Orcas are Killing Porpoises But Not Eating Them

Living up to their names, killer whales have been killing other marine mammals but not eating them for decades, and scientists aren't sure exactly why.

Orcas seemingly hunt down, harass, and often kill porpoises, but then just leave the dead animals without eating them, according to a new study in the journal Marine Mammal Science.

This behavior has been observed in the Southern Resident killer whale population, a small endangered group numbering 75 individuals that usually feed on chinook salmon.

"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 Wild Orca and co-author of the paper, said 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."

Southern Resident killer whales are found in the eastern North Pacific, off British Columbia, Washington and Oregon. This population is tiny and threatened due to the whales' reliance on chinook salmon, which have been declining in quality and quantity.

"Retrospectively, we identified and analyzed 78 episodes of Southern Resident killer whales harassing porpoises between 1962 and 2020, of which 28 resulted in the porpoise's death ("phocoenacide")," the authors wrote in the paper. "Fifty-six episodes involved harbor porpoise (Phocoena phocoena), 13 involved Dall's porpoise (Phocoenoides dalli), and the porpoise species was unreported for nine episodes."

The authors suggest three possible drivers for this behavior, the first being social play. Orcas are incredibly intelligent, and have been previously seen to play among themselves for fun, bonding with others in their social group.

"Killer whales are known to 'play' with their prey with extended handling of prey seen after the prey has been captured," Patrick J. O. Miller, a professor at the Sea Mammal Research Unit at the University of St. Andrews, told Newsweek. "When it is prey, this could be explained as a behavior to teach other group members how to handle their prey – but in this case the killer whales are not thought to actually eat the porpoises, so they aren't prey."

It's thought that the spate of attacks on boats and yachts off the Iberian Peninsula by a separate population of orcas may also an example of playful behavior.

"Sometimes SRKWs would continue engaging with the porpoise well after it had died, indicating that the killing of the porpoise was not the goal," the authors wrote in the paper. "The extended duration of porpoise-harassing incidents, some lasting multiple hours, without the termination in a set goal or outcome (i.e., none were ever knowingly consumed) might suggest that the act of interacting with a porpoise itself is, in some way, advantageous or "pleasurable" to the whales."

Another reason for the porpoise harassment suggested by the authors is that the orcas might be practicing their hunting skills.

"Postreproductive females, the most experienced hunters in the population, were involved in porpoise harassment the least, whereas juveniles, calves, and adults (perhaps teaching the youngsters) were involved more," the authors wrote.

Additionally, the porpoises targeted were often the same size as the orca's chinook salmon prey.

The third suggestion is that the attacks are actually mismothering behavior, or epimeletic behavior, with the orcas attempting to care for the weaker porpoises out of some form of empathy.

"Mismothering behavior—also known as 'displaced epimeletic behavior' to scientists—might be due to their limited opportunities to care for young," Giles explained. "Our research has shown that due to malnutrition, nearly 70% of Southern Resident killer whale pregnancies have resulted in miscarriages or calves that died right away after birth."

This strange behavior has been seen in other species of dolphin before.

"Killer whales are in the dolphin family Delphinidae, and this type of behavior has been observed with bottlenose dolphins off the UK harassing and killer harbor porpoise without eating them," Miller said. "It is not unprecedented to see larger dolphins killing but not eating smaller porpoises. One article suggested it might be a displaced infanticide behavior on the part of the bottlenose dolphins."

The exact reason for the porpoise harassment may remain unknown, the authors say, but the behavior appears to be continuing, getting passed between orcas in the population despite their diets staying unchanged.

"Killer whales are incredibly complex and intelligent animals. We found that porpoise-harassing behavior has been passed on through generations and across social groupings. It's an amazing example of killer whale culture," Sarah Teman, a marine mammal biologist at the SeaDoc Society, a program of the UC Davis School of Veterinary Medicine, and co-author of the paper, said 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."

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