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The sea otter raises her stone. The muscles in her forearms tense under the weight. With a single motion she hammers the rock down. Once. Twice. Crack! Bits of soft meat glisten as they fly free from the battered snail shells. The otter scoops up the flesh and devours the tasty morsels. As a baby she learned to use rock tools from her mother, just as her mother—and all the mothers before her—did. Her technique, refined over the years, enhances her capacity to crack even the toughest snails. With a twist of her nimble body, the otter flips and dives. The ocean washes away all traces of the rock, the shell, and the feast.

Sea otter archaeology is ephemeral. Physical records indicating how past generations of sea otters employed stone as both hammer and anvil—and how they shared this ancient craft—don’t exist. Yet new findings about how individual sea otters use tools in the present day, and how that varies across sea otter populations, have biologists talking. The discoveries are even tempting a few primate archaeologists to peek across the species divide.

“Not many animals on the planet use stones to crack food open,” explains Dora Biro, a zoologist at the University of Oxford in England who studies the emergence of culture among wild chimpanzees. She pauses to list the members of this elite group: chimpanzees, humans, long-tailed macaques, bearded capuchin monkeys, and sea otters. “Oh, and Egyptian vultures pick up rocks and drop them on ostrich eggs to break them,” she says. “Tool use is itself, very rare. Animals like [sea otters], who use percussive stone-tool technology, who pick up a rock and develop the skill to strike things with it, that is even rarer.”

Biro and her colleagues seek to identify the circumstances that lead animals to develop percussive stone-tool technology. Stone-tool technology is of interest to primatologists because it provides the earliest surviving evidence of the beginning of human tool use. “Tool use is what allows us to have this degree of dominance of the planet,” says Biro. “So in human evolutionary history, stone tools have been monumentally important.”

The ability to use stones as tools is not hardwired. Bearded capuchins are the only monkeys to use stone tools to crack open nuts, and it may take them three years or more to perfect their skill. The same holds true for sea otters. Pups learn how to use tools from their mothers when they are young, but it takes time to develop their techniques to efficiently open prey.

But it’s not that straightforward: “We see pups pound their paws on their chests without prey, so that behavior seems to be almost instinctive,” Jessica Fujii, a sea otter researcher at the Monterey Bay Aquarium in California, explains. Yet by the time they reach adulthood, around three years of age, only some sea otters use tools. “If they don’t [use tools by that age],” says Tim Tinker, a wildlife biologist at the United States Geological Survey’s Western Ecological Research Center in California, “then they won’t … and we see no difference with age after that.” So, if sea otters are going to use tools, they’ve definitely learned to by the time they’ve reached adulthood.

Poring through 17 years of observational data on prey capture and tool use from eight sea otter populations ranging from southern California to the Aleutian Islands in southwest Alaska, Fujii and her colleagues have discovered significant variation in the frequency of tool use. Their research encompassed two subspecies: Alaskan (or northern) sea otters, Enhydra lutris kenyoni (which range from the Aleutian Islands to Washington State), and California (or southern) sea otters, Enhydra lutris nereis (which range from south of San Francisco to west and north of Santa Barbara).

California sea otters turn out to be far more likely than their counterparts in the north to use tools. When examining successful dives (those that resulted in food), the researchers found that sea otters in Monterey, California, used tools on 16 percent of successful dives. In comparison, northern sea otters in Amchitka Island—part of the Aleutian Islands chain in Alaska—used tools on only one percent of their successful dives. Yet Fujii’s research demonstrates that tool use isn’t simply a subspecies difference. Other populations of Alaskan sea otters have a greater tendency to use tools than those in the Aleutians. In Glacier Bay, in southeast Alaska, for example, Alaskan sea otters used tools on 10 percent of successful dives.

A key factor in whether or not a sea otter becomes a tool user appears to be diet. In the rocky habitat of a kelp forest, sea otters tend to prefer to eat sea urchins. They quickly process these thin-shelled animals by first shoving aside the spines with their paws. Then they bite open the shell and suck out the insides. No urchin specialists use tools to open their spiky prey. “Perhaps that is why we so rarely saw [sea otters] using tools in the Aleutian Islands,” Fujii says. “There, urchins were making up 75 percent of their diet.”

