If you have a cat, you’ve probably been licked by it, and thereby experienced the raspy, sandy feel of a cat tongue. That’s due to the tongue’s being covered by many “papillae”—recurved, backwards-pointing spines made of keratin, the protein that’s in hair, hooves, and the outer layer of our skin. The papillae are in fact as hard as human fingernails, and much harder than the tongue tissue itself.
The housecat has about 300 of these papillae, but all species of felid studied so far have the spines, and they’re all about the same height: ca. 2.3 mm regardless of the size of the cat. (The authors studied, beside house cats, bobcats, cougars, snow leopards, tigers, and lions.)
All cats have papillae, but their function hasn’t been systematically studied, and thus has been speculative. One impediment is that the spines weren’t even studied morphologically, so it was thought that they were solid.
Now two scientists from Georgia have published a paper in Proc. Nat. Acad. Sci. USA (click on screenshot below; pdf here, reference at bottom, all free with the legal Chrome Unpaywall extension) that gives us more insight into cat tongues, something of great interest to us all. For one thing, they found that the spines aren’t solid, giving a clue to how they work. Be sure to see the figures and tongue movies shown here.
First, the tongues. The second series of photos show how the spines behave when the cat is licking itself.
There are four phases of grooming, shown in the figure below. As the authors describe them, they consist of “extension of the tongue, lateral expansion and stiffening of the tongue tissue, a sweep of the tongue through the fur, and retraction of the tongue in a U-shaped curl. During expansion, the spines rotate until they are perpendicular to the tongue, as shown in the high-speed film in Movie S1. This allows the papillae to stand erect to increase their contact area with fur.” As you see, a swipe takes about a quarter of a second. Go watch the movie if you can.
Cats spend a lot of time grooming themselves: the authors note that the average house cat sleeps 14 hours per day and spends about 24% of its waking time grooming itself. Therefore your moggie is licking itself for about 2.4 hours per day. Why is it doing this? Well, clearly it has some cleaning function, but whether that be to remove parasites, dirt, dead skin, or all of the above isn’t known. Thermoregulation is another possibility: since cats sweat only through their paws, spreading saliva on their fur can cause evaporative cooling of a hot cat. And maybe the papillae help the cat cut up meat or grip food better.
One clue comes from fine-resolution X-ray imaging, microcomputed tomography. This reveals that the papillae aren’t solid, but in fact hollow. There are in fact two hollow regions: at the base of the papillae, and one at the tip, where a U-shaped cross-section acts to wick up saliva by capillary action.
Here’s the hollow papillae, and then a figure showing it wicking up mock saliva (red-dyed liquid). Once in the papillae, the fluid is stable, and stays inside even when the tongue is upside down. It can them be deposited on any surface by pressing the tongue against it.
And here are photos of the papillae from six cat species:
And some figures showing how well these papillae wick up saliva:
The authors did a number of calculations, models, and measurements, and I needn’t go into the details here (they also did studies with tongues removed from dead cats—and I don’t want to know the details of how they did that). The salient results are these:
1.) The tongue of a housecat deposits about 56.6 microliters of saliva in a single lick: about 50% of the saliva on the tongue. The papillae themselves hold only about 10% of the deposited saliva, but even that allows the deposition of 48 grams (1.7 oz) of saliva per day onto the coat from the papillae.
2.) Even though most of the saliva on the tongue that gets onto the fur comes from the tongue and not the papillae, the papillae are vital. Measurements show that it’s only the papillae that can get to the cat’s skin through its two layers of fur: the thick guard hairs of the top coa, and the softer undercoat, which helps insulate the cat. Experiments with all six felids showed that, when licking, the cat compresses its fur sufficiently to allow the papillae to reach the skin. Thus, it seems, at least one function of these spines is to clean the skin and undercoat.
3.) But the deposition of saliva on to the skin (rather than the fur) also helps the cat cool itself. The estimates of evaporative cooling via saliva deposited close to the skin show that about 25% of the total cat cooling comes from skin-deposited saliva, with the remaining 75% of heat loss coming from radiation from the hairs, paws, and ears. The saliva deposited on the skin can cause a temperature difference of 17°C between the skin and the topcoat.
4.) If you have a short-haired cat, this is all good. But some breeds studied by the authors have fur too long to be penetrated by the papillae. As they say,
. . .if the papillae cannot reach the skin (), much of its fur cannot be accessed, making the cat “ungroomable.” Long-haired domestic breeds, such as Persian domestic cats, are notorious for their matted fur if not cared for properly. According to Veterinary Centers of America (VCA) animal hospitals, Persian cat owners should comb their cat daily and give it baths monthly to redistribute the fur’s natural oils . Consistent with these care instructions, two Persian breeds are the only animals to fall into the ungroomable region, as shown in the upper one-half of Fig. 3C.
Here’s that figure, showing the papilla length and fur length of various species and breeds, all of which (save the control chinchilla and the Persian cat) are able to reach and groom their skin:
So be sure to comb and wash your Persians regularly!
Finally, I think there’s a new cat-grooming brush in the offing. The authors constructed what they call the TIGR brush (tongue-inspired grooming brush) with recurved spines like the papillae. It’s shown in the video below, and TIGR is much easier to clean than is a regular human hairbrush (perhaps those recurved spines help the cat, in its mouth, remove the fur more easily, of course leading to HAIRBALLS). I sense a patent and a new product!
So the next time you see your cat groom yourself, or feel the “scratchy” sensation when your cat licks you, perhaps you’ll have a new respect for what it’s doing, and for the marvels of evolution that helped produce those hollow and recurved spines.
Noel, A. C. and D. L. Hu. 2018. Cats use hollow papillae to wick saliva into fur. Proceedings of the National Academy of Sciences.