Sea otters use rocks as tools to break open shelled prey, storing them in loose skin pouches under their armpits.

When foraging for prey like clams, mussels, or sea urchins, a sea otter will often dive to the seafloor, pick up a rock, and return to the surface. Floating on its back, the otter places the rock on its chest and smashes the prey against it until the shell breaks open. After use, the otter tucks the rock into a loose patch of skin under its foreleg, creating an improvised pocket. This behavior demonstrates foresight and tool maintenance, traits once thought rare outside primates, and highlights the otter's problem-solving abilities.