Do Sea Otters Use Tools? Rocks, Pockets and Shellfish

How sea otters use rocks: anvils and hammers

The best-known method uses a stone as an anvil. The sea otter surfaces with its catch and a rock, rolls onto its back and places the rock on its chest. Holding a mussel or clam in both front paws, it brings the shell down on the stone again and again, often at a rate of a couple of blows per second, until the shell cracks. A tough mussel can take dozens of blows.

The second method uses a stone as a hammer. Abalone cling to rocks with enormous force, and sea otters have been seen striking an abalone shell underwater with a large stone, with one observation recording 45 blows in 15 seconds. Freeing a large abalone can take several dives.

Sea otters do not seem to care much about the shape of a stone, but they choose rocks of a useful size, typically about 6–15 cm (2.4–5.9 in) across. Some otters skip the carried stone and pound mussels against fixed rocks at the water’s edge instead.

Pockets: where sea otters keep their rocks

Under each forearm, a sea otter has a loose fold of skin that forms a pouch reaching across the chest. Otters use these pouches, usually the left one, like shopping bags. During a dive they can gather several prey items, stash them in the pouch and keep both paws free to search for more.

The same pouch can carry a stone. An otter may bring a rock up from the seabed, use it on one shell, tuck it away while eating something soft such as a crab, then take it out again for the next clam. In one observation at Point Lobos, California, an otter ate 44 mussels in a single feeding session while using only six stones, a sign that otters keep a good tool for repeated use.

Other tools: shells, bottles and kelp

Rocks are not the only tools. Sea otters have been seen using empty shells, driftwood and discarded glass bottles as anvils or hammers, and pounding two live clams against each other. Some use a clam shell to help dig or pry prey out of the mud.

Kelp can also be a tool. Otters sometimes wrap a crab in kelp to keep it still on their chest while they eat other food, and they wrap themselves in kelp to stay in place while resting; see Why do sea otters hold hands? Not every clever trick counts as tool use, though: in California, some otters have learned to tear open discarded aluminum cans to get at the small octopuses hiding inside.

Learning from mother

Tool use in sea otters is learned, mainly from their mothers. A pup depends on its mother for about six months, staying with her almost constantly. Young pups play by pounding stones, shells and bits of coral against their bodies, and as they grow they copy their mother’s choice of prey and her way of opening it.

Long-term studies in California show that daughters tend to eat the same kinds of prey and use the same techniques as their mothers. Females use tools more often than males and show a wider variety of tool methods, a pattern that may come from this passing of skills down the female line. Researchers have also seen new foraging habits spread between otters in the same area.

Why some sea otters use tools more than others

Tool use varies hugely. A study that combined 17 years of observations from Alaska to California found that otters in Monterey, California, used tools on about 16% of dives, compared with about 1% at Amchitka Island in the Aleutians. Across populations, the share of otters that use tools at all ranged from about 10% to 93%.

The difference is mostly about food. In parts of Alaska, sea otters eat many sea urchins, which they can break open with their paws and teeth, and fish, which need no opening. In California, where favorite prey such as large urchins and abalone are often scarce, otters rely more on hard-shelled mussels, clams, turban snails and crabs.

A 2024 study of 196 radio-tagged southern sea otters found that tool users, especially females, could eat larger and harder-shelled prey and suffered less tooth damage than otters that relied on biting alone. That matters because sea otters with badly worn or broken teeth struggle to feed, so for some animals tool use may be essential for survival.

Archaeological traces of sea otter tool use

Sea otter tools leave marks that archaeologists can recognize. At the Bennett Slough Culverts site in California, researchers watched sea otters pounding mussels on rocks at the water’s edge over about ten years. The otters reused the same rocks as anvils, which left distinctive wear on the stones and piles of broken mussel shells below them, all cracked in a characteristic way. The study was published in 2019.

Because these piles of shell and worn rocks look different from human shell middens, the method may help scientists find ancient sea otter pounding sites along coasts where otters no longer live, and learn how long the behavior has existed.

How sea otters compare with other tool users

Sea otters are the only marine mammals that regularly use stones while foraging, and one of very few animals outside the primates known to use stones as hammers and anvils in the wild. Among primates, humans, chimpanzees, bearded capuchin monkeys and long-tailed macaques do the same. Other famous tool users include New Caledonian crows, which shape twigs into hooks. Meet more of them in animals that use tools.

Tool use helps sea otters survive where their favorite prey is scarce, while their appetite for sea urchins shapes the kelp forests around them, as explained in Why are sea otters a keystone species?