A clue to the evolution of menopause may come from older female killer whales who often take the lead in salmon hunts.
Among the whales that feast on chinook along the coast of the Pacific Northwest, females past reproductive age often lead hunting parties, especially in fish-sparse years, says Lauren Brent of the University of Exeter in England.
Male killer whales rarely live longer than 50 years, Brent says. But females can live into their 90s. Sharing their long experience in these waters may give their kin an edge in finding food. And that advantage might help push the evolution of long life after menopause, Brent and her colleagues suggest March 5 in Current Biology.
This salmon-hunting population of Orcinus orca whales is among only three kinds of mammals in which females can live long after they stop having babies themselves. The other species with such extreme postreproductive lives among females are short-finned pilot whales (but not their long-finned relatives) and humans.
The new leadership study gives the first evidence for information sharing as a possible way for old whales to boost the survival of their offspring, Brent says. Among these killer whales, both sons and daughters stick around mom even after they’re grown up, an unusual closeness for a mammal. So if their postmenopausal mom’s deep knowledge of local ecology helps them thrive and reproduce, evolution might favor longer life after menopause.
In 2012, a research paper on the same fish-hunter whale population reported that mothers seemed to enhance the survival of their adult offspring. When a female beyond her reproductive years died, any sons older than 30 faced a 14-fold jump in the risk of disappearing (presumably by dying themselves) in their first year without mom, according to research led by Darren Croft, also of Exeter and also senior author of the new work. Younger daughters faced a fivefold increase of death in the first year without their mothers.
“But how were they doing it?” Brent asks. She and her colleagues matched up fishery data about salmon abundance in the area with 751 hours of video of traveling whale groups. With records from the Center for Whale Research in Friday Harbor, Washington, expert spotters used telltale healed wounds and color-patch quirks to identify the front whale, presumably the leader, and the followers in each little hunting group. Out of 102 known individuals in these filmed journeys, the postreproductive females scored higher in leadership, increasingly so in years when salmon runs were sparse.
Males were especially likely to follow their mothers, the researchers found. This tendency might explain why the loss of older mothers had a bigger impact on male mortality, Brent suggests.
Anthropologist Sarah Blaffer Hrdy of University of California, Davis points out that the data are not so much about menopause per se as about an advantage for female whales to live longer afterward. There’s a difference, she notes, in ways of framing the discussion of how menopause evolved. It could be a matter of females stopping having young early, or — and she finds this more likely — it could be about the evolutionary advantage of females extending their life span after their reproductive years have already ended.