Sea otters use rocks to break open shells and get to the good stuff inside, the only marine mammals to frequently do this. Scientists have shown the distinctive wear produced by generations of otters can be used to track where they once lived, and to explore cultural differences among otter populations.
The sea otter once inhabited northern Pacific coastlines from Mexico to Japan, but hunting for fur reduced them to small patches. With few fossils and little reporting, we don't know much about their past concentrations and influence on their habitat.
Sea otters are so adept with stones some have even taken to juggling. The Max Plank Institute's Dr Natalie Uomini realized we could use their frequent tool use to track their past. In Scientific Reports, she shows how their stonework leaves traces that can be distinguished from other natural, or human, phenomena, under the evocative title “Wild Sea Otter Mussel Pounding Leaves Archaeological Traces”.
Otters use stones to pry abalone off rocks and as “chest anvils” to break shells while floating on their backs in the water. For archaeological purposes, however, their most interesting technique is what she calls “emergent anvil use” where mussel shells are bashed against shore rocks to break them open.
In a 10-year study at Bennett Slough, California, Uomini found mussels, which make up half of the otters' prey, were struck against rock points or ridges while the otter was in the water, rather than coming onto land. The consistent angle wore the stones down in distinctive ways, making it possible to recognize the otters' favorite stones. Tens of thousands of mussel shells were left scattered around the favored rocks. These showed a consistent pattern, with the two sides attached and a diagonal fracture on the shell's right side, consistent with Uomini's observation the otters held the mussels in both front paws, but with the right paw on top to apply the force.
"The shell breakage patterns provide a novel way to distinguish mussels broken by sea otter pounding on emergent anvils from those broken by humans or other animals," Uomini said in a statement. This can help archaeologists studying human coastal societies avoid mistaking the remains of otter activities for those of humans.
To co-author Jessica Fujii of the Monterey Bay Aquarium there are more exciting uses, however. “Stationary anvil use can be detected in locations previously inhabited by sea otters. This information could help to document past sea otter presence and diet in locations where they are currently extirpated," she said.
Otters aren't the only animals to have entered the Stone Age. Archaeology has been applied to the tools of other great apes, but never before to an animal so distant from us. The authors hope studying old otter debris, and those of other tool users like crows, will help us understand how tool use developed, and explore why some otter communities use stones more than others.