Proving the power of whiskers and paws.
The underwater world of kelp forests is a murky, mysterious place full of beautiful creatures. Some of them—sea otters—are among the most charismatic aquatic mammals. Newly published research from the University of California at Santa Cruz helps us peer into the murk and figure out how those snuggly-looking critters find food.
In the work, a rescued sea otter named Selka worked with UCSC's Sarah McKay Strobel to show off her ability to use her sensitive paws and whiskers to explore the world around her. Strobel's findings, documented in a paper published today in the Journal of Experimental Biology, show Selka proved to be much faster than humans at identifying the world around her by touch—which might illustrate how key touch is to aquatic mammals like sea otters.
Selka has a long history with the Monterey Bay Aquarium, where she is now a permanent resident after several attempts to release her into the wild early in her life. Between 2014 and 2016, however, she worked at the nearby Long Marine Laboratory with Strobel, her coauthors, and a team of volunteer helpers to help them understand how sensitive her paws and whiskers are.
With the help of some seafood treats, the researchers gradually introduced Selka to a blindfold, and she got comfortable with the other experiment equipment: a little cabinet in the side of her pool with different-sized openings. She could reach her paw directly into the cabinet, unblindfolded, and learned to distinguish between the feel of a board that had 0.08 inch grooves running vertically along it and other boards with different size grooves. She also learned how to use her whiskers, while blindfolded, to feel the difference.
Strobel says getting Selka's cooperation and not pushing her boundaries was key to the experiment design. In the end, she says, Selka “really liked doing the experiment” for the stimulus (and snacks) it provided, and even became very comfortable wearing the blindfold. It could have been very different, she says: “Imagine yourself in that situation. It could be kind of scary to have someone blindfold you and then ask you to do these complex tasks.”
But in this case, after Selka had gotten comfortable with the conditions, it was time to conduct the experiment proper. “You need to have a very controlled setting,” to do this kind of testing, Strobel says. Previously, she made attempts to observe sea otters' underwater touch behavior in the wild, but given how dark it is in the otters' murky foraging grounds, the limited visual range of cameras, and other technical concerns, she says it proved impossible. “Even in the daytime,” she says, “the water can be so murky that it almost looks like night.”
The kind of experiment Strobel conducted with Selka closely mirrors others done with aquatic mammals like manatees and pinnipeds such as seals and sea lions. With Selka in the experiment conditions she was by now used to, she distinguished between boards with differing groove widths. One of those boards, however, always had a 0.08 groove, and when Selka identified this one by pressing hard on it so it made a loud “click” noise, she got a reward. The researchers found that Selka could detect a difference down to about 0.04 of an inch. They were also surprised to learn how fast she could do so: It took her just 0.2 seconds using her paw and 0.4 seconds with her whiskers. The human volunteers who agreed to try the test took about 30 times as long to identify the difference with their fingers.
Part of Selka's speed advantage was that she didn't compare the two options: she would feel one, and if it was the 0.08 inch groove, she'd press it right away without feeling the other to compare. That “means you're remembering what the targets are,” says New College of Florida professor emeritus Gordon Bauer. He was not involved with the current experiment, although he conducted similar work on manatees. Like the manatees, he says, Selka turned a forced choice between two objects into a “go/no-go” experiment. She just remembered what she was looking for and identified it when she found it, without comparing it to other items nearby to double check.
In terms of time and sensitivity, the sea otter's abilities seem comparable to pinnipeds and manatees, Bauer says. “This is the first work that's been done on sea otters,” he says, and provides more evidence of the important role whiskers play in sensing for aquatic mammals. The otters also add a new layer with their paws, which both manatees and pinnipeds lack.
In the case of all these animals, the ability to quickly distinguish different objects by touch is very linked to memory, Bauer says. In the case of Selka, Strobel says, and sea otters more generally, that memory is key for being able to find food quickly in murky waters. Sea otters spend only one to three minutes underwater on a given dive, she says, and in that time, they need to reach foraging depths and find their tasty urchin or other food. Being able to remember how things should feel may be a key component of that, she says.
This new study adds a dimension to the current understanding of sea otters. Although they have been much-studied in the past 25 years, those studies have “focused on sea otters' lives at the surface of the water,” Strobel says. But given that sea otters eat an estimated 25 percent of their body weight per day, and all that food has to be gathered from under the water, scientists were missing a big piece of the picture, she says—what abilities allowed them to get their food in the short intervals they spent on a dive. This research points to touch being a big one.
Thanks to Selka, the conservationists who work with sea otters at Monterey Bay and elsewhere now know a little more about how their charges see—or feel—the world.