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........

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