If you were to blindfold a harbor seal, give a fish a 30-second head start, and then let the seal start swimming—well, things would not end well for that fish. Take the blindfold off and throw a pair of noise-canceling headphones on the seal, and the next fish would fare no better.
How do we know this? Because for over a decade, researchers have been doing exactly that: putting blindfolds and headphones on seals and watching them chase things. Even when they’re completely cut off from all visual and auditory input, the blubbery beasts can still home in on their prey with GPS-like accuracy. Rather than rely on sight and sound, the seals use antenna-like whiskers—precise instruments of marine carnage capable of sizing up a herring down to the centimeter.
To understand how these fine-tuned marvels work, researchers at the Massachusetts Institute of Technology have built a large-scale artificial model of a seal whisker—one that may help explain how the seal’s environmental awareness is so keen, and that might also aid in maritime endeavors like commercial fishing and pollution cleanup.
Whiskers—or vibrissae, as they’re called in this case—are modified hairs packed with nerves that relay information about direction, velocity, and the physical environment to the brain. Every mammal sports them at some stage of life, except for anteaters, egg-laying mammals like the duck-billed platypus, and humans (though we do still have vestigial whisker-moving muscles in our upper lips). Seals’ whiskers are better equipped than many other animals’ to process sensory data: While rat and cat whiskers, for example, each house around 200 nerve endings, seal whiskers each contain up to 1,500.
But as the new MIT research suggests, the real evolutionary genius behind the seal whisker is how it does—and doesn’t—respond to movement. As a seal glides through the water, its whiskers stay still, unresponsive to self-generated motion, yet remain hypersensitive to external turbulence. Wakes stirred up by moving objects in the water create vortices, or rapidly spinning whirlwinds of water; seal whiskers may sync up with these vortices and vibrate in a back-and-forth, slalom-like manner at the same frequency, allowing them to lock in and track the turbulent water to its source.
Michael Triantafyllou, the MIT engineering professor who led the research in conjunction with his former graduate student Heather Beem, attributes this nuanced biology to the unusual shape of the whiskers. Most mammalian whiskers are circular, with a consistently tapering width. Seal whiskers, on the other hand, are elliptical and fluctuate in size throughout their length, undulating in thickness like a python that’s just feasted on a burrow of mice.
Using a 3-D printer, Triantafyllou and Beem fashioned a large-scale model of a seal whisker that they attached to a moving track above a 30-meter tank of water. Dragging a long cylindrical object through water would ordinarily create vibrations and vortices, but the artificial appendage advanced easily, stirring up very little turbulence. The team then ran a second experiment, dragging a circular rod in the tank in front of the “whisker.” As the rod moved through the pool, it mimicked the vibrations of a fish swimming by; the whisker, in turn, picked up the turbulence and slalomed at precisely the same frequency.
“Now we have an idea of how it's possible that seals can find fish that they can't see,” Beem said in a statement. “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.”
Triantafyllou believes that seal-whisker hydrodynamics could be used in a host of practical applications. “They can be used, for example, to detect plumes underwater, such as in oil spills, or other plumes polluting the environment,” he said. The technology may also be able to help researchers track schools of fish, develop low-power sensors for boats and underwater vehicles, and fine-tune sea-based monitoring: “The geometry prevents these whiskers from causing vibrations, which is an advantage that can be exploited with other sensors. It is important for some sensors to not generate noise.”
Seal whiskers aren't the only biologic mechanics that have piqued the interest of engineers. Velcro was inspired by plant burrs. The designers of e-reader displays are taking cues from butterfly wings. And the U.S. Navy is studying artificial shark skin for its ability to repel barnacles, a problem that apparently costs the military an extra $250 million a year in fuel costs due to “hull drag.” Perhaps seal whiskers will one help those barnacle-free ships to sail more smoothly, to detect whatever threats and spills lurk nearby, and to more quietly survey the sea.
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