Conversely, snail specialists that use tools to crack open their thick-shelled prey tend to apply those tools to all the prey that they eat, even when they bring up urchins or other prey that other sea otters don’t use tools for. “You know that old saying, ‘If all you have is a hammer then everything looks like a nail?’” says Tinker. “If you are a tool user and all you have is a rock, then everything looks like a snail.”

In simple, deterministic terms, if using a tool increases your efficiency at eating turban snails, and you have 30 percent of them in your diet, then all sea otters everywhere whose diet is 30 percent turban snails should use tools 30 percent of the time. “But that is not the case,” says Tinker. “It increases in a very non-linear fashion with the amount of those prey types in the population’s diet.”

Tool use, as Tinker explains, grows even more complicated when you factor in how scientists believe social learning occurs. “If you are a young sea otter in California who is eating black turban snails and you are surrounded by a lot of other tool-using snail specialists, then you have a much higher likelihood yourself of learning to use tools,” explains Tinker. “But if you happen to be a young sea otter in southwest Alaska who eats snails and most of the other sea otters you see swimming around are eating urchins and not using tools, then you are much less likely to use tools yourself, even if you are eating just as many snails as that snail eater in California.”

Sea otters show impressive capacities to innovate and improvise. “Like ripping off a crab’s claw and using it to pry open the crab’s own shell,” says Tinker. Some sea otters in Monterey’s harbor, for example, pound shells against the hulls of boats or ship ladders. And inventive food gathering techniques don’t stop at the level of mothers and pups. Sometimes they spread across many sea otters in a region.

Tinker was on Amchitka Island watching sea otters in the early 1990s when large numbers of Pacific smooth lumpsuckers, Aptocyclus ventricosus, came into the shallow waters close to shore. These bizarre globular fish have a suction disk on their abdomens and normally spend most of their lives in the open ocean. Their spawning cycle is unpredictable, so they may not appear in the nearshore environment for decades and then suddenly show up to spawn. “No one had recorded seeing these fish on Amchitka since the 1950s,” Tinker explains, pointing out that none of the sea otters he was watching would have been alive at that time, so he presumes the fish were new to them, too.

Pacific smooth lumpsuckers are nest brooders. The males use their suction cups to stick themselves to the rocks where the females have laid their eggs, so they can guard them. It’s a good strategy for protecting the eggs from other fish, but not the best if the rock you happen to stick yourself to gets picked up by a sea otter. “Initially, only a couple of sea otters discovered [the lumpsuckers]. And then it just spread otter by otter like wildfire,” Tinker says. “The fish started showing up in February and by April, all sea otters were feeding on them. It dominated their diets. I actually saw a sea otter do a double take the first time it saw another sea otter swim by with one in its mouth,” he says with a smile. “It was like watching a slapstick movie.” These sea otters appeared to be learning from other otters in the population rather than just learning from their mothers.

When you strip down all this marvelous complexity, the fact that tool use occurs with similar prey types over their entire range from the western Aleutians right around to Prince William Sound, Alaska, British Columbia, Washington, and California suggests that sea otters have been using tools for a very long time. “You don’t see differences in whether they are using rock as hammer or rock as anvil,” Tinker explains. “The nature of how they use tools and what prey they use tools with is identical across their entire range.” But whether that means it goes back five to seven million years when sea otters first entered the marine environment is not known. Hints, however, might be found in the archaeological record, though not in the “rock hard” evidence we’ve come to expect from the study of ancient primates.

Researchers reveal that tool-using primates bring their food to one site and use both the site and their stones over and over again. In one 2007 study, for example, archaeologists working in Tai National Park in Ivory Coast announced the astonishing discovery of stone tools used by chimpanzees to crack nuts 4,300 years ago. The tools show use and wear patterns consistent with stone tools used by modern chimps to crack nuts, indicating that this behavior has been transmitted over more than 200 generations. Their ancestors’ stories remained in the rocks the primates once held. Unfortunately, records of sea otters’ tools are virtually impossible to find. Contrary to popular belief, sea otters do not have favorite stones that they tuck under their arms for repeat use. “Sea otters use a rock,” Fujii explains, “and then they drop it. It becomes just one of many rocks on the ocean floor.”

