Since 2020, members of a small group of killer whales have rammed into at least 673 vessels off the coasts of Portugal, Spain and Morocco — causing some to sink. The Spanish and Portuguese governments responded by tasking a group of experts with determining what was causing the whales to strike rudders, which are used to steer ships, and how to stop it.

The group, which includes biologists, government officials and marine industry representatives, on Friday released a report outlining their hypothesis: The orcas just want to have fun, and in the vast — and rather empty — open waters, the boats’ rudders are a prime toy.

“This looks like play,” said Naomi Rose, a senior scientist at the Animal Welfare Institute who was part of the working group. “It’s a very dangerous game they’re playing, obviously. But it’s a game.”