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Sea otter archaeology reveals the most smashing rocks

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. Now, a new study shows that, by borrowing techniques from archaeology, marine biologists can pick out otter “utensils” from other rocks.

Many primates have also been shown to use stone tools. Recently, researchers have blended biology and archaeology to identify patterns of wear on such tools used by apes and monkeys—dating some as far back as 700 years. 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.

Over 10 years of observations, the team identified a so-called “otter signature.” Rocks used as tools had points and ridges that were lighter in color than the rest of the rock. Next, the researchers examined 421 additional rocks in the area and found that 77 were being used to break open shellfish, they report today in Scientific Reports. Shattered mussel shells littering the rocks nearby corroborated the findings, showing telltale breakages that matched the otters’ blunt force modus operandi.

With their distinctive patterns, the stones could tell scientists when sea otters started to use tools. The researchers say understanding how long this behavior has been around and how it spread through populations could also help illuminate the broader question of how tool use in other mammals—including humans—evolved.