Otters are cute as a button, and clever too; they’re the only marine mammals known to habitually use stone tools. And as is the case with humans of millennia past, otters’ stone tool usage creates a unique archaeological record, a new study has found.

As Discovery’s Lacy Schley reports, otters are resourceful hunters that rely on a variety of methods to access hard-shelled prey like mussels and clams: they might pry the sea creatures open with their teeth, whack them against their chest or a rock placed on their chest, or bang them against a stationary rock. The rocks function like anvils, the international team of researchers behind the study explain in Scientific Reports, and the otters’ rock-smashing behavior is considered tool use because it “involve[s] the controlled use of a detached object.”

For their investigation, the researchers spent ten years observing otters as they chowed down on mussels at the Bennett Slough Culverts, a tidal estuary in California. The team found that the animals used “stationary anvil stones” for around 20 percent of the mussels they ate, and repeatedly returned to the same rocks to crack their snacks open. This in turn left distinctive wear patterns on the rocks’ points and ridges, where the otters tended to strike. The patterns clearly indicated that the stones were being hit from within the water.

The researchers also studied piles of shell fragments, or “middens,” that formed around the rocks. These too had distinct damage marks: the two sides of the shells were typically attached, with a diagonal fracture running down the right side. It is possible, the study authors say, that these patterns stem from otters being predominantly right-pawed. “Right before they hit the rock, they slightly twist the shell so that their right hand is the one that's really smashing it on the rock,” wildlife biologist and study co-author Tim Tinker tells the CBC’s Emily Chung.

The number of shells in the middens was staggering; the study authors found that a random sample likely contained pieces from as many as 132,000 individual shells. “[Otters] the most destructive things in the natural environment other than humans,” Tinker says. “There’s really nothing that can smash a clam or urchin or snail with the same sort of force that a sea otter can.”

Being aware of the unique and plentiful traces that otters create as they happily bash their time away is important for archaeologists working in coastal areas, who might need to distinguish between tool use patterns left by humans and those left by hungry marine mammals. Identifying otter activity in the archaeological record may also help researchers identify areas where the animals no longer exist. Sea otters are endangered today, their populations having been greatly reduced by the early 20th century fur trade. But they once numbered up to 300,000, spanning from Japan to the central Baja Peninsula in Mexico along the north Pacific.

Jessica Fujii, study co-author and researcher at the Monterey Bay Aquarium, says that she hopes the new study “establishes a new path for the growing field of animal archaeology.” Humans, after all, aren’t the only ones to make their mark on the record of the planet’s past.