Killer whales in North America’s Salish Sea have been filmed tearing off stalks of bull kelp, manipulating the seaweed between themselves and a partner and rolling it back and forth to remove dead skin. The behaviour, documented 30 times over an eight-day period and described in Current Biology on 23 June 2025, represents the first recorded instance of a marine mammal deliberately manufacturing a tool. Using a high-resolution drone, researchers led by Michael Weiss of the Center for Whale Research observed members of the critically endangered southern resident population biting kelp to a precise length before positioning it between their bodies. The whales—often close relatives or similar-aged peers—spent several minutes “allokelping”, a term coined by the team for this cooperative grooming. Individuals showing heavier skin shedding engaged in the activity more frequently, suggesting the improvised brushes serve both hygiene and social-bonding functions. While other sea creatures, including some dolphins, are known to employ found objects, the authors say no wild marine mammal has previously been shown to alter an item to create a tool that benefits two animals simultaneously. The discovery adds to evidence of the species’ complex culture and underscores concerns for the southern residents, whose numbers have fallen to roughly 73 as Chinook salmon stocks decline. Scientists say further observation is needed to determine whether the kelp-brush technique spreads to other orca communities or disappears if this population continues to dwindle.