Sea otters keep a favourite rock tucked in a loose pouch of skin under each forearm, carry it between dives, and use it as a personal anvil to crack open shellfish, making them one of the few animals known to keep and reuse a single tool
Sea otters carry a favored rock in a loose pouch of skin beneath each forearm, balance it on their chests like a personal anvil, and reuse the same stone across dives — a tool-keeping behavior almost unheard of outside great apes and a few birds.