originally published in the New York Times.