Harbor seals have an amazingly fine-tuned ability to detect prey. Even when blindfolded, trained seals can chase the precise path of a fish that swam by 30 seconds earlier. Scientists have suspected that the seal’s laser-like tracking ability is due in part to its whiskers.
Now engineers at MIT have fabricated and tested a large-scale model of a harbor seal’s whisker and identified a mechanism that may explain how seals sense their environment and track their prey.
The team found that a seal’s whiskers are unique in shape: even to the naked eye, an individual whisker appears wavy rather than uniform. Under a magnifying glass, the pattern is more intricate, with an elliptical cross section that varies in size along its span.
The whisker’s morphology, the researchers found, may help the seal block out the effects of its own motion as it swims through water.
“It’s like having the ability to stick your head out of a car window and have there be no noise, so that your ears don’t ring: it’s a quieting effect,” says mechanical engineering professor Michael Triantafyllou, SM ’77, ScD ’79.
Using 3-D printing techniques, Heather Beem, PhD ’15, reproduced the wavy morphology at a much larger scale. She attached the whisker model to a moving track suspended above a 30-meter-long tank of water. Then she tested the whisker’s vibration properties as it moved through the water alone and with another object.
Beem observed that the whisker’s geometry serves two main functions. First, it allows the whisker to remain relatively motionless, with little vibration, when the seal is moving through still water. It also induces the whisker to oscillate in a “slaloming” motion in response to the turbulence left by a moving object.
Beem found that the whisker slaloms between vortices like a skier zigzagging between flags. She says this response may give seals a clue to an object’s path, its size, and even its shape.
“The geometry of the whisker allows for this phenomenon of being able to move very silently through the water if the water’s calm, and extract energy from the fish’s wake in order to vibrate a lot,” Beem says. “Now we have an idea of how it’s possible that seals can find fish that they can’t see.”