National Geographic

How the cat that got the cream then drank it

Cutta_cutta

Cats have been our companions for almost 10,000 years. They have been worshipped by Egyptians, killed (or not) by physicists, and captioned by geeks. And in all that time, no one has quite appreciated how impressively they drink. Using high-speed videos, Pedro Reis and Roman Stocker from the Massachusetts Institute of Technology has shown that lapping cats are masters of physics. Every flick of their tongues finely balances a pair of forces, at high speed, to draw a column of water into their thirsty jaws.

Drinking is more of a challenge for cats than for us. They have to drink from flat, horizontal bodies of water. Even with our hands tied, we could do that just by putting out mouth at the surface and sucking, but then we have large cheeks that can form a proper seal. Pigs, sheep and horses have  the same ability, but cats and dogs do not. Their cheeks don’t extend far enough forward so they have to use a different technique: lapping.

Cat owners have watched their pets lap at water for thousands of years but when Stocker did so, his curiosity was piqued. “Three years ago, when I was watching my cat Cutta Cutta lap during breakfast, I realized there was an interesting biomechanics problem behind this simple action,” he says. The lapping motion is so fast that to fully appreciate it, you need a high-speed camera. Slow-motion films of Cutta Cutta revealed that a cat doesn’t actually scoop up its drink with its tongue in the way that a dog does. Its technique is more subtle.

For a start, it drinks only using the very tip of its tongue. As it extends its tongue, it curls the tip upwards so that the bottom side rests on the surface of the liquid, without actually breaking it.  The cat lifts its tongue, drawing a column of liquid with it. Just before the column collapses, the cat closes its mouth, captures the elevated liquid, and takes a refreshing drink.

Lappingsequence

This sequence depends on a battle between two forces. The first is inertia, the tendency for the water column to keep moving in the same way until another force acts upon it. That force is gravity, which constantly threatens to pull the water column back into the bowl. With its rising tongue, a cat uses inertia to “defeat gravity” long enough to close its mouth on the almost-collapsing tower of liquid.

Amazingly, Reis and Stocker found that this sequence was first filmed way back in 1940, in an Oscar-winning documentary film called Quicker ‘n a Wink (the cat’s at 4:42). “MIT has a center for high-speed photography started by and named after Doc Edgerton,” says Stocker. “After we had borrowed high-speed cameras several times to get shots of Cutta Cutta, conversation casually turned to the topic of our investigation. When Jim [Bales, who runs the center] heard it was the lapping of a cat, he got very excited and immediately pointed us to a 10 second video clip by Doc Edgerton, who had captured high-speed videos of a cat lapping 70 years ago! Edgerton did not attempt to explain the mechanism of lapping, but his clip remains timeless (the commentary even more so!).”

Skip forward 70 years, and Stocker and Reis’s modern (and colour) videos reveal the action in even greater detail. Reis used these to work out Froude’s number, a value that measures the trade-off between inertia and gravity. For domestic cats, the number is 0.4, close enough to 1 to indicate a good balance between the two forces.

The duo also analysed the lapping action using a glass disc to mimic the tongue-tip of a cat, and robotic stage to lift the disc at pre-programmed speeds and heights. These revealed that the cat pulls its tongue back at just the right speed and to just the right height to draw the largest volume of water into its mouth.

Stocker and Reis even found that all types of cats, from the humble house tabby to the mighty tiger, lap in the same way. He grabbed some films from a local zoo and in an astonishing case of the Internet providing useful cat videos, he scoured YouTube for videos of big cats lapping. The videos showed that just like domestic cats, the Froude’s numbers for lapping tigers, lions and leopards are all close to one. Again, inertia and gravity are play off against each other to produce the greatest possible sip, although the bigger cats compensate for their bigger tongues by lapping more slowly.

All of this started with something as simple as a cat owner watching his pet drink from a bowl and wondering how it does it. From there, the team pursued their project without any funding, or help from graduate students. They just really wanted to know the answer. If you look at the world through the eye of a scientist, even an unassuming sight like a cat drinking from a bowl can be a cool discovery just waiting to happen. Rather than killing cats, curiosity can thrive on them.

Reference: Science http://dx.doi.org/10.1126/science.1195421

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There are 18 Comments. Add Yours.

  1. Tom Noir
    November 11, 2010

    That’s really cool! But I have a follow up question. Occasionally you come across a housecat with a preference for drinking only water that is moving in some way – either their owners splash the bowl a bit or they drink out of a running tap. Sometimes the cat itself may even splash the water a bit on their own if no one will do it for them. Does this make it easier or harder for a cat to drink? One would think harder, since the mechanism they use seems to depend on a still surface. But nonetheless, some cats seem to prefer it.

