The Physics of Cat Tongues

Recent research shows that cats and dogs have one more thing to disagree on: drinking. Both species lack the power of suction and have had to find alternate ways of taking in liquid; cats, it seems, prefer a fine-tuned method. Photo courtesy of Two Tuttle’s Four Paws.

Cats’ tongues are infamous for their prickly surfaces. But a team of engineers has discovered another unique feature of this feline organ: the unexpected way it allows cats to drink.

Like dogs, cats lack full cheeks. Without the power of suction, both species must find alternate means of taking in liquid. While it was often believed that cats used a spoon-like scooping technique similar to dogs, which requires the tongue to be submerged in water, researchers at the Massachusetts Institute of Technology, the Virginia Polytechnic Institute, and Princeton University discovered that cats quench their thirst quite differently – and more delicately. High-speed imaging technology has allowed one of the engineers to videotape his cat, Cutta Cutta, performing a maneuver so fast that the human eye cannot see it. The cat extends its tongue to the liquid surface while curling the tip backward so that only the dorsal side of the tongue makes contact with the water. Without breaking the surface, the cat quickly retracts its tongue, pulling the liquid that adheres to the tip upwards in a vertical column. When the cat closes its jaws, it has successfully slurped while keeping its whiskers dry.

Cats’ ability to draw up this column of water literally defies gravity – well, momentarily that is. As the cat draws the liquid into its mouth, it must lap quickly enough so that the inertia, which keeps the water in upward motion, can overcome the gravity drawing the water back to the surface. Gravity eventually wins and pulls back most of the column but not without leaving a droplet on the cat’s tongue.

Though invisible to the naked eye, cats’ unique process of lapping liquid can be observed in these frames from a high-speed video. The cat curls the tip of its tongue backwards so that only the dorsal side touches the liquid, drawing a column of liquid into the cat’s mouth. The cat knows the exact moment to close its jaws: when gravity begins to overtake inertia and pull the water back to the bowl. Photo courtesy of Science.

To test the observations in the video footage, the engineers used a robotic device with a glass disk that acted as the tongue’s tip. Much as the high-speed imaging had revealed with Cutta Cutta, their contraption formed a vertical column of liquid and confirmed their predictions of the physics behind the mechanism. This experiment also revealed that cats’ lapping technique requires precise timing; it seems that cats know the exact moment that gravity will begin to pull the water column back to the bowl and use that knowledge as a cue to close their jaws.

The researchers suspected that this lapping finesse was not a feature of domestic cats alone but of their wild counterparts as well. After observing larger cats on YouTube and at the zoo, the engineers confirmed that wild felines use the same mechanism, only with a slower lapping frequency in order to balance inertia and gravity with their larger tongues.

A robotic demonstration of the cat’s lapping process. A glass disk, representing the dorsal side of the cat’s tongue, draws a vertical column of liquid into the feline’s mouth. Photo Courtesy of Science.

Yale Ecology and Evolutionary Biology Professor Richard Prum explains how this technique could have evolved in felines but not other mammals: “As organisms evolve and there are variations and random mutations in the structure of any part of the body, the body will discover on its own physical properties or phenomena that exist. So as soon as you have a tongue with fine enough spatial features – just the right size – to exploit the surface tension of water in this way, then you have the opportunity for this to occur.” Prum also suggests that other functions, such as taste or food handling, could have played a role in creating the tongue’s structure and that this lapping technique may simply be an incidental result. Whatever the reason, “once you’re in this zone, you might as well stay there.”

But once cats have the correct tongue, do they know the precise moment to close their jaws instinctively or from trial and error? Prum believes cats learn the fine timing but “in the same way that we can say that we learn how to walk. We stumble at first and then we have to figure it out; we’re not learning by example, but acquiring skills. And I would bet that’s what cats are doing once they have a tongue that can do that.”

Regardless of how cats acquired the refined technique, the study gives one more reason for owners to be amazed by their four-legged friends. The more technology evolves, the more opportunities we will have to find the extraordinary in the ordinary.