There’s nothing especially remarkable about archaeologists studying stone tools and shell middens—unless they weren’t made by humans. A new study suggests that sea otters create a distinct archaeological record, and it might eventually have a lot to tell us about their evolutionary history.

Sea otters are ingenious—and adorable—tool users. They often pick up small rocks and use them to crack open snails, clams, or mussels; underwater, otters also use rocks to pry abalone loose from the seafloor. Closer to shore, otters whack mussels against protruding boulders until they damage the bivalves’ hinges enough to pry them open. That percussive food preparation leaves behind a unique signature: distinctive marks on the boulder and a pile of broken mussel shells, most cracked in a particular way, piled at the bottom.

“Sea otters have created a distinct, recognizable archaeological record,” wrote Michael Haslam and his colleagues, who claim that by doing a little mustelid marine archaeology, ecologists could track past habitats of sea otter populations and maybe even the evolution and spread of otter tool use.

Sea otter stone tools

Haslam and his colleagues spent a decade, off and on, watching a community of sea otters near Moss Landing, California, and learning their technique. It’s similar to the way other archaeologists watch modern hunter-gatherers in order to better understand flint-knapping and how to interpret the shape of flakes and cores they find elsewhere. They noticed that the otters tended to crack open mussels, which they gathered near shore, on the large boulders that stuck out above the water’s surface. Other prey, like clams, usually came from deeper water, where the otters were more likely to carry smaller stones with them to use as hammers.

Here’s how it worked: an otter would dive to the bottom and emerge, triumphant, with a clump of mussels. Holding a mussel between its forepaws, the otter would bang it against the rock until the hinges had cracked enough that the shell could be pried open with tooth and claw. That sounds like a pretty simple process, but it turned out the otters were surprisingly methodical. They usually aimed for ridges or points on the rock, not flat surfaces—which makes sense, since ridges and points would focus the force of impact at a single point instead of spreading it out. They also seemed to prefer the upper parts of the boulders on the side facing the water.

When Haslam and his colleagues studied the Moss Landing boulders, they used the same checklist of criteria archaeologists use for studying stone anvils from prehistoric human sites. They found a distinctive pattern of damage to the ridges and points on the boulders. Floating debris would leave a more evenly distributed pattern of scrapes and dings on the rock, while human activity would be more focused on the landward side of the rock. This was different, showing otter material culture.

Haslam et al. 2019

Haslam et al. 2019

Haslam et al. 2019

Mustelid middens

Shell middens are piles of discarded shells and other domestic waste, like stone flakes and animal bones, left behind by prehistoric people. They're a classic staple of archaeology. And it turns out that otters have their own version of those, too. Otters are messy eaters; once an otter pries open a mussel and eats the tasty snack inside, it just lets the shells fall into the water. The result? Dense piles of mussel shells around the bases of the most heavily used anvil rocks.

If each otter at Moss Landing foraged for an hour every day and ate roughly 50 mussels, an average of around 3800 discarded shells a year should pile up in the middens between the boulders. That’s a pretty conservative estimate, since most sea otters spend much more than an hour a day foraging. And according to Haslam and his colleagues, you’d only need to pick up a few of those shells to realize they were actually sea otter artifacts.

Some earlier studies have suggested that most sea otters, like most humans, are right-handed (well, right-pawed), and though that’s definitely not a settled fact, the otters at Moss Landing showed a clear tendency to hit the rock with the right side of the mussel first. The result seems to be piles of shells with telltale radiating fractures from a point of impact near the broken right valve, but intact left valves.

Not just for primates—or even mammals

The goal of archaeology is to better understand past cultures, so what can sea otter archaeology tell us? By looking for their trademark stone anvils and shell middens, ecologists could gain a better picture of where sea otters lived in the past. We already know that they once ranged from the beaches of Baja California all the way around the northern Pacific Rim to the shores of Japan, but today they’ve been reduced to remnants of that territory. Learning more about their past habitats could help us understand how best to conserve the sea otter habitat that’s left—anvils and all.

Eventually, stone anvils and shell middens may also help ecologists learn more about how sea otters evolved this unique, handy behavior, and how it spread. And that could tell us something about how otters’ tool use is evolving thanks to our presence in their habitats; at Moss Landing, Haslam and his colleagues noticed that otters were happy to use concrete blocks, and occasionally the side of a metal culvert, as anvils.

This isn’t the first time archaeologists have looked beyond humans and our hominin ancestors. Several studies have focused on chimpanzees’ tool use, including a 4,300-year-old site where food residue still stuck to stones that chimpanzees had used to crack nuts. But primates don’t have a monopoly on tool use, and that could mean they won’t have a monopoly on archaeology for much longer. The frighteningly clever crows of New Caledonia carefully shape wooden hooks with which to scoop worms out of holes, and several other species of birds drop mollusks and nuts on stones to crack them open. Even fish in the family Labridae use underwater rocks as anvils to break open scallops and sea urchins.

Archaeology may have a lot more to tell us about the world than we realized.

Scientific Reports, 2019. DOI: 10.1038/s41598-019-39902-y;(About DOIs).