Scientists learn physics behind how cats drink water without getting wet

By Marc Kaufman
Washington Post Staff Writer
Friday, November 12, 2010; 12:49 AM

As all cat lovers know well, Felis domestica is a marvel of balance, subtlety and other hidden elegances.

Prepare to learn of another remarkable attribute: Four researchers have painstakingly filmed, analyzed and determined how it is that a cat can drink water while (unlike a dog) keeping its chin and whiskers pleasingly dry.

The answer involves an exquisite demonstration of physics: The cat, in effect, balances the forces of gravity against the forces of inertia, and so quenches its thirst.

While a dog curls its tongue like a ladle to collect the water and then pull up what it can, a cat curves its tongue under and slightly back, leaving the top surface of the tip of the tongue to lightly touch the liquid. The cat then raises its tongue rapidly, creating an upward mini-stream of water. The cat snaps its mouth shut and the water is captured before the countervailing force of gravity pulls it down.

An average house cat, the team found, can make four of these mini-streams per second.

"What we found is that the cat uses fluid dynamics and physics in a way to absolutely optimize tongue lapping and water collection," said Jeffrey Aristoff, now at Princeton University but who was one of the four researchers who began the study out of curiosity at the Massachusetts Institute of Technology.

"Nobody had ever studied it before, so nobody knew how the water went from the bowl into the cat's mouth," he said. Not surprisingly, they found that cats lap at precisely the rate that would get them the most water for the effort expended. The team's results are described in an article released Thursday by the journal Science.

As with most basic scientific research, the ultimate usefulness of this knowledge is uncertain. But it is not, the researchers say, hard to imagine some downstream applications, perhaps in robotics.

Something as complex as a cat drinking water doesn't get unraveled and turned into a paper at the nation's top science journal overnight. It was almost four years ago that Roman Stocker, an associate professor at MIT's department of civil and environmental engineering, became interested in how his cat, Cutta Cutta (or "Stars Stars" translated from an Australian aboriginal language), drank. His enthusiasm spread to Aristoff, Sunghwan Jung, now an engineer at Virginia Tech, and Pedro Reis, a physicist who works on the nature of soft solids at MIT.

"Science allows us to look at natural processes with a different eye and to understand how things work, even if that's figuring out how my cat laps his breakfast," Stocker said. "It's a job, but also a passion, and this project for me was a high point in teamwork and creativity. We did it without any funding, without any graduate students, without much of the usual apparatus that science is done with nowadays."

The four researchers went to several zoos to observe and film tigers, jaguars, lions and ocelots, and went to YouTube to find videos of bobcats, cheetahs, leopards and lionesses drinking in the wild.

They found the same basic drinking mechanism in all the cats, though the larger ones (with larger tongues) slowed their lapping to best take advantage of the physics at play - that is, the balance between upward movement of the water set off by the cat's tongue (the inertia) and the gravity pulling the water down. A lion, Aristoff said, laps about two times per second.

"In the beginning of the project, we weren't fully confident that fluid mechanics played a role in cats' drinking," said Jung, whose research focuses on soft bodies, such as fish, and the fluids surrounding them. "But as the project went on, we were surprised and amused by the beauty of the fluid mechanics involved in this system."

Aristoff explained the dynamics at work: You're in the shower and turn on the hot water. The steam starts to rise, and that upward flow lowers the pressure levels at your knees. The result is that the inside of the shower curtain will billow in toward you, unless you have some weight attached to the curtain to stop it. That interplay of motion and pressure parallels the dynamic that quenches the cat's thirst.

Although the work on cat drinking was done for professional pleasure - it wasn't funded by a grant, and the only expense was high-quality video cameras - the researchers said there could be useful implications gleaned from their "fundamental" research.

Engineers, for example, are moving into the field of "soft robots" and are working on the basic properties of nonmetallic parts that may play a role. Aristoff said there's great interest in creating robots that can walk on water, and this research could help.

This new cat-drinking research follows by 70 years related work done by Harold "Doc" Edgerton, who first used strobe lights to capture stopped action on film. His photography, which uncovered some of the secrets of how cats drink, was featured in an Academy Award-winning short called "Quicker'n a Wink." The four current-day researchers used equipment at an MIT center for high-speed photography named after Edgerton, who was a much-honored MIT professor of electrical engineering for five decades.

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