Yet one might still be able to piece together the archaeology of sea otter tool use from something more intimate—their teeth and bones. Sea otter teeth are two and a half times more resilient to chipping than human teeth thanks to the microstructure of the enamel that coats them. Even so, they are no match for the rock-hard surface of a seashell. “For a sea otter to eat turban snails using only its teeth, it’s not only difficult, it’s probably suicidal,” says Tinker. “Very quickly it will crack its teeth.”

Tinker underscores the speculative nature of the question of sea-otter archaeology but agrees that indirect inference is the only way to approach it because, unlike other tool users, the way sea otters use tools doesn’t leave a record on the tool itself. “There is clearly a selective force there and if [sea otters] were eating these thick-shelled snails a million years ago, and their teeth were no different than they are now, then it is hard to see how they would have done that without cracking their teeth if they weren’t using tools,” he says. “So … if otters lived to a certain age in the archaeological record and were eating snails, we can pretty much assume they had to be using tools.”

Clues to whether sea otters were eating turban snails in the past may lie in the Channel Islands, an archaeology site off the coast of southern California that is rich in cultural history with over 10,000 years of habitation. Remnants of Chumash civilization can still be seen in over a thousand shell middens on the islands, some with well-stratified layers—essentially slices of time—that haven’t been mixed up by gophers or other forces that jumble deposits together. “We get this beautiful stratigraphy which enables us to do fine-grained analysis through time,” says Todd Braje, an archaeologist at San Diego State University in California.

Sea otter teeth and bones are found in these middens going back at least 9,000 years. And many of the islands’ middens are completely dominated by black turban snails. Evidence that the turban snail shells were just as tough and difficult to crack then as they are now is found in the tools the Chumash themselves fashioned. “We find ‘turban smashers,’” Braje explains, describing a type of “anvil stone” the Chumash used to crack the shells open before throwing the snails in boiling water for cooking.

In the coming year, Paul Szpak, an archaeologist at the University of British Columbia in Vancouver, will examine the isotopic composition of sea otter bones found in the Channel Islands’ middens. The isotopic data will reveal the distribution of certain stable isotopes and chemical elements within the bones, shedding more light on the type and relative proportion of different foods these sea otters were eating.

Seth Newsome, an animal ecologist at the University of New Mexico, is willing to bet that otters have been using tools to process turban snails far longer than people have been on the Channel Islands or even in North America. “In the Channel Islands, human records date back only 11,000 or 12,000 years. In North America, it may be closer to 20,000 years ago,” he says. “That’s a blink of an eye in sea otter history.”

Seth believes sea otters have been using tools for most of their existence. “Sea otters have lived in the ocean for millions of years. They are small and they live in cold water. They need to eat constantly,” he explains. “To do that, they’ve evolved novel and fascinating ways of efficiently catching and processing large quantities of hard-shelled invertebrates. Tool use probably evolved early on in their evolution into the nearshore-ocean niche that they occupy.”

Piecing together the archaeology of sea otter tool use is a nascent endeavor. Like dance or song or any other behaviors that archaeologists study, the action of hitting a rock against a shell is transient. An account of the very first time a sea otter picked up a stone and whacked a snail will forever remain elusive. Over time, however, a better understanding of tool use will emerge in speculative fragments across disciplines.

Sea otter tool use is of inherent interest given its relationship to intelligence, innovation, and cultural behavior. “Culture,” says Dora Biro, “is one of the hottest topics in behavioral biology. With culture, you see different behavior in different groups. Knowledge is passed on socially between individuals. Individuals learn things socially from their mom and others. Many of these things vary regionally.”

Sea otters tick all these boxes. There is nothing robotic about a sea otter hammering a snail against a rock on its chest. The reality is much more nuanced and complex. Each motion reminds us that tool use is part of a rich history of cultural innovation that is thriving in the sea.