Humans and killer whales parted ways millions of years ago, evolutionarily speaking, yet the stately cetaceans have a lot in common with us: complex brains, close-knit family groups and, not least of all, grandmothers. Even though orca grannies may not sport white hair and glasses, they, too, go through menopause—a rarity in the animal kingdom known to affect only two nonhuman species: orcas and pilot whales. The question for biologists is, Why would a species live long past reproductive viability?

In an attempt to solve what he calls “a big evolutionary puzzle,” behavioral ecologist and author Darren Croft and his colleagues delved into 40 years of data on births and deaths among two groups of Pacific Northwest killer whales—data collected by surveying the same ocean spots every year and identifying individual whales by their markings and scars. Female orcas stop reproducing in their 30s or 40s but can live into their 90s, and offspring stay in the same group as their mother. Previously the team established that postreproductive females help their adult offspring survive and are important group leaders. “These old females actually act as repositories for ecological knowledge of when and where to find food,” Croft explains.

These findings mesh with the grandmother hypothesis, which states that evolution favored long-living grannies because they support their children in reproducing and help to ensure their grandchildren's survival. The researchers suspected, though, that the value of having older females around was not sufficient to drive the evolution of menopause (older female elephants, for example, are matriarchs yet reproduce until death).

They decided to test the reproductive-conflict hypothesis, originally developed for ancestral humans, which suggests menopause evolved because older females were related to more members of the group but were less able to compete reproductively with younger females. When the researchers tested this hypothesis with killer whales, they found that older females do indeed have more relatives in the group, and when grandmothers and their daughters breed simultaneously, the grandmothers are less likely to have offspring that survive. As a result, older females switch from reproductive competition to cooperation for the benefit of their relatives, the team reported in January in Current Biology. Croft says, however, that the findings do not negate the grandmother hypothesis—rather they add to it, offering a concrete mechanism that may have driven the evolution of menopause.

Lori Marino, a neuroscientist and expert in animal intelligence who heads the Whale Sanctuary Project and was not involved in the study, agrees. “It's typically never a single explanation for complex behavior in complex animals,” she says, adding that although humans and orcas inhabit very different environments, their similar social structures have caused their behavior to converge. Croft concurs: “The fact that we can draw these similarities between killer whales and humans—I just find that absolutely fascinating.”