Archaeologists can tell a lot by looking through prehistoric trash. Piles of chipped stones and broken shells, crushed by hungry humans long ago, can reveal where we lived, how we lived, and for how long.

Now, scientists are applying this same archaeological approach to reveal new insights into a furry, endangered mammal with a similar taste for shelled snacks.

Wild sea otters habitually crack open mussels by smashing them on stationary stones, making them one of the few animals that use tools. By observing southern sea otters engaging in such behaviors along the California coast, and then analyzing wear and tear on stones and shells deposited nearby, researchers were able to make some surprising conclusions. (See what sea otters do when no one’s looking.)

For instance, consistent cracks along the same side of the discarded mussels suggests that most otters at the site were right-handed. For decades, researchers believed that only great apes and humans had a preferred “hand,” but kangaroos, blue whales, and now sea otters continue to expand handedness in the animal world.

The size of the shell deposits and wear to the rocks also provide a baseline for estimating how many decades otters have been feeding at this site, according to a study recently published in the journal Scientific Reports. By proving the viability of these archaeological methods, future researchers can return and create a timeline of activity at this site and others.

Knowing this “could tell us more about how long [otters] have been using tools, and how widespread the tool use has been,” says study leader Jessica Fujii, a senior researcher at the Monterey Bay Aquarium.

They could also answer the question of why southern sea otters—the subspecies living in California—are more likely to use tools than northern sea otters, which are native to the Pacific Northwest and Alaska.

Archaeology turns to animals

Archaeology has strayed into the animal kingdom before, but until this study, it was focused mostly on primates such as chimpanzees.

In 2016, primate archaeologists discovered that bearded capuchin monkeys used stone anvils more than 600 years earlier than previously thought and in regions no longer frequented by that species of monkey.

In fact, primate archaeologists partnered with sea otter biologists on the new study.

Our “sea otter colleagues have been working on sea otter stone and tool use for decades,” explains Natalie Uomini, senior scientist at the Max Plank Institute for the Science of Human History in Munich. (Read more about other tool-using animals.)

“They were thinking about how far back this behavior goes and we were separately, and completely independently, thinking about how to apply archaeology to other animals.”

Now that animal archaeology has broken into marine environments, it could lead to similar findings for other tool-wielding aquatic wildlife. Tool use has been found among only a few marine animals, including dolphins, which use sponges to protect their beaks while hunting fish in coral—and possibly as love tokens.

Like sea otters, tusk fish smash any hard-shelled prey—from sea urchins to baby sea turtles—against their favorite stationary stones.

“There’s so many similarities, it’s quite unbelievable,” says Culum Brown, a biologist at the Macquarie University Fish Lab in New South Wales, Australia.