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MIT researchers unravel the physics of how cats drink

Print | Comments () Posted by Carolyn Y. Johnson November 11, 2010 02:06 PM

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By Carolyn Y. Johnson, Globe Staff

CAMBRIDGE -- Roman Stocker was eating breakfast when he looked down and saw his gray and white cat taking a drink. An idle thought brought Stocker onto hands and knees, chin level with Cutta Cutta: How did he use his short little tongue to neatly lap up water?

That seemingly banal question launched Stocker, an MIT professor who ordinarily studies how microscopic marine bacteria move, on a three-year project. He and colleagues staked out his cat's water bowl with a high-speed camera, watched tigers and lions slake their thirst at local zoos, and developed a facsimile of a cat tongue.

Cat-lovers always knew their pets were clever. But it took bright minds from the Massachusetts Institute of Technology, Princeton University, and Virginia Tech to unravel the mechanism by which cats drink without wetting their whiskers. Dogs take a straightforward approach, using their tongues as ladles to literally scoop water into their mouths. Cats, on the other hand, solve a delicate physics, fluid mechanics, and engineering problem with every gulp.

"One might think what we don't understand is quarks and particle physics and black holes ... but a lot of what we don't understand is in our everyday lives," said Pedro Reis, an MIT engineering professor who became interested in the cat question when he and Stocker discussed it on a camping trip. "Often, things that are very mundane can be an inspiration for real practical applications."

What they found was that when a cat extends its tongue, the tip curves backward into a J-shape. The tiny hairs on the middle and back of the tongue -- the ones that give a cat's tongue its characteristic sandpaper texture -- play no role in drinking. Instead, the smooth tip grazes the water, and as the cat rapidly retracts its tongue it draws liquid upward by inertia -- the tendency for matter to keep moving in the same direction. Gravity pulls the water back toward the bowl, but the cat closes its mouth at the precise moment when gravity and inertia are in balance, allowing it to swallow the maximum amount of water.

The finding, published online today in the journal Science, may seem trifling. The team did not receive funding for the work, and were motivated by curiosity. But the way a cat's tongue deftly handles liquid may help guide the thinking of people interested in soft robotics and designing systems that involve manipulating liquid.

William Kier, chairman of the biology department at the University of North Carolina at Chapel Hill, studies tongues, elephant trunks, and octopus arms, and said such basic research often has unexpected payoffs. Much to Kier's surprise, for example, he is now collaborating with engineers designing robots that mimic the soft flexibility of tentacles and trunks.

"Science is more about asking questions than getting answers," said Michael Brenner, a professor of applied mathematics at Harvard University. "People are always asking questions out of curiosity, and that's how people get new ideas to put forward new proposals."

To satisfy that curiosity, the MIT-led team borrowed a high-speed camera from the Edgerton Center at MIT, named after Harold E. "Doc" Edgerton, who pioneered high-speed photography techniques to unravel what happens in the blink of an eye. They spent hours waiting for Cutta Cutta to drink, spiking his water with a little yogurt to tempt him. When they saw, in ultra slow motion, Cutta Cutta's tongue curl into a J, pulling water up without scooping, they became determined to understand exactly what was going on.

Next they built a tongue in the lab. A glass disc mimicked the smooth tip of the tongue, and to make it quick as a cat's, they attached the mock tongue to a device a colleague had built as a prototype for an unrelated experiment on the International Space Station. With the mock tongue, they could do what they couldn't do with a housecat -- vary different aspects of lapping to better understand how it worked.

Finally, building on their observations of Cutta Cutta and on the experiments with the mechanical tongue, researchers developed a mathematical model that described cat drinking as a delicate balance between gravity and inertia. To test whether their theory was correct, they visited the Franklin Park and Stone zoos.

According to their theory, bigger cats -- with bigger tongues -- were likely to lick less frequently, making precise adjustments to their lapping that the researchers predicted in detail. They approached John Piazza, the mammal curator of Zoo New England, seeking permission to take closeup photos of some of the big cats drinking with the high-speed camera.

Piazza eagerly agreed. "We are people interested in the natural history of animals, so it's great; it adds to our knowledge and adds to knowledge we can impart to the public," he said.

There were a few experimental mishaps -- as when the scientists used a plastic water bowl and Luther, a male tiger at Franklin Park Zoo, destroyed the experimental apparatus.

But scientists found their predictions held up. Heavier animals licked less frequently. For more research subjects, they scoured the video website YouTube to find drinking bobcats, lions, and cheetahs.

Back in Stocker's condominium -- the laboratory where it all began -- eight-year-old Cutta Cutta meowed earlier this week as the scientists walked into the nook where his water bowl sat on the floor.

Stocker, patting Cutta Cutta on the head, said there are no plans for future feline research, but that the same approach will continue to be important in his laboratory.

"How you ask the question, how you go about answering it with a combination of modeling experiments," Stocker said. "How you can use technology to address apparently -- or only apparently -- simple questions."

E-mail Carolyn Y. Johnson.