Scientists have spotted a subset of killer whales using seaweed to scratch each other’s backs, marking the first known identification of “tool” usage by marine mammals.

The “southern resident” killer whales, whose behaviors are the focus of a new study, have been biting off the ends of kelp stalks, positioning the fragments between themselves and a partner and then rolling the kelp between their bodies for prolonged periods.

The study authors identified the practice via drone footage of these orcas, publishing the findings on Monday in Current Biology.

This small community of black-and-white mammals inhabits the Salish Sea, located in the inland Pacific Ocean waters between the state of Washington of British Columbia.

“What I find remarkable about this behavior is just how widespread it is in the population,” lead author Michael Weiss, of the Washington-based Center for Whale Research, said in a statement.

Weiss and his team discovered the unexpected behaviors while conducting aerial surveillance of these whales, which are members of a critically endangered population of just 73 individuals.

The researchers said they have been monitoring the whales since 2018 to better understand their social and foraging behaviors.

“While there are other killer whales around the world, the southern residents represent a genetically, ecologically, and culturally distinct population,” said Weiss, who is also affiliated with the Centre for Research in Animal Behaviour at the University of Exeter in the U.K.

Via high-resolution footage collected by their drones, Weiss and his colleagues saw that the whales were creating tools by breaking off the ends of bull kelp stalks.

They observed that whales across all social groups, both sexes and age classes then participated in the grooming behaviors although they did so with closely related whales or similarly aged partners.

The scientists also observed that those whales with more molting or dead skin were likelier to engage in grooming, suggesting a potential hygienic role, according to the study.

“Bull kelp stalk is firm but flexible, like a filled garden hose, with a slippery outer surface. I suspect these features make it an ideal grooming tool,” Weiss said.

They identified the practice dubbed “allokelping” on eight out of the 12 days they included in their study.

Weiss emphasized the element of surprise inherent in their discovery, noting that this population has been undergoing “nearly 50 years of dedicated observation.”

“To me, this demonstrates not just the power of new observation methods but also how much we still have to learn about these animals,” he said.

Co-author Rachel John, a University of Exeter master’s student, made echoed these sentiments, noting that “new discoveries can still be made” even regarding the “best-studied orcas on the planet.”

As for the possible reasons behind the allokelping behavior, senior author Darren Croft explained in a statement that “touch is really important” among primates, as this sensation “moderates stress and helps build relationships.”

“We know killer whales often make contact with other members of their group touching with their bodies and fins but using kelp like this might enhance this experience,” added Croft, a professor at the University of Exeter and the executive director of the Center for Whale Research.

Other possible reasons he cited were improvements to skin health, as well as benefits from the antibacterial and anti-inflammatory properties of brown algae like bull kelp.

Hypothesizing that allokelping is “a cultural behavior unique to southern resident killer whales,” the scientists emphasized the importance of protecting these endangered mammals.

Weiss stressed that the outlook for southern resident killer whales is “very bleak,” as the current birth rate is insufficient to sustain the population long-term. This decline is in part due to their struggle to find enough fatty Chinook salmon that they require for their survival.

“If we lose them, we lose so much more than 73 individual animals or a genetic lineage,” Weiss said. “We lose a complex society and a deep, unique set of cultural traditions.”