To understand the evolution of menopause, just look at family drama—in killer whales

Mother-daughter conflict could help explain why women live beyond child-bearing years

killer whales
If you're an orca, female competition can be killerPublic Domain

While many animals become less fertile as they age, only three species—humans, pilot whales, and killer whales—have females that regularly live well beyond their reproductive prime. These are the only species where we see grandmas acting like grandmas: they've long stopped producing offspring of their own, so they pivot to helping care for their children's children.

When something is as rare as menopause, scientists start to ask why it ever happens. Previous research has suggested that something known as the "grandmother hypothesis" is at work. When grandmas do the whole grandma thing, their grandchildren are more likely to survive and thrive. It's in their own best interest, genetically speaking, to live to a ripe, old, infertile age.

A study published Thursday in Current Biology takes this a step further: Yes, baby orcas with grandmothers are known to do better than those who only have a mother caring for them. But it might also be the case that killer whale calves that have to compete with their nieces and nephews for resources—in other words, the calves of older matriarchs who already have grown offspring and calves of their own—might not do very well.

Grandmas-long-past may have done a better job of passing their genetic material along when they stopped competing with their own daughters.

"It's easy to think that an older female will pass on their genes better by continuing to give birth in late life," study co-author Daniel Franks of the University of York said in a statement. "But our new work shows that 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."

It would have gone something like this: at some point in the orca's evolutionary history—probably relatively recently, since menopause is so rare even among their close relatives—resource competition meant that orcas who stopped reproducing earlier in life, through some biological quirk or another, actually had more surviving descendants than orcas who kept having babies until they died. Investing in grandchildren proved a better genetic strategy than constant baby-making, maybe because younger females are able to claim more food for their offspring than their mothers can, or simply because they have fewer children to worry over.

Over time, as these badass matriarchs spread their unique lack-of-fertility throughout the gene pool, this became the norm. Or something like that, anyway. Males have a shorter lifespan than females—they tend to die around the same age that female whales go through menopause, at which point the females live for decades more—so it's likely that the evolution of menopause required some evolutionarily-driven lifespan lengthening, too. More grandmas means more healthy babies in a pod.

What's fascinating about the new study is that it presents the evolution of menopause as a two way street—a tug-of-war between young females and old. It's the result of heartwarming cooperation, yes, but also of family conflict. When females of two generations breed at the same time, according to the study, the older females' calves are nearly twice as likely to die.

A 100-year-old orca known as "Granny", now thought to be deceased, was indeed an exemplary grandmother: like all killer whale pods, Granny's stayed together through the generations. Orca kids don't move away; they just stick around and have more babies, breeding back and forth between different maternal lines in the group. By the time Granny vanished in late 2016, it had likely been half a century or more since she birthed her last calf. But her longtime grandmother status made Granny the de facto leader of her pod, and her care undoubtedly helped her descendants to survive the Pacific's ever-dwindling supply of salmon.

The mystery of why, how, and when menopause evolved in whales isn't yet solved, and humans are another matter entirely. But matriarchs like Granny can help biologists get closer to figuring it all out.