A Monterey Bay sea otter with a rock anvil on its belly and a clam in its forepaws.
Jessica Fuji
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Jessica Fuji
A Monterey Bay sea otter with a rock anvil on its belly and a clam in its forepaws.
Jessica Fuji
In parts of the ocean where sea otters face stiff competition for their favorite types of food, some otters cope by using tools – such as rocks and even glass bottles – that allow them to open tougher prey that they otherwise wouldn’t. able to eat.
That’s according to a new study of sea otters in Monterey Bay, California, which looked at tool use by individual otters to see how it affected their health and nutrition.
The findings, published in the journal science, reveal how this special set of skills can increase their chances of survival in an uncertain world.
Sea otters are large marine mammals that spend their days foraging in kelp forests. They dive to the bottom to grab tasty morsels, plus sometimes rocks that they plan to use as tools. They bring them to the surface and float on their backs, using their bellies as tables as they begin to open and eat their snacks.
A female sea otter swims in Monterey Bay, off the coast of California, with an anvil-like scale on her abdomen that she will use to open the clam she holds in her front paws.
Jessica Fuji
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Jessica Fuji
A female sea otter swims in Monterey Bay, off the coast of California, with an anvil-like scale on her abdomen that she will use to open the clam she holds in her front paws.
Jessica Fuji
“Their preferred prey is usually hedgehogs and snails,” says Chris Lowe, a biologist at the University of Texas and Washington, who notes that hedgehogs and snails are relatively easy for otters to separate.
But in places where many otters live together in Monterey Bay, “unfortunately, all of these prey items are declining or have declined,” Lo says.
He says there are areas of overabundance of urchins — so-called “barren urchins” where the urchins have eaten all the algae. Since the algae have disappeared, however, these urchins no longer have good food and are therefore poor in calories. They offer little nutritional value to the otters, who are not interested in consuming these so-called “zombie hedgehogs”.
“This means otters have to eat alternative foods,” says Lo. “A lot of these alternative foods are these super-hard prey items that really require some sort of external force to penetrate them.”
Snails, for example, are plentiful in the bay, but they’re low in calories and “basically like a rock that you have to break open to eat the guts,” Lo says.
While sea otters are known to use tools, not all individuals actually use them. Some otters abandon them entirely and simply specialize in eating soft prey. Some otters use tools occasionally, while others use them most of the time when foraging.
“We were interested in the variation in the use of this tool,” Lo says, so he and some colleagues analyzed data from 196 otters in California.
These tagged otters are watched closely by volunteer ‘otter watchers’. This means researchers know what they are eating, how big and hard the prey is, and whether the otter used a tool to eat it.
It turns out that frequent tool users were able to eat harder and larger prey, according to a report in the journal Science. This was especially important for female otters, as they are smaller than males and cannot bite with as much force.
“They usually wouldn’t be able to penetrate harder prey,” Lo says. “But they use tools more than men, so they can access these new food sources.”
What’s more, the use of tools protected the otters’ teeth. The researchers were able to make evaluations of their otters’ teeth and found that tool users had less damage to their teeth from crunching hard shells.
“Without their teeth, they obviously can’t eat anything. Then they die. “What we’re suggesting is that this behavior really allowed them to continue living even though they didn’t have their preferred prey,” Lo says.
He explains that some otters learn to specialize in eating hard, low-calorie snails by using tools very often to “basically become really, really, really good at processing lots and lots and lots of snails every day.” instead of looking for high-calorie foods that don’t require tools to open but are in short supply.
“This is such an important document,” said Rob Shoemaker, president and CEO of the Indianapolis Zoo and one of the authors of a book called Animal tool behavior.
He says scientists have spent decades documenting tool use in dozens of species; tool use in sea otters, for example, has been recognized since the 1960s. But now studies like this show that this area of research is beginning to shift.
“It’s no longer about describing actual tool use or tool making,” says Shoemaker. “It describes the impact it has on that animal’s life.”
