Cats like to do things their own way—even, it seems, when it comes to drinking. Researchers have discovered that felines have their own style of lapping water. Their tongues perform a complex maneuver that pits gravity versus inertia in a delicate balance.
Surprisingly little is known about the physics of lapping. Dogs and many other animals with incomplete cheeks—who can't seal their mouths like we do to produce suction—lap up water by curling their tongues into a ladlelike shape and scooping up the liquid. Most researchers assumed felines do the same, albeit with much smaller, raspier tongues. But Roman Stocker, a biophysicist at the Massachusetts Institute of Technology in Cambridge, began to doubt this assumption one morning over breakfast. He watched his cat Cutta Cutta lap water from a bowl and began to wonder if there was more to her dainty drinking than met his eye.
On their own time, Stocker and a small crew of his colleagues filmed Cutta Cutta—and eventually nine more cats from a local shelter—with a high-speed camera. They found that, as opposed to dogs, cats rest the tips of their tongues on the liquid's surface without penetrating it. The water sticks to the cat's tongue and is pulled upward as the cat draws its tongue into its mouth. When the cat closes its mouth, it breaks the liquid column but still keeps its chin and whiskers dry (see video).
"It's a much more delicate and well-controlled mechanism than regular lapping," says Stocker. But what's most delicate is the physics behind it. To better understand the mechanics, Stocker and his team built a rudimentary robotic tongue from a glass disk about the size of the tip of a cat's tongue. The disk moved up and down over a liquid, making and breaking contact with the surface as the scientists tried out different lapping speeds.
They realized that feline lapping balances the liquid's inertia, its tendency to keep moving upward as the cat draws its tongue in, against the pull of gravity, which drags the liquid back down into the bowl. To get a satisfying drink, the cat must lap faster than gravity can overtake inertia. Good timing gives the cat the biggest drink, because the column of water is at its longest and thickest right before gravity wins out and pulls it down, the team reports  online today in Science.
"The cat seems to know exactly how fast to lap," Stocker explains. "If the cat closed its jaws sooner, it would miss some of the water. If it closed later, it would lose the whole column."
The researchers were surprised again when they took their experiment to the zoo. They filmed other species of cats, including a lion and a tiger, and found that they, too, exploited inertia with pinpoint lapping. Because these cats are taller and have bigger tongues, they need to lap more slowly to keep inertia and gravity in balance, but the mechanism is the same one used by their tiny domestic cousins.
"This study has a really big cool factor," says neurophysiologist Rebecca German of the Johns Hopkins University School of Medicine in Baltimore, Maryland, who researches mammalian feeding mechanisms. "It's elegant; they found something new in a really simple fashion."
So far, this drinking behavior has been seen only in felines, but robots could soon join their ranks. Hillel Chiel, a neurobiologist at Case Western Reserve University in Cleveland, Ohio, who works to develop "soft" robots that mimic natural behavior, says feline lapping could give robotics experts a new way to approach problems that require delicate handling of liquids, such as oil cleanups. "I think this study will definitely have an impact." Chiel says he's now "sort of sorry" he doesn't own cats. "I think I'd like to watch them lap."