In California, otters turn to tools to eat prey in Monterey Bay

Sea otters on the West Coast are turning to resource use to exploit richer prey and gain a competitive edge, a new study shows.

In Monterey Bay, an incredible rebound in sea otter populations has made their favorite prey scarce — and prompted them to pursue tougher options, according to findings published Thursday in Science.

“Historically speaking [otters] prefer sea urchins and abalone – prey items that require no tools to break open,” said lead author Chris Law, a researcher at the University of Texas at Austin.

“But those populations have declined enormously. And a lot of it has to do with the otters eating them all.”

That decline has been a boon to coastal ecosystems. Sea urchins are voracious consumers of kelp, the undersea algae forests that perform a similar function under the sea as tree forests on land.

Otters, which eat three times as much per pound of body weight as a typical mammal of their size, are “remarkably efficient predators,” says ecologist and co-author Tim Tinker of the University of California, Santa Cruz.

By eating their favorite sea urchins, which would otherwise graze on kelp and coastal marshes, otters have helped restore both ecosystems, according to findings published in Nature in January.

That’s the culmination of a great conservation success story, Tinker noted. To survive in the cold waters of the North Pacific, sea otters have the densest fur of any mammal: up to a million hairs per square centimeter, which is fifty times that of a chinchilla – or a thousand times that of a human .

This dense fur fueled a two-century-long commercial hunt in which trappers killed hundreds of thousands of otters—creating corporate giants like the Hudson’s Bay Company and ultimately reducing Central California’s otter population to an isolated colony of just fifty individuals.

As scientists now understand, otters are a “keystone species,” meaning they have an impact on ecosystems far greater than what their numbers would usually suggest. Their decline in recent centuries caused a ‘cascade’ as hungry sea urchins, freed from their main predator, ate through the kelp forests.

As a result, the coastal ecosystem underwent “a phase shift from kelp forests to deforested arid sea urchins,” a 2009 paper found. As the kelp forests disappeared, the numbers of filter-feeding mussels and barnacles declined, as did populations of fish such as greenfinches. Bald eagles switched from a balanced diet to one consisting solely of seabirds such as gulls; seagulls in turn switched from eating fish to invertebrates.

Sea otters swim near shore at Elkhorn Slough in Moss Landing, California, on April 12, 2018. Marine biologists at the Monterey Bay Aquarium have observed that sea otters rehabilitated and released into Elkhorn Slough have helped restore the eel grass beds and ecosystem. (Photo by Paul Chinn/The San Francisco Chronicle via Getty Images)

Edge of extinction

In 1911, with otters on the brink of extinction, Congress banned the fur trade, and by the time it passed the Marine Mammal Protection Act in 1972, the local population had risen again to 1,000.

Now they’re closer to 3,000 — putting them somewhere close to “equilibrium” with their local prey sources, Tinker said.

Their increasingly specialized diets — an exclusive focus on snails or clams — “and the increased use of tools are some of the interesting behavioral manifestations of this process” of recovery, Tinker said.

The tools in question are, in human terms, very simple: rocks, glass bottles, shells, or the sides of boats or docks.

But these make an excellent anvil on which a hungry otter can crack open a mussel (rare but very nutritious) or a sea snail (not high in calories, but very common) and scoop out the tasty meat.

These tools have allowed resurgent otter populations to do something that was little reported in the years when otters had an abundance of sea urchins they couldn’t eat: specialize on hard-shelled mollusks.

No one knows whether this specialization – or the use of tools – is really new to sea otters. “Their tools don’t really fossilize,” Law said. “So in terms of when their tools first appeared – no idea.”

“It would be super cool to find out,” he added.

Sea otters split from their freshwater relatives about five million years ago – about as far back as humans split from the other great apes.

It is also not clear whether the choice to specialize in specific prey – as sea otters now do – is new. The last time sea otter populations were this high was during the height of the fur trade, and the modern study of marine ecology did not exist.

In other words, no one alive today saw Monterey Bay when it came close to its carrying capacity for otters—at least, until now.

However, one thing is clear from the research: Whether it’s a new behavior or not, sea otters’ ability to tap into new resources could mean the difference between life and death as competition for their preferred resources increases.

Female sea otters sensitive to tool use

That’s especially true for female sea otters, who the scientists found are particularly sensitive to tool use.

“Females have a weaker biting ability, so they are unable to break open harder or larger prey,” says Law.

By using tools, “they can really access these harder, larger prey that they might not otherwise be able to get to by biting alone.”

That’s important because female sea otters are also the ones who have to give birth to, care for and raise the next generation.

This is a long and energetically demanding process that only ends when the pups have learned to forage for themselves – which often means adopting the strategy used by their mother.

“Anything that increases feeding efficiency will really help these otters survive and raise pups until they are ready to go,” Law said.

This gender disparity in tool use is not unique to otters. Female bonobo chimpanzees, chimpanzees and dolphins also use tools more often than their male counterparts.

Sea otters are seen together along the Elkhorn Slough in Moss Landing, California on March 26, 2018. (AP Photo/Eric Risberg)

By using tools, female otters were able to break open prey twice as hard as they would otherwise have been able to, the researchers found.

For men, using tools does not necessarily open up new resources: their hard jaws can break open snails or mussels without help.

But it does save their teeth. Both male and female otters suffered less dental damage when they used rocks, boats or bottles as anvils to crack open their clams or crabs – which meant fewer cracked teeth and ultimately a longer life.

This adds otters to “a growing list of examples of species” that use tools to minimize injuries while hunting – such as birds that drop hard-shelled prey from a height to break it open, or dolphins that sponge use them as a kind of glove to protect their prey. snouts as they pull bottom-dwelling fish from the sand.

Broadly speaking, Tinker said, the otters’ recovery and their surprising behavioral change may hold lessons for the broad swaths of the California and Oregon coasts where they have not yet recovered.

In those areas, “their ecosystem functions are still absent,” Tinker said.

“But as we look at future environmental change and the need to build resilience in coastal ecosystems, we think sea otters could be an important part of that.”

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