Sea otters found to be “touch specialists”

Oct 2, 2018

Sea otters spend much of their day devouring whatever ocean snacks they can hunt down. They rely on their digestion to keep warm in the chilly Pacific Ocean, so they need to get their paws on at least ten pounds of food each day.

But finding clams, abalone, and shrimp in the murky waters off California’s Central Coast is a tricky task. While otters can use the full portfolio of their five senses in air, most of those senses are much less useful when an otter dives for food underwater.

Researchers from the University of California at Santa Cruz recently looked into otters’ sense of touch — zooming in on both paws and whiskers —to understand how otters find so much food each day. Their results appeared in the Journal of Experimental Biology in September.

The studies focus on one otter, a research animal officially named Otter 595. She’s affectionately called Selka, after the half-seal, half-human mythical creatures. And Selka’s tactile abilities were clearly a strength.

“With either her paws or her whiskers, Selka’s sense of touch is comparable to a lot of other species that we consider as touch specialists,” said Sarah McKay Strobel, a sensory ecologist at UC Santa Cruz who worked on this study.

At the outset of this study, Strobel had a thesis that touch was a crucial sense for otters — many of their other senses don’t seem to provide much help for an otter hoping to find a clam.

“Otters can see relatively well, but if you consider the environment they live in,” Strobel said, “it's really hard to see more than a foot or two ahead of you, regardless of if you're a human, or a seal, or a sea otter.”

Hearing isn’t a strong suit for otters, either. They struggle to pick up small signals against the ocean’s constant background noise.

It’s a similar story for an otter’s sense of smell.

“A sea otter is holding its breath every time it's diving, so it can't necessarily sniff and smell things in the strictest sense,” Strobel said.

To get a handle on Selka’s tactile precision and speed, Strobel used a set of plastic plates covered in vertical grooves, similar to the texture of corduroy pants. First, Selka learned that one of the plates — the plate with two-millimeter wide grooves—was the right plate to choose. Then, Strobel ran tests to see how well Selka could choose between that special plate, and others with slightly different grooves.

The tests happened in an experimental set up that’s almost like a little otter-sized drive-thru window.

At the beginning of each test, Selka’s drive thru door slid open. But instead of placing an order, Selka felt two different textured plates for as long as she liked. In some tests, she used only her whiskers, while in others, she used only her paws.

When Selka chose a plate by pushing it down, she got instant feedback.

“If she got it correct, then she got a big juicy shrimp,” Strobel said. Selka’s drive-thru window also played an approving bell tone for all right answers.

For wrong answers, Selka still got a bit of food, but the apparatus played a quick “Nope” sound.

But Selka heard more bell tones than “Nope” sounds. Her responses were accurate, and she was also speedy.

“Selka was solving the problem with her paws in less than 200 milliseconds, sometimes even less than 100 milliseconds,” Strobel said. “And with her whiskers she was less than 400 milliseconds, so less than half a second.”

In that time, Selka could tell the difference between grooves with teeny tiny differences—less than a millimeter for both her paws and her whiskers.

“We don't necessarily know what exactly she's picking up about it, but it's something about the way that the plate feels to her, the way that the texture feels to her,” Strobel said.

Humans took much longer to choose between different plates as well as Selka could. Undergraduate volunteers in the study needed a few seconds or more to choose between two plates. But that makes sense, Strobel says, because people don’t feel their way to meals, the way otters do when they dive.

“When you start to account for all these things that a sea otter needs to be able to do within 30 to 90 seconds while holding its breath, you can imagine why it might be very important to use your sense of touch to find things and catch things really, really quickly,” Strobel said.

And finding food matters for the otter population in California as a whole, said Mike Harris, an environmental scientist for the California Department of Fish and Wildlife. Southern sea otters are fully protected in California, and they’re listed as threatened under the federal Endangered Species Act.

Sea otters’ biggest threat, Harris says, is shark bites. But food availability matters, too.

“Along most of the Big Sur coastline otters have reached a point where they're essentially at carrying capacity,” Harris said. 

Carrying capacity means there’s not enough food available for more otters to survive. But understanding how otters find food could help researchers and conservation experts continue recovering their population in the future, Harris said.

As for sea otters’ sensitivity to different textures, Harris isn’t exactly surprised. Otters clearly use their paws and whiskers a lot—but knowing that they can use their sense of touch so quickly is fascinating, he said.

When otters seem to be fiddling around in the ocean, they’re also collecting lots of information from their paws and their whiskers—using touch quickly enough to keep up with their voracious appetites.