Menopause is all too familiar to women, but in other species it’s remarkably rare. Last year, scientists reported that females in a single population of chimpanzees live long past their reproductive years. But aside from chimps and humans, researchers have found clear evidence of menopause in only five species — all of them whales.
Scientists have long debated why menopause evolved. Perhaps it provided an evolutionary edge to females, or maybe 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 being pregnant at the same time as their daughters, avoiding resource conflicts that would hurt both of their offspring.
Samuel Ellis, a biologist at the University of Exeter who led the study, published in Nature, said that whales may have evolved menopause for the same reasons that humans did.
“Perhaps this is such an unusual strategy that there’s only one way to get there,” Dr. Ellis said.
In the vast majority of species, females keep producing eggs throughout their lives. That pattern makes sense in terms of natural selection. The more offspring that a female can successfully raise throughout her life, the more copies of her genes get passed down to future generations. Even long-lived females typically fit this pattern: Female elephants, for example, stay fertile into their 60s.
Five whale species — killer whales, false killer whales, beluga whales, short-finned pilot whales and narwhals — don’t fit that pattern. Female killer whales, for example, generally breed only 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 on the surface. But the other menopausal species live far from shore and spend a lot of time diving.
“So many of these species are so cryptic,” Dr. Ellis said. “The ocean is a big place.”
Rather than chasing after the whales, Dr. Ellis and his colleagues tried to squeeze insights from data that marine biologists have already collected. Sometimes groups of whales wash ashore in mass strandings, for example. As marine biologists examine the bodies of their animals, they make estimates of their age 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 don’t go through menopause, such as dolphins and sperm whales. Using statistical equations, Dr. Ellis and his colleagues estimated the average life span of the whales, the number of offspring they produce and how long they remain fertile.
In the species that don’t go through menopause, female whales fit the same trend: Bigger whales tended to live longer.
A different pattern emerged among the five menopausal species. The female whales stayed fertile for as long as you’d predict for a whale their size. But they then lived, on average, for 40 years beyond their predicted life span.
This finding suggests that menopause did not evolve thanks to mutations that shortened the reproductive years of the whales. Instead, natural selection must have favored mutations that added more years to the animals’ life after their reproduction stopped.
So what sort of evolutionary advantage could have come from these new reproductive behaviors? One possibility is that older females no longer give birth at the same time as their own offspring are giving birth. That way, they don’t come into conflict. In the long term, the researchers suggest, avoiding this conflict would allow the menopausal whales to pass down more of their genes.
Instead of coming into conflict with their offspring, the older female whales could provide help. In previous studies on killer whales, researchers have found that older females lead their pods on long journeys. In studies of people, too, researchers have found that grandmothers can provide extra food that raise the odds of their grandchildren’s survival.
The fact that only five whale species are known to have evolved menopause suggests that this advantage can only be gained under certain circumstances. Dr. Ellis speculated that a species needs a particular social life, in which females stay in a group for a long 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 research, said that Dr. Ellis and his colleagues made ingenious use of what data they could find.
“I think it’s astonishing how much we know about whale demography, given that they live in the ocean,” she said.
She said their hypothesis is plausible, but also pointed out that they could analyze relatively few whales.
“I think we do need to be very cautious about this kind of work,” Dr. Sear said. “It’s really interesting and informative, but it does not provide conclusive evidence on why menopause evolved at all.”
