Anyone who has been around a cat has undoubtedly felt the roughness of a cat tongue. On bare skin, cat tongues feel rough and bumpy, like sandpaper. Turns out, there's more to those bumps. They're not actually bumps, and the roughness helps cats groom themselves and drink water.
Cats use their tongues...a lot. National Geographic says that cats spend up to one-quarter of their waking hours cleaning their fur. Cats are able to do such a good job cleaning themselves thanks to the structure of their tongues. That roughness that might feel like bumps are actually tiny spines, called papillae.
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Cat tongue anatomy
Researchers studying cat tongues compared tongues with domestic cats with those of their wild counterparts. Previous research from 1982 suggested that the papillae on cat tongues had the shape of a hollow cone. But one researcher, David Hu, a bioengineer at Georgia Tech, wanted to take a closer look. After looking at the tongues of domestic cats, lions and tigers, bobcats, cougars, and snow leopards, Hu realized that the the papillae actually curved backwards towards the throat.
This subtle difference impacts cats' ability to drink water more effectively—a hollow cone can't wick up water on contact using surface tension, but a curved spine can. These spines are curved and hollow-tipped, almost like miniature cat claws, or fangs. As cats groom themselves, the shape of the spines helps readily transfer large amounts of saliva from their mouth to their fur. When cats are lapping water from a water bowl (or the fresh, cold glass of water you put on the coffee table for yourself), this feature of cat tongues helps them drink water more effectively.
Hu's research shows that each papilla can wick 4.1 microliters, a very small number that is just a fraction of a water droplet. But when grooming, this translates to the transfer of 48 milliliters from tongue to fur, or about a fifth of a cup of water.
Cats can't use suction to pull water from their water bowl into their mouths like humans can when we drink, either with our mouths on a cup or through a straw. Their mouths open too widely for that, and the water would flow out. A different team of researchers from Virginia Tech studied high-speed video to better understand how cat tongues pull water into their mouths when they drink.
They flick the surface of water with the topside tip of their tongues and then lift the tongue very rapidly. When gravity acts on that, the researchers say, this creates a "column" of water, which the cats "bite" at just the right moment to drink when it gets into their mouths.
These researchers discovered that when house cats are lapping water, they do so about four times a second. Lions and tigers, on the other hand, lap more slowly, but they are able to pull in more water per lap due to the greater size of their tongues. Tigers have the same type of spines to help them drink that your house cat does, they just have a lot more of them due to an increase in body size in general.
Your cat's water bowl
When your cat is slaking her thirst from her water bowl, she's not aware of it, but she's fighting gravity and inertia with each of her four laps per second. And, it turns out, cats are better at getting water into their mouths than dogs are.
Dogs use their tongues more like a spoon, to simply "scoop" water into their mouths. For a cat to use its tongue and the inertia of its movement to get the maximum amount of water into its mouths as possible requires cats to lap at just the right speed to keep the water from sliding off their tongues due to gravity.
These researchers, including Roman Stocker, a professor of civil and environmental engineering at the Massachusetts Institute of Technology, said that cats seem to know just how rapidly to lap to pull water in most effectively. These researchers, who published in the journal Science, observed that cats and dogs drink very differently.
Dogs use their tongues as a spoon of sorts. But the "scoop" that cats create with their tongues stays mostly empty as they drink water. How do they do this? It's because they are only touching the very surface of the water, and then the papillae are doing the rest of the work, to pull the water into its mouth.
A robotic cat tongue
These researchers built a robotic tongue to further test their results. Since larger cats like lions and tigers have larger tongues, the cats would need to lap more slowly to find the right balance between inertia and gravity. the robotic tongue didn't let the researchers down . . . lions and tigers lap less than two times per second, about half the rate of your house cat.
Your cat's tongue may be flesh and blood, but it's movement is optimized almost as well as a robotic tongue would be. Stocker's research established that the mathematical ratio between gravity and inertia of a cat lapping is almost exactly one, no matter the cat's size, indicating an almost perfect balance.
Next time your cat is drinking from her water bowl, count the number of laps and think about how long it would take you to slake your thirst at her rate that Stocker found of 0.1 milliliter of liquid per lap, or just 5 teaspoons a minute.
- National Geographic: How cat tongues work—and can inspire human tech
- PNAS: Cats use hollow papillae to wick saliva into fur
- PBS: Ever wondered why your cat’s tongue feels like sandpaper?
- NPR: Freaked Out By Your Cat's Scratchy Tongue? Don't Be! It's Keeping Them Cleaner.
- Live Science: Study Reveals Physics of How Cats Drink
- MIT: The surprising physics of cats’ drinking