On a spring afternoon in 2017, Alexis Noel stood at her lab bench. In one hand, she held a slab of raw pork; in the other, a freshly dissected tongue.
Noel, a biomechanist at the Georgia Institute of Technology, had spent the past several months puzzling through the ragged architecture of the domestic cat tongue. These little pink organs are covered in mysterious, backward-facing spines shaped like tiny claws, nothing like the sandpaper texture often used to describe them. Noel was testing her latest theory on the structures—that they might help big cats lick scraps of meat off bones—by laboriously scoring the top of a supermarket pork chop with a tongue from her university’s Applied Physiology department.
It did seem the spines were tearing through the flesh. But when Noel eventually meandered over to the lab’s sink to scrub the tongue clean, she noticed something strange: Even as the biggest chunks of flesh fell away under the force of the water, the spikes were stained a bright red. When Noel peered under the microscope, she found that little bits of pork had lodged themselves beneath the tips of the tongue’s tiny grappling hooks. The spines were hollow—a perfect structure, Noel would realize, for holding liquid.
Noel’s findings, published today in the journal PNAS, demonstrate how the dimpled little spines, or papillae, that coat cat tongues serve to shuttle saliva from mouth to skin. It’s a subtle superpower that helps cats clean and cool their bodies with every lick.
Cat tongues really get put through the ringer: Domestic breeds spend up to a quarter of their waking hours engaged in fastidious grooming—and for good reason. “If you count all the hairs, a cat has the surface area of a ping-pong table,” explains senior author David Hu, a mechanical engineer at the Georgia Institute of Technology. “The hairs are a huge network of stuff… and it’s all places you can get blood, pollen, or dander.”
But that fateful pork chop told Noel there had to be more to the story.
When she 3D-scanned the tongue of a house cat, Noel noted hundreds of hooked structures that arched back towards the cat’s throat like one-way traffic spikes. The papillae at the front of the tongue were the largest and most likely to contact the fur during grooming. Each of them clocked in at just a couple millimeters tall, with the approximate elasticity of a human fingernail, and was hollow in two spots: at the base, where it attached to the surface of the tongue, and at the tip, forming a little U-shaped cavity like the nib of a quill pen.
The resemblance was no coincidence. When Noel next extracted an individual papilla and touched it to a spot of red dye, the spine sipped up a smidge of the liquid—just as the tip of a quill draws from an inkwell. “Discovering that these spines were hollow was a mind-blowing discovery,” Noel says. “It was contrary to what everyone was saying up until this point.”
This straw-like structure, Noel realized, might be the cat’s key to siphoning spit from mouth to skin. In the feline world, saliva is powerful cleaning tool: It can extricate dirt and break down contaminants in fur. One of spit’s other important functions, however, may also be its coolest. Evaporation of a liquid from the surface of skin removes heat, but cats don’t have sweat glands on most of their bodies. So to keep from overheating, felines might deploy a DIY option—like a tongue that’s a super-efficient conveyor belt for liquid.
To test this theory, Noel calculated the amount of saliva carried by the tongue of a domestic cat. With every sweep, she found, 300 or so papillae contact feline fur, carrying a total of 4.1 microliters of saliva. If that sounds small, it should: It’s the equivalent of one-tenth of a drop from a typical eyedropper. Even en masse, the saliva in papillae cavities would account for only 5 percent or less of the total fluid on the top of the tongue, with the lion’s share on the surface of the tissue itself or clinging to the walls of the spines.
The papillae soon proved their use, however. The force of grooming compresses the fur close down to the skin, pushing air out from between individual hairs. But for even the most ardent of lickers, a fuzzy gap remains between the top of the wall of hair and the skin below. It’s at this interface that the papillae shine: Only they are fine enough to wriggle down between the lattice of fur. Repeated contact between hair and papillae probably dislodges saliva from the grooves, allowing saliva to trickle down towards the cat’s warm body, Hu explains, though the exact mechanics of this dispensation await further research.
Cats spend about two and a half hours each day raking their tongues through their coats, executing each lick in under a second apiece. According to Noel’s calculations, the drag-and-drop spit strategy distributes about 50 milliliters of saliva across a house cat’s body each day, and could account for up to a quarter of a cat’s temperature regulation. Based on measurements taken with a thermal camera, Noel also found that grooming could generate a temperature drop of up to 17 degrees Celsius (31 degrees Fahrenheit) through the evaporation of saliva from skin.
As long as papillae are taller than the height of a cat’s compressed fur, the system of grooming is pretty foolproof. Curious to see if this held true across species, Noel next measured the spines of five other very diverse cats: bobcat, snow leopard, cougar, tiger, and lion. To her surprise, the papillae of all six species were all about two millimeters long, despite massive differences in size (an average lion weighs 30 times more than a typical house cat). It turns out that, big or small, most felines have highly compactable coats: Across 19 species, nearly all could cram their fur down to a height compatible with these petite papillae. The only exceptions the researchers found were two breeds of “ungroomable” Persian cats whose coats—even when smashed down—put the skin just out of spitting distance. Persians were purposefully bred to be “obnoxiously fluffy,” Noel notes. The mismatch between pelt and papillae may account for why Persians need to be combed and bathed so regularly: Their tongues can’t access all of their fur.
Persians aside, though, this counterintuitive uniformity may say a lot about cat tongue evolution. “The fact that these papillae don’t scale up as the cat gets bigger is one of the most exciting things about this paper,” explains Margaret Rubega, an evolutionary biologist at the University of Connecticut who was not involved in the research. “It suggests that the size of these little spines is important in how they work. Selection has acted on these to create these forms even though the rest of the animal is changing.”
And this research likely only scratches the surface of what papillae are capable of. Cats use their textured tongues as taste-testers, thermostats, grooming devices, and toilet paper; these organs likely also play a role in helping felines grip objects, clean wounds, and—harkening back to that slab of raw pork—get the most out of their food. What’s more, there are other types of papillae on the cat tongue still waiting to be dissected, says Sunghwan Jung, a bioengineer at Virginia Polytechnic Institute and State University who did not participate in the new finding.
“The tongue is the way cats interact with the world,” Hu says. “I think there are still a lot of unstudied functions.”
Future technologies that simulate these structures could help veterinarians and cat owners administer topical medications to finicky felines, or even suck allergen-laden spit out of cat fur. Noel is already in the middle of patenting a cat-inspired hairbrush with top-notch detangling capabilities that’s easier to clean than typical combs with straight bristles (and, in case you were wondering, it’s way less weird than the tongue prosthetic Licki Brush that caters to pet owners eager to lick their cats).
Unsurprisingly, Noel’s invention is the spitting image of a giant, spiny tongue.