Illustrated by: Perrin Ireland

Why would a group of primate archaeologists want to study the mouths of ancient sea otters? Turns out, the teeth of these animals may give us clues about how a species enters into the esteemed realm of tool users. Once thought to be the domain of humans only, tool use has since been documented in several species—including some sea otters that crack open snails with rocks or the hulls of ships, or even use a crab’s own claw to scrape out its meaty insides.

As reported in Hakai Magazine, scientists don’t know how long otters have been doing this. Tool use in sea otters isn’t universal. It varies, depending on where they live and what they eat. Interestingly, when a new type of fish starts swimming in otter habitat, otters quickly develop methods for eating it. This leads scientists to think that their tool use is not purely instinctual but rather knowledge that gets passed between them. This could mean sea otters have some sort of culture that spreads from place to place and through time as they teach their skill to their offspring.

Tool-using cultures have also been seen in primates. In 2007, archaeologists working in the Ivory Coast found stones that chimpanzees had used to crack nuts 4,300 years ago. After examining the wear and tear of the stones, the scientists concluded that the ancient chimps used the tools in a similar way to the nut-cracking chimps of today—suggesting that the behavior has been passed down through more than 200 generations.

Studying otter tools—even ones used this millennium—is trickier. Sea otters tend to drop the rocks they use to smash food after one use, so there’s no wear and tear. The rock that was once a hammer sinks to the bottom of the ocean, where it looks a lot like a…rock.

This is where the teeth come in. The archaeologists assume the sea otter diet hasn’t changed too drastically over the last few million years. For instance, they likely ate sea snails back then, too. But if tool use had yet to develop among these ancient otters, biting into a snail could break a tooth, possibly shortening the otter’s lifespan. If an otter lived to a certain age with no cracked teeth, however, scientists think they can deduce that it had been using tools—then dropping them into sea for no one to find.

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