Sea otters are the smallest marine mammals, but they have the best motor skill, which means they can perform a difficult action quickly and skillfully with their paws. They are known for using stones and kelp as tools to acquire food and avoid floating away. Some parts of their body contribute to their tool-using skills. With the largest lungs in the otter family, they can stay afloat on their backs to manipulate tools. At each of their axilla, the equivalent of human’s armpit, there is a pouch in which they store food or tools. Moreover, they are the only marine mammals that catch prey with their front paws instead of their teeth. This characteristic, combined with their handiness with tools, makes them particularly special. Being one of the few marine mammals with tool-using skills, sea otters have three continuous and distinct methods of using stones and kelp as tools for hunting.

        First, sea otters use stones as hammers to obtain food. When they are hungry, they will dive into the sea and hunt for shellfish, such as urchins, crabs, and mussels. (See picture 1) However, some shellfish are stuck on rocks, and thus sea otters are not able to get them off with bare paws. To acquire the food, they will swim to the bottom of the ocean, pick up a stone, and use it as a hammer to knock the shellfish off. Furthermore, they do not show preference for smooth or rugged stones. Nonetheless, they tend to choose large ones that are between the size of six to fifteen centimeters. (See picture 2)