Cat fanciers know the drill: Cats are perfect. They pose with precision as if arranged by a stylist, they clean themselves with languid but exact precision, and they scheme smart, sly scenarios with perfect nonchalance. They are also the poster pet for grace and elegance. They seem to defy gravity, as well as reason, with their exquisite balance–they are phenoms in the physics department. This we know. But until recently it wasn’t known that this perfection is evident all the way down to the way in which they lap liquids.
Researchers at MIT, Virginia Tech and Princeton University analyzed the way domestic and big cats drink and found that felines of all sizes take advantage of a perfect balance between two physical forces. The results were recently published in the online issue of the journal Science.
It had been known that when cats drink, they stick their tongues straight down toward the liquid with the tip of the tongue curled backwards to form a scoop, so that the top part of the tongue touches the liquid first. The new research reveals that the top surface of the cat’s tongue is the only surface to touch the liquid. Cats, unlike dogs, aren’t using their tongues like spoons after all. Instead, the cat’s lapping mechanism is much more subtle and elegant. (But don’t tell that to the cats.) The researchers observed that the smooth tip of the tongue barely brushes the surface of the liquid before the cat rapidly draws its tongue back up. As it does so, a column of milk forms between the moving tongue and the liquid’s surface. The cat then closes its mouth, pinching off the top of the column for a nice drink, while keeping its chin dry. Of course, none of that unsightly milk on the chin.
And here’s where the cat’s innate sense of physics comes into play. The liquid column is created by a subtle and perfect balance between gravity, which pulls the liquid back to the bowl, and inertia, which in physics means the tendency of the liquid to continue moving in a direction unless another force interferes. The cat instinctively knows just how quickly to lap in order to balance these two forces, and just when to close its mouth. If it waits another fraction of a second, the force of gravity will overtake inertia, causing the column to break, the liquid to fall back into the bowl, and the cat’s tongue to come up empty.
Knowing the size and speed of the tongue of various cats they studied, the researchers then developed a mathematical model involving the Froude number, a dimensionless number that characterizes the ratio between gravity and inertia. For cats of all sizes, that number is almost exactly one, indicating a perfect balance!
So the next time your puss is looking smug on the couch, just remember, she is a pretty perfectly purring machine after all.