Sea Otters Are Among the Only Animals to Keep and Reuse the Same Tool

Sea otters possess loose flaps of skin under their forearms that act as built-in pockets. Inside, they store their favorite rock—a flat, palm-sized anvil used to smash open shells while floating on their backs. Once an otter finds the perfect stone, they will carry it for months or even years, tucking it safely away between dives.

This behavior is exceptionally rare. While chimps and crows discard their tools after a single use, sea otters retain theirs, placing them on a very short list of animals that keep a specific tool long-term. In fact, researchers can often identify individual otters by their signature rocks. Because female otters are smaller, they rely on these tool-belts the most to crack open tough prey.