Why do whales go through menopause? – 03/15/2024 – Science

Menopause is very familiar to women, but in other species it is rare. Researchers have found clear evidence that females of five whale species also go through this stage.

Scientists have long debated why menopause evolved. Perhaps it provided an evolutionary advantage to females, or perhaps it was a side effect of some other beneficial feature of their lives.

In a new study of five species of whales, it is argued that menopause offered these animals an evolutionary advantage. It would prevent, for example, older females from becoming pregnant at the same time as their daughters, avoiding resource conflicts that would harm the offspring of both.

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

In the vast majority of species, females continue producing eggs throughout their lives. This pattern makes sense in terms of natural selection. The more offspring a female can successfully raise over her lifetime, the more copies of her genes are passed on to future generations. Even long-lived females generally fit this pattern: elephants, for example, remain fertile until they are 60 years old.

Five species of whales — killer whales, false killer whales, belugas, short-finned pilot whales and narwhals — do not fit this pattern. Female orcas, for example, generally reproduce until they are around 40 years old, but can survive into their 90s.

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

Instead of chasing whales, Ellis and his colleagues tried to extract information from the data that marine biologists have already collected. Sometimes groups of whales strand themselves en masse on the coast, for example. By examining the animals’ bodies, marine biologists make age estimates and perform autopsies on the females to see if they are pregnant or still producing eggs.

The researchers collected data for the five species of whales that experience menopause and 27 others that do not, such as dolphins and sperm whales. Using statistical equations, they estimated the average life expectancy of whales, the number of offspring they produce and how long they remain fertile.

In species without menopause, females follow the same trend: larger whales tend to live longer.

A different pattern emerged among the five species that went through menopause. The female whales remained fertile for as long as you would expect for a whale their size. But then they lived, on average, 40 years beyond their expected life expectancy.

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

So what kind of evolutionary advantage could have arisen from this reproductive behavior?

One possibility is that older females do not give birth at the same time as their own offspring are giving birth. This way, they do not conflict. In the long term, the study authors suggest, avoiding this conflict would allow whales that go through menopause to pass on more of their genes.

Rather than conflict with their offspring, older female whales could provide assistance. In previous studies of orcas, older females were found to lead their herds on long journeys. In human studies, researchers have also found that grandmothers can provide extra food that increases their grandchildren’s chances of survival.

The fact that only five species of whales are known to have developed menopause suggests that this advantage can only be obtained under certain circumstances. Ellis hypothesizes that a species may need females to remain in a group for a long time and to be 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 research, said Ellis and his colleagues made ingenious use of the data they could find. “I think it’s amazing how much we know about whale demographics, considering they live in the ocean.”

She said their hypothesis is plausible, but also pointed out that they may have analyzed too few whales. “I think we need to be very cautious with this type of work,” Sear said. “It’s really interesting and informative, but it doesn’t provide conclusive evidence about why menopause evolved.”