Sea otters use tools to open hard-shelled prey, saving their teeth, research reveals | Animals

Floating on its back in the waters of California’s Monterey Bay, a sea otter takes a shelled animal and strikes it against a rock sitting on its chest to break open the prey.

This behavior, documented in footage from researcher Chris Law, is seen in relatively few animals and allows the otter to access food without damaging its teeth. A new study, which will be published in the journal Science on Friday, sheds light on the threatened species’ tactics.

Researchers found that when there’s a decline in their preferred food sources, such as abalone and sea urchins, sea otters that use tools are able to consume larger prey like crabs and clams and reduce dental injuries. Most sea otters that do this are female, according to the study. That is probably because the tools allow them to overcome a smaller body size and weaker biting ability to meet calorie demands, said Law, a postdoctoral researcher at the University of Texas at Austin, who led the study.

Long brown otter sliding on its back atop perfectly smooth, dark blue water, lifting a rock the size of half a fist and slamming it onto a larger rock balanced on its belly. The video is on autorepeat.
Otter using a rock anvil to open the shell of a sea animal. Video by Chris Law.

The study looks at sea otters in Monterey Bay on California’s central coast, which is home to the southern sea otter population. The animal once occupied waters from Alaska to Baja California, until the fur trade drove them to near extinction. The US Fish and Wildlife Service has explored the idea of reintroducing sea otters along the west coast. The population slowly increased due to conservation efforts in the 1970s, and there are roughly 3,000 in California today.

Groups like the Center for Biological Diversity have backed sea otter restoration, citing the important role the animals could play in helping restore the region’s crucial, yet decimated, kelp forests.

In central California, sea otters’ preferred prey are sea urchins and abalone, which are typically easier to break open, but those species are declining, Law said. Because of that, they will more often seek out crabs, clams and mussels as well as marine snails. The hard shells of the snails can damage the otter’s teeth if they attempt to open them with their mouths, the study points out.

“There’s fishing and habitat destruction, so their favorite prey are gone and they have to switch to alternative preys,” Law said. “What we found is that this behavior actually allows them to switch to those prey.”

The study authors and volunteers followed 196 otters, who were equipped with radio tags, for their research.

The sea otters primarily use rocks as hammers, but will also use shells and trash, and occasionally, boats or docks, said Law, an evolutionary biologist who completed the research as part of his dissertation as a doctorate student at the University of California at Santa Cruz.

The study established, for the first time, that those tools helped reduce tooth damage for the otters, which is essential to their survival. If an otter’s teeth develop too much damage, the animal could face starvation.

The research also found that females who used tools were able to consume prey that were up to 35% harder compared to males who used tools. It is not clear whether tool use is increasing among otters, but the behavior is beneficial for the animal.

“This behavior really allows them to eat other prey items and in an environment where that’s depleted. It really just showcases how it’s important for their overall survival,” Law said.

“If there are no urchins and abalone for them to eat and they are faced with other prey types they can’t open, they can’t survive.”


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