Why female killer whales go through menopause
The New York Times
For anyone with ovaries, menopause is a fact of life — seemingly mundane, perhaps, in its inevitability. In fact, menopause is a biological rarity, one scientists haven’t managed to fully explain. Only three species outlive their fertility: humans, killer whales and short-finned pilot whales. Figuring out what commonalities exist among these species might help scientists understand why menopause happens.
A new study on killer whales, published earlier this month in Current Biology, suggests reproductive conflict between mothers and daughters may have played an important role in the evolution of menopause. Analyzing four decades of data on killer whales in the Pacific Northwest, the authors found that when mothers and daughters breed around the same time, the calves of the older females had higher mortality rates than those of the younger females.
Female killer whales typically start reproducing at age 15, and stop in their 30s and 40s. Yet they can live to be more than 90, meaning they might spend up to two-thirds of their lives not birthing any offspring. In the framework of evolution this seems to make little sense: One might expect that female killer whales that continue reproducing throughout their lives would pass on more of their genes.
But the unique demography of killer whale social groups may motivate younger females to invest more competitive effort into reproduction, tipping the costs and benefits of reproduction for older females, said Darren Croft, a professor of animal behavior at the University of Exeter in England and an author of the new paper.
Killer whale groups are matriarchal, with sons and daughters living with their mothers, but not their fathers, throughout their lives.
“When females are born, they have a relatively low relatedness to the males in their group, because their father isn’t around,” Mr. Croft said. “But as a female starts to reproduce, her relatedness to males increases, because her sons stay with her.”
Because young females are related to fewer males in their pods, the researchers believe they have less of a stake in the success of the wider group, and therefore invest more resources into competition, perhaps by hoarding food for themselves and their offspring,or fighting with other group members. But older females who have more offspring, and thus share more of the group’s genes, are more likely to cooperate by sharing food and knowledge.
To test their hypothesis, the researchers analyzed demographic data over 43 years for 200 whales. This unparalleled database of killer whales was an important strength of the study, said Ruth Esteban, who researches killer whales with Conservation, Information and Research on Cetaceans in Spain, and did not participate in this research.
The scientists found that as younger females aged and had offspring, they indeed became more related to their pods. Then, the researchers looked for instances in which mothers and daughters bred simultaneously. In those situations, calves born to older mothers were 1.7 times as likely to die in the first 15 years of life as those born to the younger mothers.
Mr. Croft’s team thinks reproductive conflict may also underpin human menopause.
There is by no means consensus, though. Many other theories exist, including the grandmother hypothesis, partly developed by Kristen Hawkes, a professor of anthropology at the University of Utah. She believes the primary reason women live so long after menopause is because they help improve the survival of grandchildren, which helps pass their own genes on.
But others contest the grandmother hypothesis as a single explanation for menopause. The benefits females get from helping grandchildren, who carry only a quarter of their genes, are “far too small to ever favor stopping reproduction in the first place,” said Michael Cant, a professor of evolutionary biology at the University of Exeter and one of Mr. Croft’s co-authors.