Orcas in the North Pacific have been seen "massaging" each other - rubbing pieces of kelp between their bodies.
Using drones, researchers filmed the animals selecting and biting off the kelp, then placing the tube-shaped piece of seaweed onto the back of another whale.
The scientists think the massages might have a health or hygiene function, but they also believe they are a form of social bonding.
The findings, published in the journal Current Biology, appear to be an example of tool use in killer whales - with the animals fashioning a tubular piece of seaweed and using it for a specific, planned purpose.
"Let's call it a kelp massage," explained Prof Darren Croft from the University of Exeter and the Center for Whale Research in Washington State.
