Smithsonian.com

Cats use inertia to bring milk to their mouths (photo courtesy of flickr user invisible monsters)

The 1940 documentary short “Quicker’n a Wink” fascinated people with its slow-motion imagery of things like the beating of a hummingbird’s wings; it won a 1941 Academy Award. One of the revelations from the movie was that a cat curls its tongue backwards into a “J” when it goes to take a drink of liquid, letting the top of its tongue touch the surface first.

But kitties aren’t using their tongues like ladles, scooping up water, says a new study published yesterday in Science. Cats simply have to brush their tongues along to the surface of the liquid and let the power of physics bring it into their mouths.

The scientists used high-speed imaging to watch cats—including one researcher’s own pet, Cutta Cutta—lapping up milk, sometimes spending hours just waiting for the cat to get thirsty. Each cat would dip its tongue towards the liquid, just brushing the surface, not piercing it. When it lifted its tongue, the liquid adhered to the tip of the tongue and was drawn upward in a column (as in the above photo), thinning as the cat drew its tongue into its mouth. Just before the column would break, the cat would close its mouth and trap the milk, storing it in cavities inside and swallowing after every three to 17 of these lapping cycles.

This small act seems to defy gravity, but in actuality the cats have figured out how to keep a delicate balance between inertia—the tendency of the liquid to keep moving in the same direction—and the gravitational forces pulling the milk back into the bowl. Domestic cats don’t bring up a lot of liquid in each lap, only about a tenth of a milliliter, but they do it quickly, at a rate of about four laps per second.

When the scientists watched high-speed and YouTube video of larger kitties, such as lions and tigers, they found that the bigger cats’ tongues worked the same way, but they lapped at a slower rate. The researchers were able to develop an equation that predicted lapping frequency based on animal mass.

“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…suggests that the cat chooses the speed in order to maximize the amount of liquid ingested per lap,” says study co-author Jeffrey Aristoff, a mathematician at Princeton University. “This suggests that cats are smarter than many people think, at least when it comes to hydrodynamics.”