A small population of orcas in the Pacific Northwest has been harassing and killing young porpoises for decades. The odd behavior is not hunting—these orcas just eat fish—and as a result, it has long perplexed scientists.
Now, after analyzing 78 recorded interactions between 1962 and 2020, a team of researchers has come up with three plausible explanations for the bullying-like activity: social play, hunting practice or “mismothering” behavior, in which the orca tries to care for the other animal. The scientists published their findings last week in the journal Marine Mammal Science.
Southern Resident killer whales (Orcinus orca) are an endangered population of orcas in the North Pacific, made up of only about 75 individuals in three separate pods. The animals primarily prey on Chinook salmon, which have also been declining in number because of overfishing, habitat loss and dams that impede their migrations. Though they can sometimes grow larger, these salmon typically reach about 3 feet and 30 pounds—roughly the same size as the orcas’ porpoise targets.
“I am frequently asked, why don’t the Southern Residents just eat seals or porpoises instead?” Deborah Giles, a Southern Resident killer whale expert at the nonprofit Wild Orca and co-author of the paper, says in a statement. “It’s because fish-eating killer whales have a completely different ecology and culture from orcas that eat marine mammals—even though the two populations live in the same waters. So, we must conclude that their interactions with porpoises serve a different purpose, but this purpose has only been speculation until now.”
Orcas are cognitively complex animals that engage in playful activities to bond with one another. Individuals in the Pacific Northwest have been known to play with buoys and crab traps, while a group of Iberian orcas has been ramming into boats in what some have called playful behavior (though researchers aren’t entirely sure of the animals’ motivation).
The authors hypothesize in the new study that harassing porpoises may be another way the orcas entertain themselves, especially because they don’t seem to have an intended outcome in mind—the orcas continue engaging with porpoises even after they’ve killed them.
The team’s second hypothesis, hunting practice, is closely tied to play. Often, killer whales will catch and manipulate their food before eating it, which may allow young members to learn and practice hunting skills. Mammal-eating Bigg’s killer whales, for example, sometimes kill surplus amounts of harbor seals, which might allow calves to learn to hunt, per the study.
“We’ve heard of orcas who show their young how to get food in different ways. And sometimes they show them how to do it, and then they don’t actually eat the animal,” Lori Marino, a neuroscientist who studies cetacean intelligence and was not involved in the study, tells Insider’s Grace Eliza Goodwin and Maiya Focht.
The most experienced hunters—the post-reproductive female orcas—were the least involved in harassing porpoises, per the study. Instead, it was mostly juveniles, calves and adults that might have been teaching the behavior.
However, play and practice for hunting can sometimes be indistinguishable, so harassing porpoises may serve both purposes for the Southern Resident killer whales, the authors write.
The researchers’ last explanation is mismothering behavior—or mis-attempts at caring for porpoises they may view as weak or ill. Such empathetic behavior has been documented in killer whales before, such as in one case where a Southern Resident mother carried her dead calf for 17 days.
“Mismothering behavior—also known as ‘displaced epimeletic behavior’ to scientists—might be due to their limited opportunities to care for young,” Giles says in the statement. “Our research has shown that due to malnutrition, nearly 70 percent of Southern Resident killer whale pregnancies have resulted in miscarriages or calves that died right away after birth.”
The authors say in the statement that researchers may never fully know why orcas are bullying porpoises, but the action does highlight the importance of preserving their Chinook salmon prey.
“Porpoise-harassing behavior has been passed on through generations and across social groupings. It’s an amazing example of killer whale culture,” co-author Sarah Teman, a biologist at the University of California Davis School of Veterinary Medicine, says in the statement. “Still, we don’t expect the Southern Resident killer whales to start eating porpoises. The culture of eating salmon is deeply ingrained in Southern Resident society. These whales need healthy salmon populations to survive.”