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 (h[papillae] < h[fur]), 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:
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.
h/t: Gravelinspector
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Noel, A. C. and D. L. Hu. 2018. Cats use hollow papillae to wick saliva into fur. Proceedings of the National Academy of Sciences.
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For anyone out there with cat allergies: when I had cats I found that the build up of saliva residue on a cat’s fur is a major trigger. When I began having allergic symptoms I would know it was time to wash the cats and after doing so the red eyes and runny nose would clear up.
That is why we have antihistamines with Pseudoephedrine and other things so we don’t have to wash the cats all the time. Of course the politicians think we are are running meth labs so we have to show id and sign for the pills.
That’s one of the weirder differences between UK and US. While the knowledge of how to make Crystal Meth and the like is perfectly well spread in the UK (I knew of it being available on the underground market in the 1980s), the cultural concept and political terror (not to put too fine a point on it) of the “meth lab” just doesn’t exist. Consequence : no restrictions on sale of drugs containing ephedrine/ pseudoephedrine.
On the other hand, the person wishing to commit suicide by overdosing on paracetamol-containing drugs will have to visit at least three shops to stock up. Which can take a whole 20 minutes. It’s weird, as a difference, because the drugs themselves and the chemistry are the same.
Absolutely fascinating. I have two questions:
1. Since the papillae are made of keratin, do they wear down and grow back like fingernails and hair, or are they permanent? It seems from the diagram and description that they are in some ways like a cat’s claws, only hollowed out.
2. Why have some mammals evolved to meticulously groom themselves, while other animals — often very similar mammals in similar ecosystems or even the same regions. With something like a sloth, the answer is obvious: the sloth has a symbiotic relationship with all the crap that ends up on it, like moss. But what about other mammals that don’t groom and don’t have a symbiotic relationship with the parasites/bacteria/mold/other threats that might affect them?
Oh, and just a comment about this sentence: “Thermoregulation is another possibility: since cats sweat only through their paws, spreading saliva on their fur can cause evaporative cooling of a hot cat.”
This is definitely not the case for any of the cats I’ve ever owned. All of my cats have loved the heat. My current cat sleeps under the bed in the master bedroom once we turn the central heating during the fall because there’s a vent under the bed and the bedskirt traps it underneath. It must be 110 degrees under there. He also sleeps on the heated floor in the bathroom.
I used to know a cat, (Lawrence) who chose to doze in a greenhouse – in the summer!!
You could carry him out; he would walk back in.
I don’t know why, but a cat whose name is Lawrence just sounds so adorable to me. If he was named Jacob or Brian or whatever, I wouldn’t care, but Lawrence…I don’t know why. It’s just one of those things.
In this case, clearly channelling Lawrence of Arabia.
Keep clear of motorbikes!
Oh, and when my cat is sleeping on the heated floor and I come in to take a bath, he leaves (he’s very easily annoyed). The second I get out, he comes back and goes to sleep on the floor again.
Got to get those 14 hours a day in!
WOT I FOUND OUT [PROBABLY ERRORS]
Cat’s claws are hollowed out too & yes they must obviously wear down & grow back.
LINGUAL PAPILLAE: There’s four types in the tongue. One of them is called FIILIFORM PAPILLAE which is the most numerous & they are responsible for giving the tongue its texture & sense of touch. Unlike the other kinds of papillae, filiform papillae do not contain taste buds. They cover most of the front two-thirds of the tongue’s surface & are made up of irregular connective tissue cores with a keratin–containing epithelium which has fine secondary threads. Heavy keratinization of filiform papillae occurs not just in cats. It is a form of body tissue & thus it must grow & replace itself.
CLAWS: Claws grow from the base just like fingernails & hair. I am guessing that ‘tongue claws’ grow in exactly the same way. The structure of claws is such that the keratin is layered like an onion thus layers peel off with use revealing a new sharp edge – in effect claws are self-sharpening with use just like ‘magic’ chefs knives where the steel is structured to flake like flint & similar siliceous stones. That’s why you’ll not see a blunt claw on a healthy, active cat. Scratch posts act to remove ‘dead’ keratin to maintain the clawiness of the claw.
SLOTHS: All terrestrial mammals groom themselves including all six species of sloth – sloths are clean animals as clean as any other mammal – there is a co-evolutionary relationship between the sloth & the algae of the genus Trichophilus with the algae living in special grooves in sloth hair. Sloths can pick out ticks etc without the algae getting in the way & they eat the algae anyway. There is a specific moth species that uses the algae on sloths for something too – no time to find out what’s going on there!
