The word is out among boaters along the coasts of European countries: Look out for orcas. For more than a year now, killer whales have been regularly causing trouble for boaters near France, Spain, Portugal, and elsewhere, reports NPR. Nobody has been injured, but the same can't be said for boats: Two were sunk off Portugal's coast just last month, per Portugal Resident. The unusual behavior has prompted warnings such as this one in Yachting Monthly: "Orcas have been nudging boats to bring them to a halt, and gnawing at their rudders, leaving sailors shaken and stranded, in many cases relying on salvage tows to get them back to shore."
So what's going on? Scientists aren't sure, but theories floated in the NPR story and at Live Science are interesting—and somewhat innocent. For example, Renaud de Stephanis of CIRCE Conservacion Information and Research tells NPR he doesn't think the orcas mean harm. His guess is that they've come to like the feel of water pressure from boat propellers. "What we think is that they're asking to have the propeller in the face," de Stephanis says. If they come across a sailboat with its engine off, "they get kind of frustrated and that's why they break the rudder."
Live Science suggests something along those lines as well—that the boat ramming might be a temporary "fad." It seems that orcas have a documented history of picking up behavioral quirks from one another, as when Puget Sound orcas went through a phase in the late 1980s of "wearing dead salmon like hats," per the story. It cites a 2004 scientific paper on that very thing, which explained that a female orca did it first, and the other orcas mimicked her. The fashion trend lasted a summer. Researcher de Stephanis speculates that the orcas involved in the recent boat encounters are mostly juvenile males who will outgrow the "game" eventually. (Read more orca stories.)