Yet the multigenerational field study, published Wednesday in the journal Current Biology, is the first time scientists have observed that this preferential treatment extracts a permanent toll on whale mothers. With dwindling food supplies in the Pacific, the strategy is now adding to concerns over the orca population’s overall future, researchers said.

“For their whole life, these sons are imposing a cost on their mother, and this is highly unusual,” Sam Ellis, an animal behavioral researcher at England’s University of Exeter, who co-wrote the study, said in an interview. The study is also the first time scientists have documented a lifelong toll on mothers in any large mammal species that usually reproduces more than once.

Researchers studied decades-worth of census data that tracked 40 female orcas living off the Pacific coast of Washington state and British Columbia since 1982.

They found that female orcas who had a son were about 50 percent less likely than those who had daughters to have another whale baby survive its first year of life, in any given year. That held true even after the sons reached adulthood, from 12 to 15 years old.

In contrast, researchers found no evidence that raising female offspring — who achieve complete independence from their parent and do not demand continued feeding as adults — reduce a mother’s future reproductive chances.

“What the killer whales seem to be doing is having a son, and investing in that son for the rest of their life,” Ellis said. “That is regardless of whether the son is 3 or 30.”

“It’s unusual because you would expect under most scenarios your best option as a female is to have your offspring, get them big and strong, and then have another offspring,” Ellis added. When orca mothers catch large salmon, they feed portions to their infants — and any adult sons, he said.

The strategy of orca whale moms feeding males into adulthood may have an evolutionary motivation — as these sons often go on to father many more offspring of their own.

By giving male offspring preferential treatment into adulthood, mothers may increase their chance of raising a son who grows up to become the dominant member of the group, Ellis said. In any given year, it is the oldest male in a group of orca whales — or occasionally the two oldest — who will father all the offspring in a group, Ellis explained.

Only 73 southern resident orcas exist in the wild. Scientists just discovered that 3 of them are pregnant.

So by raising her son to become the dominant group member — even if it means neglecting her female offspring and sacrificing her future child-rearing capacity — a mother killer whale could still be increasing her chance of becoming a grandmother to many.

“From a female’s point of view, if you invest in your son — if you keep your son alive, and keep them big, you might hit the jackpot and have loads of grand-offspring from your son,” Ellis said.

But less available salmon means orca moms may now be feeding their sons at the cost of their own survival. “When in the past there were lots of salmon — this strategy of investing everything in your son was a really great idea,” Ellis said.

Orca vs. shark: Rare drone footage shows killer whales mauling great whites

But “the amount of salmon available is just declining” in the Pacific, he added. “If there was more salmon around, this increased investment in males would matter less — because there’s still enough food for females to both invest in their sons and invest in themselves.”

There are just 73 southern resident killer whales alive in the Pacific, a population that has been in decline since the early 1990s.