OTHER MAMMALS: Rhino, elephant & no doubt many other animals take water baths followed by mud baths followed by a good old scratch on a tree stump or termite mound – the whole routine designed to remove ticks from ‘armpits’ & groin. I assume the mud acts as a sticky material which removes parasites as it is scraped off.
CARNIVORES: Gore is smelly. I would expect ambush predators to spend more of their ‘energy/time budget’ grooming to increase their success rate in the hunt. I assume that horses & other herbivores have much less time to groom because of the low energy content of their foods. Horses, cattle engage in a lot of mutual grooming with horses pairing up to nibble at those hard to reach places & cows lick each other which is believed to help somewhat with parasites.
CAT THERMOREGULATION
In dogs & cats the eccrine [sweat] glands are only in the foot pads & the nose leather. The total gland area is too small to be useful for thermoregulation. If your cats groom their fur then it is indulging in thermoregulation as well as cleaning.
Cats also thermoregulate by drinking water, changing location, reducing/increasing activity, stretching the body to increase exposed skin area or exposing their tummy – in reverse they’ll form a loaf of bread with extremities tucked in. I expect they also can increase pulse rate to move blood faster. They don’t pant much.
So I expect the cat on the mat under the bed is still thermoregulating one way or another or multiple ways
Regarding 3, I’m just saying that I don’t think cats grooming themselves has anything to do with thermoregulation. Also, while cats claws are hollow, they do have veins if you go even just a bit too far (I’m sure you know this). That’s the distinction I was trying to make, but I definitely didn’t do so clearly.
Anyway, this was all excellent information, as you often provide when I post questions. Thanks for this one and thanks again for previous answers!
It’s how I learn new stuffz! 🙂
Hmmm, I’m not sure I like that analogy, because it conflicts with what I’ve seen examining flints under the microscope. Even down to the hundredth-mm level, they’ve got a very uniform structure with no laminations, cleavages etc. On the micron level (thousandth-mm) – better than my microscopes or preparation technique and budget – they’re reported to have a micro-granular structure with very fine, but not wholly crystalline structures in a sea of nearly glassy material. The presence of a percent or so of water in the mix is probably important.
What you’re probably thinking about (sorry, but my degree was in “Geology and Mineralogy” – looking closely at rocks is a bad habit I have) is the fracture habit that both cherts (flint is a chert) have of breaking to give a “conchoidal” fracture surface which includes a sharp (almost to the molecular level) edge at the line of breakage. But I think really that is more a function of the homogeneity of the material and the lack of crystalline planes of weakness allowing the material to express it’s fracture in response to the stress fields at the time of fracture, rather than being interfered with by the variations of strength in the crystallisation structure. Conchoidal fractures are actually relatively uncommon in minerals.
Thanks! What I thought was going on: Flint, chert laid down in sedimentary layers & then hardened under high temp/pressure to be like the leaves of a book. Cleaving occurs in the plane of the ‘pages’ leaving a sharp edge.
Flint in particular (“chert” is a broader church) seems to form as a seabed gel (under what conditions, for what reasons? Now there is a damned good question.) which has at least some mobility (it frequently encloses and partially replaces fossils of benthic or infaunal (burrowing) organisms). Quite how it then solidifies – well it was a mystery when I was a student, and I’ve not heard of it having been satisfactorily solved.
Certainly you get “beds” of flint across the clifffs, which coarsely follow time-surfaces. Indeed, they can be correlated to a degree across fields and even regions (which given their habit of totally trashing drill bits ($100,000 trash) until the late 1990s, was why I had to pay damned close attention to the stuff). But what recurring features of seabed chemistry they reflect … is still TTBOMK not known.
So many questions. Is the cat family unique in this way? Why are dogs’ tongues so darn smooth? Are cats’ tongues used for scraping flesh off of prey? I have seen cats lick their prey. My cat used to wash after eating, and it would smell like cat food.
I don’t remember where I heard it, but I’ve always thought that big cats like lions do indeed use their rough tongues to scrape bits of flesh off of bones. However, I also assumed that big cats = big tongue papillae (and really rough flesh-scraping tongues), which does not appear to be the case.
Yes the ‘tongue knives’ act as scrapers of prey flesh. There is a video showing a cat tongue being pulled through a store-bought piece of pork & the meat is easily ripped to shreds by the papillae. The papillae are scoop-shaped in cross section – little shovels.
