Orcas that grab baby porpoises and play with them to death

For sixty years, orcas in the Salish Sea off Washington state and British Columbia have been observed to harass baby porpoises, tossing them out of the water to each other for hours. The orcas' behavior often kills the young porpoises, but they don't eat them, perplexing scientists. Link to the article in Live Science here.

"I always liken it to a group of friends in a swimming pool and they have a ball between them and the object is to keep the ball above the water," study lead author Deborah Giles, an orca researcher at the University of Washington and research director at the non-profit Wild Orca, told Live Science.

The orcas belong to a small and endangered population called southern resident killer whales that live off the Pacific coast of North America. They are fish-eaters, meaning it's unlikely they are playing with porpoises to consume them — deepening the mystery of why these orcas do it.

"In some cases, you can barely see that there was any interaction," Giles said. "You might see teeth marks where the [killer] whale was clearly gently holding the animal, but the animal tried to swim away, so it scraped the skin."

Out of 78 documented cases of orcas harassing porpoises, 28 cases resulted in death, according to a study published Sept. 28 in the journal Marine Mammal Science. I wonder how many of the young porpoises who survived the interaction later died from injuries and trauma.

Giles theorizes that porpoise-tossing in a form of social play, strengthening social bonds and reinforcing how to hunt cooperatively. She also theorizes that it may be "mismothering behavior": a clumsy and dysfunctional way of providing "care" for the baby porpoises, due to lowered birth rates in the orca population.

Last spring it became a social media fad to cheer the orcas off the Iberian coast who taught each other how to attack and capsize yachts. I doubt porpoise-tossing is a localized cultural behavior that will draw many human fans.