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Cats Get Gravity Assist When Drinking

November 12, 2010

It’s an occurrence that happens so fast it cannot be followed by the human eye.  However, researchers at MIT, Virginia Tech and Princeton University announced on Thursday that they have solved the mystery of how cats lap liquids with such elegance.

Using high-speed cameras, the researchers found that domestic cats and larger felines like tigers use their tongues to delicately draw up water without breaking the surface of the liquid, harnessing a perfect balance between the physical forces of gravity and inertia.

Scientists previously believed that when cats insert their tongue into a bowl of liquid, the top surface of the tongue touches the liquid first, then the tip curves like the capital letter “J” to form a ladle.

However, using high-speed videos the researchers discovered that the top surface of the cat’s tongue is the only surface to ever touch the liquid, and that there is no ladling effect whatsoever.

Rather, the cat’s lapping mechanism is far more subtle and elegant, with the smooth tip of the tongue barely brushing the surface of the liquid before the cat quickly draws its tongue back in. 

As this happens, a column of liquid forms between the tongue and the liquid’s surface.  The feline then closes its mouth, pinching off the top of the column for a nice drink — keeping its chin dry all the while.

It turns out the liquid column is created by a delicate balance between gravity, which pulls the liquid back to the bowl, and inertia — the tendency of the liquid to continue moving in a direction unless another force interferes.  

The cat instinctively knows precisely how fast to lap in order to balance these two forces, and just when to close its mouth. In fact, waiting just another fraction of a second would allow the force of gravity to overtake inertia, causing the column to break, resulting in the liquid falling back into the bowl.

Household cats average about four laps per second, with each lap bringing in about 0.1 milliliters of liquid, while larger felines such as tigers lap more slowly to maintain the balance of gravity and inertia.

The researchers used observational data gathered from high-speed digital videos of domestic cats and a range of large cats (tiger, lion and jaguar) from Boston-area zoos.

They also gathered additional data by analyzing YouTube videos of large cats lapping.  Viewing these videos significantly slowed down, the researchers determined the speed of the tongue’s movement and the frequency of lapping.  Once they knew the size and speed of the tongue, the researchers then developed a mathematical model involving the Froude number, a dimensionless number that characterizes the ratio between gravity and inertia.  They found that for cats of all sizes, that number is almost exactly one, indicating a perfect balance.

The researchers also created a robotic version of a cat’s tongue that moves up and down over a dish of water, allowing them to systematically explore different aspects of lapping.

“The amount of liquid available for the cat to capture each time it closes its mouth depends on the size and speed of the tongue. Our research “” the experimental measurements and theoretical predictions “” suggests that the cat chooses the speed in order to maximize the amount of liquid ingested per lap,” said Jeffrey Aristoff of Princeton’s Department of Mechanical and Aerospace Engineering, who studies liquid surfaces.

“This suggests that cats are smarter than many people think, at least when it comes to hydrodynamics, said Aristoff, adding that the diverse scientific backgrounds of the researchers helped benefit the study.

“In the beginning of the project, we weren’t fully confident that fluid mechanics played a role in cat’s drinking. But as the project went on, we were surprised and amused by the beauty of the fluid mechanics involved in this system,” said Sunghwan Jung of Virginia Tech’s Department of Engineering Science and Mechanics.

“Our process in this work was typical, archetypal really, of any new scientific study of a natural phenomenon. You begin with an observation and a broad question, ‘How does the cat drink?’ and then try to answer it through careful experimentation and mathematical modeling,” said Pedro Reis of MIT’s Department of Mechanical Engineering, a physicist who works on the mechanics of soft solids.

“To us, this study provides further confirmation of how exciting it is to explore the scientific unknown, especially when this unknown is something that’s part of our everyday experiences.”

The results of the study were published in the November 11 online issue of the journal Science

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