A female killer whale and her newborn calf Reuters

Granny hasn’t been seen since last October, and those who know her suspect that she’s dead.

She was an orca, or killer whale—part of a 24-strong group called J-pod that lives in the Pacific Northwest. That pod has been studied by scientists for over 40 years, and Granny has always been a common sight, identifiable by the small half-moon notch in her back fin and the grey saddle patch behind it. Based on those sightings, and on Granny’s family ties, researchers have estimated that she was at least 74 years old at the time of her disappearance, and as possibly as old as 105.

The exact length of her life will never be known—her body hasn’t been found and is probably forever lost to the Pacific. But what’s clear is this: Granny had stopped reproducing sometime before humans got to know her. In the last four decades, she has never given birth to a calf. “I find that absolutely incredible,” says Darren Croft from the University of Exeter. “Her longevity isn’t the interesting thing;  it’s her life without a calf.”

Almost all animals reproduce until they die, even very long-lived ones like elephants and blue whales. As far as we know, just three species buck the trend: killer whales, short-finned pilot whales, and humans. Women go through menopause between 45 and 55 even though they live for a long time more; the same is true for hunter-gatherers without access to modern medicine. Likewise, female killer whales stop reproducing in their 30s or 40s, but might live for decades more. That’s an evolutionary mystery. Why give up so many chances to pass your genes to the next generation?

This question is really two separate ones. First, why stop reproducing early? Second, why survive beyond that point?

Let’s start with the second. A popular idea called the “grandmother hypothesis”, proposed in the 1960s, says that older women play a vital role in helping to feed, raise, and teach their children and grandchildren. In doing so, they’re still ensuring the success of their genetic legacy. There’s some evidence for this in humans: one study of Finns and Canadians found that women who live longest after menopause end up with more grandchildren, because their daughters are better at having and raising kids.

The same is true for killer whales. We know this because scientists led by Ken Balcomb have spent the last 40 years monitoring the lives, family ties, and deaths of the Pacific Northwest orcas—Granny included. In 2012, Croft, Balcomb, and student Emma Foster used that census to show that mothers clearly help their children to survive—and their sons in particular. If a male orca’s mother dies, his odds of joining her in the following year go up by three times if he’s younger than thirty, by eight times if he’s older than 30, and by a whopping 14 times if the mother was post-menopausal.

So mother orcas, especially older ones, are somehow helping to keep their sons alive (and older sons in particular). Croft’s colleague Lauren Brent worked out how in 2015. Using hundreds of hours of video footage, she showed that older females guide their pods—especially males—to hotspots of salmon, which make up the vast majority of their diet. “These whales live on a knife edge and this knowledge about when and where to find food is vital for keeping the group alive,” says Croft. That’s partly why Granny’s death was so sad. “She had a very important role to play, and her pod is in a really difficult place.”

But none of this explains why female orcas (or humans, for that matter) should forgo reproduction early. Sure, they use their knowledge to help their families, but why not continue to expand those families by having more children? After all, that’s what elephants do. The oldest matriarchs guide their herds to sources of water, and teach them the right anti-lion strategies, all while continuing to reproduce. In humans, orcas, and elephants, grandmothers matter—yet only in the first two are they also infertile. Why?

The answer lies not in cooperation, but in conflict. Animals that raise their young in groups don’t have infinite resources. There’s only so much food, attention, and care to go around. So adding an extra child to the mix—another mouth to feed, another body to watch—is partly an act of competition. And the outcome of that competition depends on the family structure of each species.

Among ancestral humans, sons tended to stay with their mothers while daughters moved out to join new families. When one such daughter begins her new life, she’s initially unrelated to anyone around her. With no relatives to help, her best way of passing her genes to the next generation is to have children. As she does, she becomes increasingly related to her new family group, and shares more and more genes with them. Now, she can also enhance her genetic legacy by helping her own relatives raise their children. She may even be better off doing so; if she had any more children, they would be in direct competition with her existing descendants.  

So, as Rufus Johnstone and Michael Cant from the University of Cambridge showed in 2008, evolutionary theory predicts that that older women are better off if they stop reproducing and focus on helping their kin. Because of the family dynamics in our species, menopause should naturally evolve.

That’s not true for elephants. In their societies, daughters stay at home, while sons leave. This means that females are surrounded by female relatives throughout their lives, and theory predicts that their strategies shouldn’t change with time. A matriarch’s best bet is to carry on reproducing for as long as she can. (Some sources wrongly claim that elephants go through menopause; some individuals might stop reproducing early, especially in captivity, but the species as a whole does not.)

Killer whales (and short-finned pilot whales) have different social structures again, where both sons and daughters stay with their mothers. Females will happily mate with outsiders, but they always return to their birth pod to raise their calves. This means that young females are relatively unrelated to the males in their group; after all, their fathers come from different pods. But as she gets older, her relatedness to nearby males increases as she accumulates sons and grandsons. This leads to essentially the same pattern as in humans: female orcas become increasingly more related to their neighbors as they get older.

Croft’s group, together with Cant and Johnstone, have now confirmed this pattern by looking at the shifting family ties of the Pacific Northwest orcas. And by building a mathematical simulation, they predicted that it leads to the same outcome: Orcas ought to evolve menopause, because younger females benefit more from having children, while older ones benefit more by helping their offspring.

That’s also evident in the records. The team showed that if two generations of killer whales breed at the same time, the death rates for calves born to older mothers are 1.7 times higher than for those born to younger mothers. “This explains why it is a better option for older females to stop having more calves and instead focus on helping their existing calves, for example by using their lifetime of accumulated knowledge to lead the family to good feeding grounds,” says Hazel Nichols from Liverpool John Moores University, who studies mammal societies.

Why do the new calves of older mothers do so poorly? It’s not clear. These animals share food, so perhaps younger females are hoarding more for themselves, or sharing exclusively with their own young. Or perhaps they’re less likely to babysit or suckle another female’s child. “There’s this tug-of-war between older and younger females,” says Croft. “It’s not just that the older ones are stepping back and saying: It’s your turn. It’s also that the younger generation is chomping at the bit to reproduce. And this reproductive conflict, when taken together with the benefits of grandmothers, can unravel the mystery of why menopause evolved.”

That was only possible because Balcomb and others have spent decades amassing data on the Pacific Northwest orcas. “It’s probably the only data set where we can test the generality of the evolutionary theory that was developed for humans,” says Croft.

Rebecca Sear from the London School of Hygiene and Tropical Medicine, who has studied menopause in humans, agrees. “Working with humans, I am incredibly fortunate in being able to download data at the click of mouse, so I’m filled with awe at the work involved in collecting this kind of demographic data on this kind of species,” she says. “There isn’t an overwhelming amount of empirical evidence in humans that older women really suffer higher costs in reproductive conflict with younger women, but that may be because few studies have actually tested for this yet.”

It’s hard to do so for humans, because modern medicine has so utterly reduced mortality and increased longevity. With killer whales, it’s almost the opposite. “We’ve fished the salmon out, polluted their water, and disturbed them with industrialization,” says Croft. “They’re really, really struggling.” And now, with Granny gone, her children and grandchildren face an even more uncertain future.

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