How cats master the physical universe when they lap up milk

When cats drink, they never splash or make a mess. It turns out there's a solid scientific reason for that. As a group of MIT engineers discovered, feline lapping requires a finely-tuned mastery of fluid dynamics.

It turns out that cats' sense of balance and grace extends even to their drinking habits. Until this study, people thought that cats lapped by curling their tongue backwards, so it hits the water like the tail end of a lower-case "g", and then curled it forward so it scooped up water like a spoon. But when MIT researchers studied slow-motion video of their own cats, as well as videos of big cats on YouTube, they found out that cats are too good to use spoons.

Instead of scooping the water, a cat's tongue speeds along the surface of it, pulling a layer of water upwards. The motion of the tongue creates an unbroken column of water that gushes up towards the mouth of the cat and lets the cat take a sip of water. When the motion of the tongue stops, the water stops coming, and the cat doesn't get splashed.

It sounds like a simple trick, until you try it. The researchers constructed a model cat tongue that lapped up water. In order for it to do so effectively, it didn't just have to mimic cat behavior - it had to grapple with the Froude number. The Froude number is a ratio that takes into account both gravitational forces and inertia. When cats lap up liquid, the Froude number hovers very close to one, meaning that the inertia of the water being whipped upwards is balanced with gravitational forces pulling it down. No splashing. No dry tongues.

The robo-kitty-tongue, pictured moving in slo-mo below, had to vary its speed depending on its size.

Big tongues mean a lot of water pulled up, which means a lot of inertia. If a tiger laps at the speed of a housecat, four times a second, it will splash itself. If a domestic cat imitates a tiger, it won't get anything to drink. Jeffrey Aristoff, a mathematician, admires the calculated engineering involved:

"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. This suggests that cats are smarter than many people think, at least when it comes to hydrodynamics."

So when your cat looks at you with contempt, it's just because your furry pal finds your grasp of fluid dynamics pathetically limited. Also, it wants fishies. Now.

Via MIT.