Sea otters are the smallest marine mammals, but they have some incredible adaptations to thrive in the ocean. Unlike seals and whales, they don’t have a thick layer of blubber to keep them warm. Instead, they rely on the densest fur in the animal kingdom—over a million hairs per square inch—to trap air and insulate their bodies.
But staying warm isn’t their only challenge. Sea otters feast on tough-shelled prey like clams, mussels, crabs, and sea urchins. Their powerful jaws and specialized molars help them crush some food, but for the hardest shells, they need a little extra help. That’s where their special pockets and their favorite rocks come in.
Researchers have observed sea otters using their chosen rocks in two ways: either as an anvil, placing the rock on their chest and smashing shellfish against it, or as a hammer, using the rock to dislodge stubborn prey like abalone from underwater rocks. Some otters have even been seen using two rocks in tandem—one as a hammer and the other as an anvil—demonstrating a remarkable level of problem-solving and dexterity.
But perhaps even more impressive is how the otters cherish some rocks.
Under each forearm, sea otters have flaps of loose skin that function like built-in pockets. These pouches allow them to store extra food while diving, preventing the need to return to the surface too often. But what really sets these pockets apart is that many sea otters keep a special rock tucked away in them—a tool they use over and over again. Yep, otters have a favorite rock.
Unlike other animals that use tools occasionally, sea otters show remarkable consistency. Once an otter finds a rock that works well, it often keeps the same one for months or even years, storing it in its pocket when not in use. This behavior is not perfectly understood but it suggests not just intelligence, but also a sense of attachment to a particular tool.
Of course, it’s probably not a case of emotional attachment towards the rock, but more a sense of practicality; once you find a rock that works well, why give it up?
Scientists believe that sea otters select their rocks carefully. They tend to prefer stones that are just the right size—large enough to be effective but small enough to fit in their pockets. The ability to reuse a trusted tool gives them a major advantage in breaking open hard-shelled prey without wearing down their teeth.
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Females are more likely to use tools
Not all sea otters use tools equally. Studies have shown that females are more likely to rely on rocks than males. This is likely because female otters are smaller and have weaker bites, making it harder to crack open prey with just their teeth. Tool use allows them to access harder, larger food items and conserve energy—especially important for mothers raising pups, which requires a significant amount of calories.
“Sea otters vary in how often they use tools,” said Chris Law, a postdoctoral researcher and an Early Career Provost Fellow at UT Austin who led the study while a graduate student at UC Santa Cruz. “The females are likely using tools to overcome their smaller body size and weaker biting ability in order to meet their calorie demands. Raising pups takes a lot of energy, and the females need to be efficient in their foraging. The study shows that tool use is an important behavior for survival.”
Interestingly, other intelligent animals show a similar trend. In species like chimpanzees, dolphins, and bonobos, females tend to use tools more frequently than males, likely for the same reasons: efficiency and energy conservation.
Otter mothers also play a key role in passing down tool-use knowledge.
Sea otter pups don’t start life knowing how to use tools. Instead, they learn by watching their mothers. Since female otters are the primary tool users, their young often pick up the habit as they grow. Pups spend six to seven months with their mothers, during which they observe her diving, foraging, and using rocks to crack open food. However, studies have shown that even orphan otters have a propensity towards using tools and can also learn by themselves.
Otters play a key role in their environment
The use of tools is rare in the animal kingdom. While primates like chimpanzees and capuchin monkeys use sticks and rocks to obtain food, and certain birds like crows and parrots have been known to manipulate objects, sea otters are one of the few non-primate species to exhibit such consistent and advanced tool use. It’s important that they do it, too. They play a critical role in marine ecosystems preying on sea urchins that feed on kelp forests, and also have been shown to have positive effects on estuary habitats.
Despite their cleverness and adaptability, sea otters face numerous threats. Once hunted to near extinction for their luxurious fur, they are still classified as a threatened species. Today, they face challenges from habitat loss, pollution, and climate change. Oil spills are particularly devastating for sea otters because their fur, their only protection against the cold, becomes matted and loses its insulating ability.
However, when otters make a comeback, the entire habitat starts to improve. Protecting them is not just about saving a single species—it’s about preserving the balance of marine ecosystems. By keeping sea urchin populations in check, sea otters help maintain healthy kelp forests, which provide habitat for countless marine species and play a role in carbon sequestration, helping to fight climate change.
The fact that sea otters carry and reuse a favorite rock offers a fascinating glimpse into their intelligence and adaptability. They deserve not only our admiration but also our protection, as their survival is deeply intertwined with the health of marine ecosystems.