Using archaeology to uncover sea otters’ historical habitats

When a hungry sea otter uses a rock to crack open a tasty mussel, it’s doing something unique among marine mammals: using a stone tool. Researchers are now revealing how this behavior makes it possible to study sea otters’ past through the lens of archaeology.

A sea otter cracks open a mussel shell using a shoreline “anvil rock”.

Sea otters use large stationary stones like anvils, to smash open mussel shells. Their hammering leaves distinctive marks on the rock. An ideal place to document this behavior is just a short drive up the coast from the Aquarium, near Elkhorn Slough.

It’s an area that Monterey Bay Aquarium researchers like Jessica Fujii have monitored for years. Southern sea otters are a threatened species, and many of the orphaned pups reared by the Aquarium’s one-of-a-kind surrogacy program end up joining the wild population in Moss Landing Harbor. Years of data on sea otter foraging gathered by Aquarium researchers showed otters like to dine near the Bennett Slough culverts at the north end of the harbor.

“We have observations spanning about ten years in this location, both on wild animals that have no tags, and also sea otters that were raised at the Aquarium and successfully transitioned to the wild,” says Jessica, who has also monitored wild sea otters (while avoiding brown bears) in Alaska.

By pounding mussel shells on shoreline rocks, sea otters create unique marks on the rocks that identify them as a favorite habitat.

“I usually know as soon as I pull into the road if there are otters around because there are tourists with cameras enjoying seeing them up close,” she says. Walking carefully so as not to disturb the animals, Jessica takes up a spot 30 or 40 feet away. “There’s a little fence you can just lean up against, watch the otters and collect data,” she says.

This work proved key for researchers from Germany’s Max Planck Institute for the Science of Human History. Their collaboration with the Aquarium and the University of California, Santa Cruz led to the new sea otter archaeology publication in Scientific Reports.

Animal archaeology

For years Jessica has been studying the different and often fascinating ways sea otters use tools — and sea turtles their own flippers — to forage. Her research turns heads in what she calls the “small but growing” realm of animal archaeology, which focuses on primates and other animals’ tool use.

Sometimes, wild animals using tools can leave physical evidence hinting at their species’ history.

Many animals, including the New Caledonian crow, are known to use tools. Photo courtesy James St. Clair, University of Western Australia

For instance, capuchin monkeys have been shown to use stone tools to crack open nuts for centuries, if not millennia, while New Caledonian crows craft tools out of twigs to extract delectable bugs from hard-to-reach holes.

A conversation with the archaeologists ensued. Would it be possible to use the techniques of animal archaeology to study marine mammals?

In cases where an otter picks up a small rock from the seafloor, cracks open its next snack, and then chucks the rock back into the water, the answer was: probably not. Often, the otter is hundreds of yards offshore, making the rock difficult if not impossible to retrieve.

But at the Bennett Slough culverts, the stones are fixed objects along the shoreline. So when the team from Europe came for a month-long visit, Jessica and Tim Tinker from the US Geological Survey pointed out the location where they could observe the otters’ pounding behavior close to shore. The team captured videos of otters cracking mussels against the anvil rocks.

Mussel shells collected by researchers at the Bennett Slough site show distinctive breakage patterns.

Upon closer examination of the area, the visiting researchers discovered peculiar piles of damaged shells.

“From that, they were able to match the behaviors they were observing with the damage patterns they were seeing on these rocks,” she says. “But they wanted to be able to link their observations to more long-term behavior records. That’s when we pulled out the observations already collected just by happenstance at that location, both by UC Santa Cruz and by the Aquarium.”

Ultimately, the team found enough evidence to demonstrate that otters are responsible for a distinctive pattern along the points and ridges of stone nearest the water — marks that, in the past, could have been misattributed to humans.

A new tool for studying otters

“Showing that we can use archaeological methods with a marine mammal is pretty exciting,” Jessica says. “I feel like it opens doors to new research opportunities.”

The next question facing researchers is whether they can find other locations in the sea otters’ historical range where mussels and anvil rocks are found together. It might be around Elkhorn Slough — or it could be somewhere else entirely, since sea otters once ranged all the way from from Mexico’s Baja Peninsula to Northern Japan.

Sea otters position rocks on their chests. as well as shoreline anvil rocks, to crack open shellfish.

Such a finding could show that hungry sea otters were using similar stones as anvils elsewhere, perhaps before they were nearly wiped out by fur hunters in the 1800s. The biggest hurdle, Jessica says, is how much human activities have changed the coastline.

In addition to documenting that mussel-pounding leaves distinctive marks on anvil rocks, the team also stumbled across an interesting “accidental finding”: Every sea otter they observed relied on its right paw to control and smash its food, just as most humans might use their right hand.

“This gets into the idea of ‘handedness’ that we see in other animals, preferring a right or left side,” Jessica says. “We didn’t intend to look for that.”

From an evolutionary standpoint, this raises more questions. Though the Aquarium has been observing sea otters for decades, Jessica says this finding is new.

“There’s still a lot we don’t know about sea otters,” she says. “But the clues they leave, whether in ancient middens or on pounded rocks, help us better understand their essential role in coastal ecosystems.”

You can watch sea otters using anvil rocks in the video below, and learn more about Monterey Bay Aquarium’s work to study and recover California’s sea otters on our website.

Featured photo: Sea otters feast on mussels at the Bennett Slough culverts in Moss Landing Harbor, where researchers used archaeological approaches to study favored sea otter habitat. Photo © Michael Haslam

—Daniel Potter

One thought on “Using archaeology to uncover sea otters’ historical habitats”

  1. On 3-17-19 i took my skiff back to San Nicolas Island and i can tell you that after the rampage by the fin-fisher last Sept, the entire south side of the island remains denuded! The remaining strands of kelp are flourishing–never seen longer, singular strands of kelp– There is ZERO canopy left along the entire back side of the island. I hope you do a kelp survey, or know of an organization that does this because this loss of canopy at the island precludes the establishment of any otter population in the future! You can write off San Nicolas island as a last ditch potential sanctuary in the Southern habitat. I will know your commitment to the survival of the sea otter by your decision to open a dialogue. This was a tragic and historical loss, i hope that you care–YOU have a following, a VOICE in this struggle, i am just one man…


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