Why do whales go through menopause?

Menopause is all too familiar for women, but in other species it is remarkably rare. Last year, scientists reported that females in a single population of chimpanzees live well past their reproductive years. But aside from chimpanzees and humans, researchers have found clear evidence of menopause in only five species: all whales.

Scientists have long debated why menopause developed. Perhaps it offered women an evolutionary advantage, or perhaps it was a side effect of some other beneficial feature of their lives.

In a new study looking at the biology of the five whale species, researchers argued that menopause gave the animals an evolutionary advantage. For example, it could have prevented older females from becoming pregnant at the same time as their daughters, thus avoiding resource conflicts that would harm both their offspring.

Samuel Ellis, a biologist at the University of Exeter who led the study, published in Nature, said whales may have developed menopause for the same reasons as humans.

“Maybe this is such an unusual strategy that there’s only one way to get there,” said Dr. Ellis.

In the vast majority of species, females continue to produce eggs throughout their lives. That pattern makes sense in terms of natural selection. The more offspring a female can successfully raise throughout her life, the more copies of her genes are passed on to future generations. Even long-lived females tend to fit this pattern: female elephants, for example stay fertile into your sixties.

Five whale species – orcas, false killer whales, beluga whales, short-finned pilot whales and narwhals – do not fit that pattern. For example, female killer whales generally only breed until about age 40, but can survive into their 90s.

Killer whales are relatively easy to study: they often swim in coastal waters and spend a lot of time at the surface. But the other menopausal species live far from the coast and spend a lot of time diving.

“So many of these species are so cryptic,” said Dr. Ellis. “The ocean is a big place.”

Instead of chasing the whales, Dr. Ellis and his colleagues gain insights from data that marine biologists have already collected. For example, sometimes groups of whales wash up during mass strandings. As marine biologists examine the bodies of their animals, they make estimates of their ages and perform autopsies on the females to see if they are pregnant or still producing eggs.

Dr. Ellis and his colleagues collected data for the five species of menopausal whales, along with 27 related species that do not go through menopause, such as dolphins and sperm whales. Using statistical equations, Dr. Ellis and his colleagues determined the average lifespan of the whales, the number of offspring they produce and how long they remain fertile.

Among species that don’t go through menopause, female whales fit the same trend: larger whales tended to live longer.

A different pattern emerged for the five menopausal species. The female whales remained fertile for as long as you would predict for a whale of their size. But then they lived an average of forty years longer than their predicted lifespan.

This finding suggests that menopause did not arise thanks to mutations that shortened the whales’ reproductive years. Instead, natural selection must have favored mutations that added more years to the animals’ lives after their reproduction stopped.

So what kind of evolutionary advantage could this new reproductive behavior provide? One possibility is that older females no longer give birth at the same time as their own offspring. This way they don’t come into conflict. In the long term, the researchers suggest, avoiding this conflict would allow the menopausal whales to pass on more of their genes.

Instead of coming into conflict with their offspring, the older female whales could provide assistance. In previous studies In killer whales, researchers have found that older females lead their groups on long journeys. In studies Researchers have also discovered that grandmothers can provide extra food among humans, which increases the chances of survival of their grandchildren.

The fact that only five whale species are known to have developed menopause suggests that this advantage can only be achieved under certain circumstances. Dr. Ellis speculated that a species requires a certain social life, in which females remain in a group for long periods of time and are closely related to younger members of the group.

Rebecca Sear, a demographer at the London School of Hygiene and Tropical Medicine who was not involved in the study, said Dr. Ellis and his colleagues made ingenious use of the data they could find.

“I think it’s amazing how much we know about the demography of whales, considering they live in the ocean,” she said.

She said their hypothesis is plausible, but also pointed out that they were able to analyze relatively few whales.

“I think we have to be very careful with this kind of work,” said Dr. Sear. “It’s really interesting and informative, but it doesn’t provide conclusive evidence as to why menopause occurred in the first place.”