Kobus Cilliers | On 26, May 2019

Darrell Mann

Cats love to groom themselves almost as much as they love to sleep, spending up to one-quarter of their waking hours cleaning their fur.

The secret to their self-cleaning success? The spines on their tongues are curved and hollow-tipped, according to a paper published this month in the journal PNAS. These tiny spines, called papillae, can transfer large amounts of saliva from mouth to fur, which not only cleanses Fluffy down to her skin but also lowers her body temperature as the saliva evaporates.

“A cat tongue works like a very smart comb,” says David Hu, a bioengineer at Georgia Tech and senior author of the new paper.

The results don’t just provide insight into how one of the world’s most popular pets stays clean. They’ve also inspired a new type of brush, called the TIGR (the Tongue-Inspired Grooming) brush. Studded with small, curved flexible spines just like those on a cat’s tongue, the TIGR prototype readily removes loose hair or fur from humans and felines alike but can be cleaned with the simple swipe of a finger. It may even possibly make cats less allergenic by removing excess dander, Hu says.

The researchers found that these scoop-shaped papillae are what allow cats to get saliva right down to their skin, which could inspire new approaches to cleaning and depositing fluids on all kinds of hairy, furry, and fuzzy surfaces. “Transporting liquids is a problem for animals and engineers,” says Sunghwan “Sunny” Jung, a bioengineer at Cornell University who was not involved in the research. “This paper shows that scientists can use the physics of basic animal behavior to answer fundamental questions.”

Hu’s PhD student, Alexis Noel, first became interested in cat grooming after watching her cat’s tongue repeatedly get snagged as it licked a microfiber blanket. Noel had seen her cat licking itself plenty of times before, but as she watched it try to groom a fluffy blanket, she began to think about the process with fresh eyes.

The scoop-shaped papillae cover a cat’s tongue and give it its sandpapery texture. It was these spines that got tangled in Noel’s blanket. Noel wanted to understand why these papillae were so good at getting knots out of fur (and blankets). A quick skim of the research literature revealed that scientists had paid almost no attention to the biomechanics of cat grooming. Hu and Noel decided to change this. First, though, they would need cat tongues. Obtaining samples from deceased domestic cats was straightforward. Getting the tongues of wild cats, not so much.

“There aren’t a lot of tongues just sitting around,” he says.

After hassling zoos and animal reserves for months, they finally had enough samples. With tongues from six cat species—domestic cat, bobcat, cougar, snow leopard, tiger, and lion—Noel and Hu got a closer look at the papillae, zooming in with a micro-CT scanner. A 1982 paper reported that cat papillae had the shape of a hollow cone, but newer technology used by the Georgia Tech researchers revealed that the spines actually curved backwards towards the throat.

The difference sounds subtle, but it’s not, Hu says. The curved shape of the papillae allows it to wick up water on contact, using surface tension—something that a hollow cone couldn’t do.

“At this small scale, these differences really matter,” Hu says.

Although each papilla may only be able to wick a fraction of a water droplet (4.1 microliters, to be precise), over the course of a day, the tongue of a domestic cat transfers an average of 48ml to its fur, about a fifth of a cup of water.

The researchers discovered that papillae’s orientation wasn’t fixed. High-speed videos of three short-haired domestic cats grooming showed that the papillae rotate as the cat’s tongue encountered knots in its fur. This rotation let the spike probe even deeper into the fuzzy snarl and ultimately work it loose.

This flexibility, Hu says, is the key to what allows such relatively short spikes to clean not only the longer, sparser outer layer of fur but also the thick, down-like undercoat next to the skin. The researchers’ measurements revealed that even relatively light pressure from the tongue during grooming allowed all the species of cat to clean themselves down to the skin. The one exception? A Persian cat, a domestic breed that needs to be brushed daily to prevent mats from forming.

Understanding how cats stay clean was one thing, but as an engineer, Hu wanted to go further. When his kids got lice, Hu spent hours searching the pharmacy for the right brush to remove the nits, and then combing through their hair to remove every last trace of the lice. A quick Internet search revealed that combs hadn’t changed much in tens of thousands of years. Hu’s work with Noel made the pair think that perhaps the cat tongue could inspire something better.

“Looking for new materials by studying how cats groom is very cool—it shows you don’t have to go deep into the forest to find something useful,” says Sylvain Deville, an engineer at the French National Center for Scientific Research, who wasn’t involved in the research. Using a silicone-based polymer, the pair 3D-printed a small, flexible brush about the size of two fingers held side-by-side. The spines on the brush were simply a scaled-up version of cat papillae. When Noel and Hu compared how well the TIGR and a human hairbrush could remove knots from a pelt of faux nylon fur, the TIGR removed more knots with less force than a standard brush. Noel could also remove the loose fur from the brush with the simple swipe of a finger, as opposed to painstakingly picking the hair out with tweezers.

It could also mean a better brush for cats, some of whom despise existing brushes currently on the market. The softness and flexibility of the TIGR may provide something closer to the cat’s own grooming experience that’s more tolerable for them—and their owners.

From a contradiction-solving perspective, the cat tongue offers up a master-class. Putting aside the fact that it also has a whole bunch of other functions to perform besides grooming, the grooming function alone should be full of compromises: we want fur to be unknotted (so the tongue needs to grip it hard), but we don’t want the fur to stick to the tongue (so the tongue needs to not grip it hard); we want to penetrate right down to the skin but we don’t want papillae to be long; we want to pick up fluid and also deposit fluid; we want to groom the fur in one direction, but untangling knots requires movement in multiple directions. Here’s how we might start to contain some of that complexity in the Contradiction Matrix:

The results of which seem to make a pretty good instruction-set for the solution evolved by the cat:

Principle 1, Segmentation – segment the surface of the tongue into lots of papillae

Principle 3, Local Quality – different papillae are different sizes at different positions on the tongue

Principle 4, Asymmetry – the papillae use multiple dimensions of asymmetry, and also face ‘backwards’ towards the throat so as to achieve stickiness in one direction and ‘not-sticky’ in the other

Principle 14, Curvature – the scoop-shape of the papillae, and the papillae rotate as they encounter knots

Principle 15, Dynamics – the papillae move and have a varying orientation

Principle 17, Another Dimension – the papillae stick up from the surface of the tongue

All in all, from an evolutionary perspective, while every biologist usually declares the brain or eyes as the most amazing outcomes of the evolution process, I’d like to add the cat’s tongue to the list of candidates for the most sophisticated system on the planet. I think I might even be able to put up with my cat’s habit of licking my nose from now on. It’s not annoying, it’s evolutionary genius.