+1
Second video down in THIS LINK
I too am doubtful about the thermoregulation part of the theory. Perhaps they do it in extreme heat, which I haven’t observed. Even then, I would expect conservation of water to take priority. As I see it, cats thermoregulate like most creatures, they move to where the temperature suits them better. Of course, the cooling due to grooming will probably cause them to seek a warmer spot than when they are not grooming.
I don’t know if the study mentions this but I find that their grooming makes quite a noise. My wife and I hate it when one of our cats grooms itself on our bed when we’re in it. The noise and vibration bothers my wife particularly.
I enjoy it when my cat joins me in bed, takes several minutes to clean herself and then settles down. Sort of calming, like rain is to some people.
Yes, it isn’t so bad when we’ve just gone to bed though my wife doesn’t even like it then. I think it is when it happens in the middle of the night, or in the morning before we’re ready to rise, that it can be bothersome.
I lost my cat the hard way. He didn’t come home one day. Still I sometimes awake thinking I feel him on the bed washing. Then gloom descends.
That is sad. I fear that too as we have coyotes in our area. Our security camera even caught them in our back yard! This is why we don’t let them out at night but even then it’s a gamble.
No the coyotes have humans in their area
Both are true but only humans can know it and say “in our area” but I get your point.
Thanks Paul considerate reply bud
“Why are cat tongues spiny?”
That’s a rather teleological title, don’t you think?
I’m no biologist but isn’t it just a handy shorthand? If one believes in determinism, and feels that knowledge should inform our conversation all the way to this level, then there are no causes at all. Or just the one big one.
Huh?
My interpretation:
“All the way”: Biology has this thing about being accused of non-causality teleology (“striving for goals”) while also promoting shorthand and eagerly adopt adaptive just-so stories. While other sciences lack all of that: “why” is just asking for mechanisms obeying causality.
“The … one”: Religion?
It may be shorthand but the article isn’t about why cat tongues are spiny; it’s about why spiny tongues are useful.
“Why are giraffes tall?” is one question; “why being so tall is useful” is another question.
I think the licking also makes them feel good. Several of our cats lick each other and the recipients of the licking sure look happy and tranquil.
It seems a pretty common evolutionary outcome that organisms are “programmed” to experience something like pleasure or contentment when performing behaviors that are beneficial to them and vice versa.
Well put.
Which leads to the question whether the consciousness of organisms generally is like what humans experience. What is it like to be a bat, or a cat. Does the bird building a nest feel something like pleasure or contentment each time a twig is inserted? Certainly there must be some continuity between the early hominids and modern humans. Otherwise, as the argument goes, you could just be an android soullessly moving according to software rather than feelings of pleasure or pain. Does a flatworm feel pleasure moving according to the chemistry in it’s skin and eyespots? Does a bean plant feel content when phototropism kicks in at the first red wavelengths of the rising sun? Write out your answer on a single page of singly lined notebook paper and submit it by the end of the week. Class dismissed.
We don’t have the knowledge and technology yet to tell for sure but here’s how I look at it. We observe animals’ gaze, movements, reactions in terms of our own. When they correlate well, we assume their inner workings are likely similar.
I’d give that answer a B+. B for being reasonable, and + for agreeing with me. 😎
I like your grading scale. I’m stealing it.
Does a creature have to be conscious to experience pleasure, fear etc? And what is consciousness anyway? I think ‘drives’ do not require whatever consciousness is to function & it seems reasonable to me that the brain/mind of all creatures contains ‘drives’ to seek sustenance, reproduce etc.
If consciousness is real it needs a tighter definition than ‘self awareness’ [or whatever] & I see no reason why consciousness is required for ‘pleasure’ or ‘fear’ to be experienced. ‘Drives’ are presumably part of what leads to consciousness, but ‘drives’ must exist first.
Does the above make sense? I’m unsure!
I go with Daniel Dennet. Consciousness is just what it feels like to be a brain. It is the perspective alone, that of a brain experiencing its own operation, that makes it seem mysterious.
You seem to be saying animals are more like soulless zombies then. Drives only. Humans, on the other hand have a complex relationship with our drives which we call consciousness.
Drives are urges to do something like scratch my elbow. Pleasure is the feeling you get when the drive is designed to be repeated, like eating a bowl of ice cream with nuts and chocolate syrup. And a cherry. And some whipped cream.
I can’t disagree if you’re going to top your argument with whipped cream!
