The Physics of How Cats Drink

About four years ago, Roman Stocker was watching his cat, Cutta Cutta, drink from his bowl. And Stocker, an engineer who studies the biophysics of ocean microbes, wondered exactly what physical forces allowed the cat to lap. So he and Pedro Reis, a colleague in the Department of Civil and Environmental Engineering, worked with Sunghwan Jung, SM ‘95, PhD ‘07, and Jeffrey Aristoff ‘04, PhD ‘09, who were then at MIT, to shoot and study video of domestic and big cats as they drank. Their research, published in Science, found that felines of all sizes take advantage of a perfect balance between two physical forces.

The researchers saw that when a cat drinks, it extends its tongue straight down toward the bowl with the tip curled backward, so that the top of the tongue touches the liquid first. They later learned that Harold “Doc” Edgerton’s 1940 film of a cat lapping milk showed this same phenomenon. But the new videos took things further, revealing that the top of the tongue is the only surface to touch the liquid. Cats, unlike dogs, don’t dip their tongues in like ladles. The smooth tip barely touches the surface before the cat draws its tongue back up. As it does so, a column of liquid forms between the moving tongue and the liquid’s surface. The cat then closes its mouth, pinching off the top of the column and keeping its chin dry.

When the cat’s tongue touches the liquid surface, some of the liquid sticks to it, much as water adheres to a human palm when it touches the surface of a pool. But the cat draws its tongue back up so rapidly that for a fraction of a second, inertia overcomes gravity, which is pulling the liquid back toward the bowl. The cat instinctively knows just when this delicate balance will change; it closes its mouth in the instant before gravity stops the moving liquid from continuing to follow the tongue. If the cat hesitated, the column would break, the liquid would fall back into the bowl, and the tongue would come up empty.

The researchers analyzed high-speed digital video of domestic cats like Cutta Cutta (watch it above or at www.technologyreview.com/cat-drink) as well as lions, a tiger, and a jaguar. By slowing the videos, the researchers established the speed of the tongues’ movement and the frequency of lapping. Knowing the size and speed of the tongue, they then developed a mathematical model of lapping, which involves the ratio between gravity and inertia. For cats of all sizes, that number is almost exactly one, indicating a perfect balance. Domestic cats average four laps per second, but big cats, whose tongues are larger, lap more slowly to achieve the same result.

To better understand the subtle dynamics of lapping, the researchers also created a robotic version of a cat’s tongue that moves up and down over a dish of water. The model allowed them to systematically explore different aspects of lapping and, ultimately, identify the mechanisms at work.