Twenty years ago, in the waters off the coast of Washington State and British Columbia, an adult female killer whale (dubbed K16 by those who know her well) gave birth to a son, K35.

"These two have an extremely close social relationship," says Michael Weiss, the research director at the Center for Whale Research in Friday Harbor, Washington.

It's hard for Weiss to think of a time when he didn't catch the pair hanging out in the same group, and often immediately next to each other.

"[They're] just this pair of whales that are basically each other's best friend," he said.

Weiss has observed the mom and son pair spending a lot of time close together, touching and floating at the surface — and sharing salmon.

Male orcas are massive, and so are their appetites. They're also less maneuverable, which may make it harder for them to catch prey. All this means that a male like K35 needs help getting enough food.

So his mom will often dive down, says Weiss, "catch a salmon, and bring it up to the surface and actually bite half of the fish off and leave that half for her son. So she's sharing a huge amount of food."

And here's the striking thing — since K35 was born two decades ago, his mom has never had another calf. Contrast that with other females in the population.

"Some whales started reproducing at the same age around the same time," says Weiss, "and they had daughters. And they've produced three or four offspring."

It's not just K16. In a paper out this week in the journal Current Biology, Weiss and his colleagues looked back across four decades of life history records of Southern Resident killer whales in the Pacific Northwest. The trend was clear:

"Killer whale mothers pay a really huge cost to take care of their sons," says Weiss. That cost is that they have fewer offspring. "And they do this throughout their son's life and never really stop paying that cost to keep their sons alive."

In fact, when a mom dies, her son usually perishes within a year or two. "We think that is largely because they're seeing a huge reduction in the amount of food they get," explains Weiss.

Weiss can't think of another animal that makes this never-ending investment when it has the option of reproducing multiple times. So why would these orca moms sacrifice so much for their sons? Weiss argues the potential payoff is huge.

"K35 is now one of our biggest males in the population," says Weiss. "He's grown big and healthy and looks good."

That means he's ready to become a father.

"Honestly, I wouldn't be surprised if some of the next few calves that get popped out in the population are his," says Weiss.

The result is that his mom would become a grandmother multiple times over. Her genes would end up in a bunch of calves. And because they'll be born into other pods, she wouldn't have to spend any effort raising them.

"It's an amazing piece of work," says Eve Jourdain, the director of the Norwegian Orca Survey. Jourdain, who wasn't involved in the study, is hoping to conduct a similar research project in Norway.

"There could be direct comparisons possible across populations," says Jourdain. "And only then can we start getting a better understanding of how important it is for the conservation of those different populations."

It's worth mentioning that this strategy of moms investing so much in their sons has a dark side. Southern Resident killer whales like K16 and K35 are in trouble. In recent years, the population has cratered to just 73 animals. And what these orcas really need right now is more reproductive females.

"That's how you keep a population of slow-breeding animals going," says Weiss. "So investing in sons for a population like ours that is so stressed is really not ideal."

Weiss worries that this maternal strategy, which served this population so well in the past, could raise their risk of extinction — that the kind of lifelong bonds he's seen between K16 and her son may tether these creatures to an uncertain fate.

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