I should have ended with the cherry on top. 😎
Life sure is complicated. It seems likely to me, based on what we have discovered about our universe so far, that consciousness is a spectrum rather than simply has it or doesn’t have it. Though consciousness isn’t even all that well defined I am sure that if I have this property then my cat surely does as well, though to a different degree. Or at least different in some characteristics. And similarly with other animals.
Plants? Doesn’t seem plausible as they don’t seem to have anything analogous to a brain and it seems pretty certain that consciousness is a property of brains.
Insects? Seems unlikely as their brains are so small it seems quite plausible they don’t have the capacity. But perhaps they are conscious to some very rudimentary degree.
But, what exactly is consciousness? How do we differentiate or separate out consciousness from other cognitive abilities? For example, I say my cat is conscious but to a lesser degree than I am, but is that actually the case? She seems to have a perfectly clear awareness that she is herself and that other things are not. So, is she as conscious as I am but lacking the cognitive abilities that I have that allow me to contemplate all this? Conversely, could an organism have cognitive abilities comparable to humans’ without being conscious, and what would that look like? It seems likely to me that consciousness and higher order cognitive abilities are linked in some way.
One aspect of human level consciousness not seen in other organisms is extensive recursion. Along with a detailed memory, this makes much of our thinking “meta”, ie, our thoughts and words are almost always followed by thoughts about the initial thoughts, and thoughts about the secondary thoughts, etc. A dog or cat seems to be pretty much tied to one or two levels of recursion so that they seem less creative in their response. As a result they are usually quicker to respond, which was undoubtedly valuable in the setting from which they recently evolved.
But are those aspects of consciousness or are they separate cognitive features? Couldn’t we make a computer that is capable of recursion to an arbitrarily high number of levels? And solid state memories and the computer hardware and software to use them are far better than human memory.
Certainly we could build computers with all those capabilities and more. The problem with trying to simulate humans with a computer is, we are a tightly bound system of senses, hormones, emotions, and calculations. Building a system like that would be so hard it would be much easier just to use sexual reproduction and make another human. 😎
More fun too.
Well, yes and no. The fertilization is a walk in the park, but you still have to feed data into the little jelly roll for 20 years until it’s of any use.
lol!
Yeah, that part does get tough.
Mother-cat used to clean the catlings – no nappies that can be used without opposable thumbs – and the sensation probably takes them back mentally to the nest. Since many of the characteristics of the domesticated cat are neotenic (retention of juvenile characteristics into the adult lifestyle and/ or anatomy), and that seems to be an easy route of development to take, then I can easily see this sensation being retained into the adult. Particularly if the cat grows up in the presence of another cat – which would not naturally be the case in adult wildcats.
Lions living in prides with other adult cats is a very unusual situation in the context of Panathera as a whole.
I forwarded this post to my daughter who is a DVM.
“DVM”?
Dilatory Vocational Motorcyclist?
Discretionary Vibrating Monologist?
Dog’s Voluntary Mother?
Doctor of Veterinary Medicine.
I’m sure she’ll love your guesses.
Doh! I should have guessed, but I can’t even remember what the equivalent UK qualification is.
The tongue papillae of felids seem to be multifunctional. Fur wetted during self-grooming may facilitate the removal of fleas. Fleas lose their mobility in wet cat fur and become very easy to pick off with tweezers. Something similar may happen when cats self-groom. Cats will also lick meat, and the papillae may help remove meat tissue from tendons and bone.
Just to be clear, the study was undertaken in Jimmy Carter’s Georgia, at Georgia Tech, and not Joseph Stalin’s Georgia.
Interesting. Amazing that we are still learning a lot about creatures that are so close to us!
There are fantastic images of lions with their mouths open, and huge lumps of animal meat dangling off their lolling tongues at vertical angles. Their tongues are like velcro, and the meat just hangs there like it’s been glued on. They effectively carry the meat around with their tongues.
I really enjoyed this article, Jerry. It made me look closer at something with which I am so familiar and tend to take for granted = )
Ah, keratin. Ooh, hollow.
Possibly, but WEIT has shown videos that when cats drink water they can just lap it up, while dogs have to do a tongue twist.
At a guess, the cat family evolving smaller species can be eased by this. (Reversely, small stature dogs may have disproportionally longer tongues – and drool a lot. ;-))
Interesting!
Podcast on this topic
https://sciencesessions.podbean.com/mf/play/bsuj4e/alexisNoelDavidHuPodcast.mp3