Sea Otters Keep a Favorite Rock in Their Armpit Pouch
Sea otters love eating hard-shelled sea urchins and clams, but they do not crack them open with their teeth. Instead, they store a favorite anvil stone in a loose pocket of skin under their forelegs. They carry this preferred rock on multiple diving trips, resting it on their chests while floating on their backs to hammer open their catch.
The Anatomy of the Sub-Arm Pouch
Sea otters possess a distinctive anatomical feature beneath their front limbs: a loose fold of skin extending across the chest under each foreleg. While often described colloquially as an armpit pocket, this flap functions as an essential, built-in storage compartment. In the wild, an otter uses these pouches to temporarily secure prey harvested during deep dives to the sea floor, allowing it to collect multiple food items in a single trip rather than surfacing after every single capture.
Beyond carrying food, these skin pouches serve as personal holsters for inanimate objects, most notably stones. A sea otter that discovers a stone with the right size, shape, and heft for cracking shells will frequently tuck that specific rock into its pouch. By storing the rock securely against its body while swimming and diving, the otter retains access to an effective tool without having to search for a replacement anvil every time it catches a hard-shelled invertebrate.
Anvils on the Chest: Mechanics of Tool Use
Sea otters are among the very few non-human mammals documented to use tools routinely in the wild. When feeding, a sea otter floats on its back at the surface, turning its torso into a stable, buoyant platform. It retrieves its stored stone from under its arm, balances the rock on its chest or abdomen, and grasps a hard-shelled prey item with both forepaws. It then repeatedly strikes the prey against the stationary rock until the protective exterior fractures.
This anvil technique is remarkably efficient for opening heavily armored marine life, including clams, mussels, and crabs. Otters also employ stones underwater in a different manner, using rocks to hammer against well-anchored prey such as abalone to dislodge them from rocky substrates before bringing them to the surface. By utilizing stone tools as both hammers and anvils, otters exploit food sources that would otherwise be inaccessible or cause severe damage to their teeth.