But there’s something even more special about Shachi: At 40 years of age, she is not just a matriarch, but a grandmother. Shachi gave birth in 2005 to a female called Eclipse, who a decade later produced a male named Nova. Eclipse, at 10, was the youngest female ever known to reproduce, and she did so after a period of low numbers of Chinook salmon — a fish the orcas rely on.

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“Most of the calves that were born in that period did not survive,” said Michael Weiss, a behavioral ecologist at the University of Exeter in the United Kingdom who has been studying this whale population since 2012. “But [Nova], the son of this small, inexperienced mother, is still growing, looking healthy, and is one of the most active, social members of the pod.”

How did Nova make it? Weiss gives credit to Grandma Shachi, who often stuck by Nova’s side as his mother was off foraging. Shachi “seems to have really taken on a major caregiving role,” he said.

The dividends paid by Shachi’s extra effort with her grand-offspring were not unique, according to a study published Monday on the “grandmother effect” in this orca population. If a southern resident grandmother orca dies, her grand-whales are much more likely to perish within two years, it found. That death sentence becomes even more probable if she dies when Chinook salmon are sparse, suggesting that active grandmothers act as a sort of buffer between their grandbabies and famine.

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Using more than 40 years’ worth of observational data, the researchers pieced together family relationships, births, deaths and salmon abundance statistics to show that the grandmother effect is a powerful evolutionary strategy. So powerful, in fact, that the death of a grandmother can negatively affect a grand-whale’s chance of survival even after it has reached adulthood.

“The grandmother effect that we have shown also appears to impact whales for their entire lives,” said Daniel Franks, a biologist at the University of York and senior author of the study, which was published in Proceedings of the National Academy of Sciences.

Apart from humans and resident killer whales like those in the Salish Sea, just three other species — all whales — are known to undergo menopause, which ends the ability to reproduce. It’s not known whether more free-ranging populations of killer whales, known as transient orcas, experience menopause.

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And for a long time, scientists struggled to explain why evolution would favor animals that can live long past their reproductive prime.

Take the former leader of J Pod, an orca known as Granny, or J2. Scientists say Granny may have been well over 90 years old when she disappeared from the Salish Sea in 2016. Female orcas stop reproducing at about 45 years of age, meaning Granny spent about a half-century swimming around not doing the one thing evolution wants animals to do — make copies of themselves.

But in the context of the grandmother effect, Granny’s reign makes sense.

Granny “was always the leader, typically traveling anywhere from 100 yards to a mile in front of the rest of the pod,” said Weiss, a co-author on the study.

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“She was the oldest female in J-Pod, and it was pretty obvious a lot of the time that she was in an elite position,” said Bradley Hanson, team leader for the marine mammal ecology program at the National Oceanic and Atmospheric Administration’s Northwest Fisheries Science Center.

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Hanson, who was not involved in the study, said it offers more evidence that orcas’ social structure “is extremely important in the survivorship of the population.” That knowledge, he said, could aid efforts to recover populations of resident orcas, which are considered endangered in the United States and Canada.

The grandmother effect becomes even more pronounced after an orca has gone through menopause, the study found. The researchers surmise that this is because post-reproductive grandmothers have more time and resources to share when they’re not taking care of their own calves.

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“I think this is incredibly exciting,” Franks said.

Other research has found evidence for the grandmother effect in humans, but the new study is the first to document it in a nonhuman menopausal species. And while scientists often caution against anthropomorphism, it provides another point of comparison between us and these intelligent and emotionally complex marine mammals. Last summer, a female in Shachi’s pod, Tahlequah, offered a mesmerizing display of the species’ capacity to mourn, carrying the body of her dead calf for 17 days.

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But there are words of caution embedded in the new findings. Just 73 resident killer whales remained in the Salish Sea as of July. According to the Environmental Protection Agency, dams, agricultural runoff, overfishing and other human-made problems have decimated the whales’ primary food, Chinook salmon.

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“This study is one of many that are a warning of the danger to these whales if salmon populations are continued to be mismanaged,” Franks said. “When the salmon are not doing well, the killer whales do not do well, and there is very little time left to take action.”

All of which makes whales like Shachi even more irreplaceable, the researchers say.

“I have seen her go from a mother, to a grandmother, to the leader of the pod,” Weiss said. “I feel immensely privileged to be able to see such a dramatic life story play out.”