Killer whales are known for exceptional intelligence, displaying complex social structures and sophisticated communication.

New research provides fresh evidence, documenting how the marine mammals use stalks of seaweed as tools to groom each other — as in, “You scratch my back and I’ll scratch yours.”

Using drones to observe a population of killer whales in the Salish Sea, part of the Pacific Ocean between Washington state and British Columbia, researchers noticed that these predators engaged in a behaviour they named “allokelping”, one of the few known examples of tool use by marine mammals.

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The killer whales find large stalks of a type of seaweed called bull kelp, either attached to the seabed or floating at the surface.

They then bite off the end of the stalk, position it between themselves and another killer whale and roll the kelp between their bodies.

The researchers hypothesise that the behaviour promotes skin health while strengthening social bonds. Other populations of killer whales have been observed rubbing their bodies on smooth stone beaches, possibly to remove dead skin.

“Most examples of tool use in animals involve solving ecological problems, such as gaining access to food,” marine biologist Darren Croft of the University of Exeter in England, executive director of the Centre for Whale Research and co-author of the study published this week in the journal Current Biology, said.

“For example, chimpanzees use sticks to fish for termites.

“What’s remarkable about this discovery is that the tool — the kelp — is used not to obtain food but to facilitate social interaction.

“This kind of socially motivated tool use is extremely rare in non-human animals and has previously only been observed in a small number of primates, usually in captivity.”

The researchers documented the behaviour among both male and female killer whales of all ages. It likely plays an important role in their social lives, Croft said.

There are some other examples of tool use among marine mammals.

Sea otters use rocks and other hard objects to crack open shells to get at the meat inside. And certain dolphins use marine sponges to protect their snouts and stir up the seabed while foraging.

The researchers said the behaviour by the killer whales goes one step further because they modify an object for use as a tool.

“While this is not the first documented case of cetacean or marine mammal tool use, it is — as far as we know — the first case of cetacean tool manufacturing with tool use,” study co-author Rachel John, a University of Exeter graduate student studying killer whale behaviour, said.

“The whales are not just finding perfect lengths of kelp in the environment, but rather actively modifying larger intact stalks of kelp to create the pieces they are using for allokelping.

“Another key part of what makes this behaviour so unique is the fact that they are manipulating the kelp co-operatively with a partner without the use of hands or any hand-like appendages.

“They use their mouth to position the kelp initially, but after that they only use the momentum and pressure of the core of their bodies to maintain contact with each other and the kelp between them.”

The behaviour is known only among this killer whale population.