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.”