“We were amazed when we first noticed this behavior,” said Michael Weiss, research director at the Center for Whale Research in the U.S. state of Washington. What started as a puzzling observation in drone footage revealed something unexpected: killer whales are making tools from a type of seaweed called kelp.

The behavior, dubbed “allokelping” (kelping with another whale), represents the first evidence of toolmaking by marine mammals. The finding is detailed in a recently published paper in the journal Current Biology.

Southern resident killer whales (Orcinus orca ater) in Washington’s Salish Sea have been observed biting off the ends of bull kelp stalks, positioning the trimmed pieces between themselves and a partner, then rolling the kelp between their bodies for extended periods.

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

The discovery emerged when researchers upgraded their drone equipment in 2024, allowing them to capture high-quality, zoomed-in footage of the whales. Rachel John, a master’s degree student studying animal behavior at the University of Exeter, U.K., was the first to spot the unusual behavior.

“I was watching one of those really close-up videos when I saw something that looked like a brown stick between two whales that were in contact with each other,” John said. “We didn’t think much of this at first, but we had also noticed pairs of whales staying in contact with each other for long periods — several minutes at a time.”

As the research team collected more observations throughout the 2024 field season, they realized this wasn’t random play behavior. The whales were deliberately selecting intact kelp stalks, grabbing the end, and using “a jerk of their head and the drag from the top of the kelp and their teeth to actually separate out a small piece of kelp,” typically about 2 feet (60 centimeters) long, according to Weiss.

What makes this behavior particularly remarkable is how the whales coordinate their movements. “It’s quite incredible when you think about the size of these animals and the length of the [kelp stem] that they are fashioning and the way that they are coordinating their movements together in such synchrony,” Charli Grimes, a researcher from the University of Exeter, said during a press briefing.

The scientists observed allokelping on eight out of 12 study days, with whales of all ages participating. Mothers and calves, siblings, and whales of similar ages were most likely to engage in the behavior together, suggesting it serves both social bonding and hygiene functions.

While most animals use tools to obtain food, orcas are using a tool to solve a different challenge. “It might be a health problem or a health challenge around skin, but also … it seems very likely it’s related to a social challenge, a need to have those sorts of intense, meaningful social interactions,” said Darren Croft, a professor and whale researcher at the University of Exeter, noting that this discovery breaks new ground in animal tool use research.

The whales appear to use the kelp to help exfoliate dead skin, which can be challenging for marine mammals in cold water. Researchers found tentative evidence that whales with more molting skin were more likely to engage in allokelping behavior.

“Brown algae like bull kelp also have antibacterial and anti-inflammatory properties that may provide further benefits to the whales,” Croft said.

The research represents 50 years of study on what Weiss calls “the best-studied orcas on the planet,” yet demonstrates how much remains to be discovered about these well-known marine mammals.

“Thus far, there is still a pretty small sample size of allokelping, but if, as the authors suggest, this behavior relates to social skin care, it’s probably been ubiquitous for some time,” Monika Wieland Shields, who directs the Orca Behavior Institute, also in Washington, but who wasn’t involved in the research, told The Guardian. “This research demonstrates the new behavioral, cultural and social insights that can come from relatively new techniques like drone observation.”

The southern resident population of orcas is listed as endangered in both the U.S. and Canada. As of July 2024, only 73 individuals remain in the population observed for this study. Weiss described this number as “critically low,” since they don’t interbreed with other orca populations.

“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.”

The whales face multiple threats, including declining populations of Chinook salmon (Oncorhynchus tshawytscha), their primary food source, due to overfishing, climate change and habitat destruction. Pollution and noise from human activities further stress the whale population.

Adding to these concerns, the bull kelp forests where the whales source their grooming tools are themselves under threat. “Bull kelp needs cold water to thrive, and so as the oceans are getting warmer … there are more and more die-offs of bull kelp within the southern residents’ critical habitat,” Weiss said. “Protecting the future of these kelp forests, where the killer whales make their tools, may be important in preserving this unique culture and ensuring the southern resident killer whale population continues for generations to come.”

Banner image two whales allokelping, with a small length of kelp stem visible between them. Center for Whale Research, NMFS NOAA Permit 27038

Liz Kimbrough is a senior staff writer for Mongabay and holds a Ph.D. in Ecology and Evolutionary Biology from Tulane University, in New Orleans, Louisiana where she studied the microbiomes of trees. View more of her reporting here.

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Citation:

Weiss, M. N., John, R. E., Caro-Ruiz, A. M., Ellifrit, D. K., Grimes, C. A., Redmond, T. A., … Croft, D. P. (2025). Manufacture and use of allogrooming tools by wild killer whales. Current Biology, 35(12), R599-R600. doi:10.1016/j.cub.2025.04.021

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