At least 78 times over the last 60 years, southern resident killer whales have done something baffling: they have chased down porpoises, pushing the smaller cetaceans with their snouts, holding and carrying them in their mouths, balancing them on their heads or bodies or between the pectoral fins of two killer whales swimming side by side, occasionally tossing the porpoises into the air, and sometimes killing them.

But the orcas have never eaten the porpoises.

“The bottom line is we don’t know why they do it,” says Deborah Giles, science and research director of the nonprofit Wild Orca, who co-led a new study investigating the behavior.

Instead what emerges from these puzzling encounters between two sleek-bodied cetaceans is the importance of a different kind of creature: Chinook salmon, which make up 50–100% of the southern residents’ diet, depending on the season.

“It’s a reminder that these animals could catch other food sources,” study team member Joe Gaydos, science director of the nonprofit SeaDoc Society, says of the southern residents, who historically have spent a lot of their time in the inland waters of the Salish Sea. “It’s not going to happen. They are salmon eaters. They’re going to be salmon eaters. We’ve got to recover salmon.”

In the new study, 20 scientists from 16 different organizations assembled accounts of porpoise harassment from researchers, whale watch organizations, marine mammal stranding response programs, and the public. They looked for reports in databases of government and whale research organizations, peer-reviewed papers and other written accounts, and directly from people who witnessed the events.

All three pods – J, K, and L – that make up the southern resident community have engaged in porpoise harassment, the researchers report in a paper published September 28 in Marine Mammal Science.

Most of the incidents involved between two and four killer whales, but some were carried out by a lone orca and a few involved groups of 15 to 25. The orcas most often targeted harbor porpoises, occasionally Dall’s porpoises, and usually a single porpoise at a time. More than one-third of the encounters resulted in the porpoise’s death.

With just 75 individuals in the population, the southern residents are endangered, and one of the main threats to their survival is lack of their preferred Chinook salmon prey. But transient or Bigg’s orcas who also inhabit the Salish Sea and who do eat porpoises as well as other marine mammals have been on the increase as the southern residents have declined. So, many people who hear about the orca-porpoise interactions wonder if the southern residents are chasing and killing porpoises to test out a new menu item, Giles says.

It's something she has wondered herself in the past. “Part of me really wants them to take a bite, you know, and eat this thing and be less hungry,” she says.

But the new analysis pretty definitively rules this explanation out, as no southern resident was ever observed to eat or even nibble on a porpoise.

Nor do the orcas appear to be killing porpoises in order to eliminate a competitor for their increasingly scarce food. If that were the case, the researchers would expect the orcas to dispatch the porpoises quickly and efficiently. But the killer whales sometimes toy with a porpoise with for up to three and a half hours, sometimes even after the smaller animal has died. And porpoises eat very little salmon, in any case.

By analyzing which whales participated in each episode of porpoise harassment and what they did, the researchers aimed to draw conclusions about the why of the behavior. For example, if it were mainly adult males who engaged in porpoise harassment, it might be some kind of display meant to impress females. But this wasn’t the case.

“Whales of all swims of life were doing it,” says study co-leader Sarah Teman, a graduate student at the University of Washington in Seattle, who worked on the analysis as a research assistant with SeaDoc Society. Only the older matriarchs seemed relatively uninterested in taking part.

Three hypotheses emerged as plausible explanations for the southern residents’ harassment of porpoises: play, practice hunting, and misdirected caretaking behavior.

The porpoise harassment often looks playful to human observers (although the porpoises, to be sure, don’t experience it that way). And juvenile orcas are the most frequent participants.

The orcas disproportionately target newborn and young porpoises, the researchers found. These animals are similar in size to adult Chinook salmon, lending credence to the hunting practice hypothesis.

For predators like killer whales, the line between play and hunting practice may be a fuzzy one, Teman says. “Killer whales, I’m sure, get enjoyment out of hunting. That’s such a huge part of their life history.”

Then again, some of what the orcas do with the porpoises looks similar to the way they interact with their own young. Mothers and other family members sometimes lift calves out of the water on their back, head, or rostrum. Killer whales are also known to carry ill or deceased calves, as the orca known as J35 or Tahlequah did over the course of 17 days and perhaps 1,000 miles in the summer of 2018.

J35 has been observed harassing porpoises three times – but all prior to 2018. Given the sparse data, the researchers say they couldn't draw any conclusions about why individual whales engage in this behavior.

Nor are the three hypotheses mutually exclusive. The motivation “could be different from whale to whale, from maybe day to day,” or could depend on the whale’s age or sex, Teman says.

The study illustrates how a novel behavior takes root in and is transmitted through an orca population, says Alfredo López Fernandez, who is part of the Grupo de trabajo Orca Atlántica studying orcas off the coast of the Iberian Peninsula, and who was not involved in the new work.

Porpoise harassment behavior was first seen in L pod and appears to have spread to K and then J pods. Today, all three pods exhibit the behavior about equally. Most of the incidents have involved individuals from three L pod and four J pod matrilines – the basic unit of resident killer whale society, consisting of a female orca and her direct descendants. But the behavior is widespread in the population, with members of 19 out of 23 matrilines participating.

“I think this is a good example of killer whale culture,” says Gaydos – that is, behavior that is passed down through observation and imitation, rather than genetically determined. “It started with specific animals and then moved to other animals within the matriline.”

Culture also explains why the southern residents see salmon as food – but never porpoises, no matter how hungry they may be. 

The new paper comes on the heels of widespread coverage of another instance of puzzling orca behavior, the Iberian orcas’ interactions with, ramming, and sometimes sinking of small boats.

There’s no direct connection between the Iberian and southern resident orcas nor between their strange-to-humans behaviors, López Fernandez and the southern resident researchers all emphasize. But the difficulty of determining the reason for porpoise harassment among southern residents, the most deeply studied orca population in the world, highlights the importance of sustained, close attention to understanding different orca cultures.

“I think that the more we study populations of whales around the world, the more we're going to learn intriguing things about them,” Giles says. “And in some ways, it's just going to make them even more mysterious.”