Why do cetaceans experience menopause?

In humans, menopause is a well-known phenomenon. However, it is extremely rare in other species. Last year, a team of scientists revealed that females in one group of chimpanzees lived long after their fertility phase had ended. But apart from chimpanzees and humans, researchers have only found clear evidence of menopause in five species, all from the cetacean order.

Scientific debate about the causes of menopause dates back a long time. It may have provided an evolutionary advantage to females or been a side effect of some other beneficial element in their lives.

In a new study of the biology of five cetacean species, researchers argue that menopause gave the animals an evolutionary advantage. For example, it would have prevented older females from becoming pregnant at the same time as their daughters, which would have prevented conflicts over resources, harming the offspring of both.

Samuel Ellis, a biologist at the University of Exeter and director of the study published in the journal natureHe claims that menopause can occur in cetaceans for the same reasons as in humans. “There's probably only one way to get to this point, since it's an uncommon strategy,” Ellis says.

Live another 40 years

In the vast majority of species, females spawn throughout their lives. It's a pattern that makes sense from the point of view of natural selection. The more offspring a female can successfully raise in her lifetime, the more copies of her genes will be passed on to future generations. Even the females of the longest-lived species follow this pattern: elephants continue to be fertile into their 60s.

There are five species of cetaceans (orcas, false killer whales, beluga whales, short-finned whales, and narwhals) that do not follow the above pattern. For example, female killer whales tend to only reproduce until they are 40 years old, but can live for more than 90 years.

Killer whales are relatively easy to study: they usually swim close to shore and spend a lot of time at the surface. Other species that reach menopause live in areas far from the coast and spend a lot of time submerged in the sea. “There are a lot of things that are terribly vague,” Ellis says. The ocean is so vast.”

Instead of pinning down cetaceans, Ellis and his team tried to extract information from data already collected by marine biologists. Sometimes there are groups of cetaceans crowding the beach in a mass stranding, for example. When marine biologists examine animals' bodies, they make estimates about their age and perform autopsies on females to determine if they are pregnant or still ovulating.

Ellis and his team collected data on five postmenopausal cetacean species, as well as 27 species not associated with menopause, such as dolphins and sperm whales. Using statistical equations, the research team estimated the average life expectancy of cetaceans, the number of offspring, and the duration of their fertility.

In species that did not menstruate, females followed the same pattern: larger cetaceans tended to live longer.

On the other hand, another pattern emerged in the five postmenopausal species: the females were about as fertile as one would expect from cetaceans of their size. But then, they lived on average 40 years above their average life expectancy.

This result suggests that menopause did not arise from mutations that shortened the cetaceans' reproductive phase, but rather that natural selection appears to have favored mutations that added years to the lives of females after they stopped reproducing.

Avoid competition

What kind of evolutionary advantage might these new reproductive behaviors confer? One possibility is that older females did not give birth at the same time as their daughters. In this way conflicts between them were avoided. The researchers suggest that avoiding these conflicts in the long term would have allowed postmenopausal females to pass on a greater number of their genes to their offspring.

Instead of struggling with their daughters, older females can devote themselves to helping. In previous studies on killer whales, researchers found that older females lead the group on long-distance trips. In human work, it has also been found that grandmothers can provide complementary foods to their grandchildren, increasing their likelihood of survival.

The fact that only five species of cetaceans have developed menopause suggests that this advantage can only be obtained under certain conditions. Ellis speculates that this species must have a type of social life in which females live in groups for long periods of time and associate closely with younger members of the group.

Ellis and his team made brilliant use of the data they were able to find, says Rebecca Cyr, a demographer at the London School of Hygiene and Tropical Medicine, who was not involved in the research. “The amount of things we know about the demographics of whales seems impressive to me, considering that they live in the marine environment,” he says.

Cyr believes the team's hypothesis is plausible, but he also points out that the researchers were able to analyze a relatively small number of cetaceans. “It seems to me that we have to be very careful in this kind of work,” he adds. They are very interesting and useful, but by no means provide conclusive evidence about the causes that led to the onset of menopause.