The Washington Post via Getty Images Is it evolutionary biology that grandmothers babysit?

What do menopausal women have in common with killer whales? Tread lightly there, friend.

An international research team from the universities of Exeter, Cambridge and York (U.K.), the Center for Whale Research (U.S.) and Fisheries and Oceans Canada is tackling one of evolutionary biology’s biggest puzzles: Why is it that in only three species do females live past their reproductive age ― humans, killer whales and pilot whales?

The prevailing theory has been that grandmothers ― both human and whale ― are needed to help care for the offspring of their adult children. But Darren Croft, co-author of a study recently published in “Current Biology,” told The Huffington Post that there may be something else at play: Mother-daughter conflict.

To produce the strongest gene pool possible, older females have to stop competing with their daughters. When grandmothers help raise their grandchildren, at least for whales, those offspring do better than the calves raised by just a mother alone, according to the study. Hence, it’s in a whale population’s best genetic interest for older females to stop producing more offspring, since their genetic line is more likely to survive if they just help out with the grand-calves.

There could be a similar explanation for why human females lose the ability to reproduce long before their death, according to Croft. But there’s more.

Reproductive conflict between generations is the newest piece of the puzzle. Older female whales may stop reproducing because their calves don’t fare as well in the competition for resources ― food. The calves with younger mothers do better and have higher survival rates.

In the new study Croft and his colleagues used a long-term dataset on wild resident killer whales of the Pacific Northwest. The authors found that when older and younger females breed at the same time, selection will favor younger females. Likewise, it’s better for older females ― those younger females’ mothers and grandmothers ― to compete less and cooperate more.

When mothers and daughters breed at the same time, the calves of the older females are 1.7 times more likely to die than the calves of younger females, the study found.

The bottom line, Croft says, is that menopause is no accident. Rather, it’s an evolved trait driven by both cooperation and conflict in family groups.

Killer whales start reproducing around age 15 and enter menopause around age 30 to 40. But they live well into their 90s. “In our previous work,” Croft said in an email to HuffPost, “we have shown that these old females boost the survival of their adult offspring by helping them to find food.” He added, “This however did not explain why they stop reproducing. Our new work shows that a key mechanism proposed to explain menopause in ancestral humans is also at play in killer whales.”

“In humans, we know that grandmothers provide important benefits to their grand offspring ― which can explain the benefits of the long female lifespan seen in humans,” he added. “[But] what has been difficult to explain is why females don’t keep reproducing until the end of life.”

If an old female killer whale reproduces, her late-life offspring suffer from being out-competed by her grandchildren. This, together with her investment in helping her grandchildren, can explain the evolution of menopause, said the study.