Sea otters—the only marine mammals known to use stone tools—eat on the go by cracking open mussels, sea urchins, and abalone with rocks, using their furry chests as anvils.
The findings made researchers wonder whether such methods could also be used on sea otters.
Many sea otter rocks get discarded to the sea floor, but some turn up on beaches, where the otters bang their shelled prey against boulders protruding from the sea.
One such place, an estuary in central California called Bennett Slough, offered researchers an opportunity to examine the otters’ feeding behavior—and the rocks they dropped after smashing up dinner.
With their distinctive patterns, the stones could tell scientists when sea otters started to use tools.