  2. khan
    November 11, 2010

    Cats that have been weaned too young will lower their lower jaw into the water dish.
    —————————————-
    I have had cats that preferred running water when they could get it.

  3. Al Feersum
    November 11, 2010

    “Faster than a goose in high wind!” Class!

    @1 – Tom, dunno about whether their preference is down to the mechanism, or just their own obsessive desire to show you up. Mine prefer rainwater to tapwater, yet my daughter’s cats prefer to drink out of the tap while it is running, and my mother’s cat? She prefers to drink out of the bath, and often demands water be put in it if empty.

    I suspect it’s down to the fact that they know that they are our superiors… and we will slavishly bend to their every whim…

    Maybe they prefer running water because it tastes better, in the same way that it is recommended that tea is made with freshly drawn water.

  4. Glenn
    November 11, 2010

    The TV show was called “You asked for it!”.
    They settled an argument on whether a cat lapped forward or backward by filming one drinking with a (at the time) high-speed camera and slowing down the film.
    Congratulations on repeating the experiment with easily obtained equipment and claiming to be the first. Think I’ll try dropping objects of differing mass off a tower.

  5. Ed Yong
    November 11, 2010

    Or you could try working on your reading comprehension first…

    Nowhere did the authors claim that they were the first to film this. In fact, they explicity state that it was done in 1940, as did I.

  6. Walter S. Andriuzzi
    November 11, 2010

    “killed (or not) by physicists”. The geeky side of me is so loving this sentence! (And wait, there’s no non-geeky side of me)

  7. Elia Ben-Ari
    November 11, 2010

    Great story, Ed. I especially liked your ending! And how nice that no grad students or postdocs were harmed in these experiments.

  8. Charles Sullivan
    November 11, 2010

    I’m guessing that drinking would be even easier for cats if they were ever to live on Mars (in a pressurized structure of course), what with the lower gravity and all.

  9. Mark Young
    November 12, 2010

    What is the upward force? Is it a bounce from a downwards thrust,or is it to do with surface tension and hydrogen bonds?

  10. Shade
    November 12, 2010

    @ charles
    The lower gravity could very well mess the whole thing up, as Ed pointed out, its very well blanced, and to mess with a key element in that balance would probably have them drinking through their nose as much as anything else.

    Loved the end note Ed, totally agree, the world we live in is made of wonder to those who have a will to look.

  11. Andrew Wilson
    November 12, 2010

    It would be interesting to see whether the cat could learn to adapt to the lower gravity; they must have some flexibility to cope with liquids with varying inertia so presumably they learn this action. Developmental cat studies on Mars ahoy!

    I love that old video about high speed filming. I’m so going to use that in lectures on motion capture :)

  12. Rich Langsford
    November 12, 2010

    Fascinating study, and really enjoyed the entire Pete Smith Specialty clip…haven’t seen one of those since I was a kid! I didn’t see any links to the new, color, high speed clips featuring Cutta Cutta. Are these available online?

    Oops! Never mind…I found them here

    http://web.mit.edu/preis/www/videos/Movie_cutta_12x_slower_reis_jung_aristoff_stocker.mov

    and here

    http://web.mit.edu/preis/www/videos/Movie_Cutta_67x_slower_reis_jung_aristoff_stocker.mov

    and the glass tongue simulator thingie here:

    http://web.mit.edu/preis/www/videos/Movie_experiment_100x_slower_reis_jung_aristoff_stocker.mov

    Thanks for the great blog!

  13. Wayne Stoller
    November 12, 2010

    I don’t suppose the fact that a cat’s tongue has much deeper “pockets” between Papillae and would have more adhesion for the water to draw it up, has much to do with the difference between Cats and Dogs. Water cohesion is interesting also.

  14. Ed Yong
    November 12, 2010

    @Mark Young – it’s the initial pull from the cat’s tongue, which probably has to do with the microstructure that Wayne describes in 15. I think the paper says something about surface tension not being involved, but I don’t have it on me right now to check.

    @Rich – Thanks for finding those vids.

  15. Bishnu Marasini
    November 13, 2010

    Very rough surface found on tongue of cat family might act as sucker creating vacuum.

  16. Bryan Eastin
    November 13, 2010

    The focus on inertia in this article seems odd to me. It’s sort of like saying that the trick to shooting a basketball is in balancing inertia and gravity. Kind of, but getting the basketball moving in the first place is pretty important too. That feeling may just reflect my surprise at the cat’s ability to form such a nice column of water by flicking its tongue, however. Also, inertia is not considered a force; forces are things that, if unbalanced, change velocities.

  17. Ryan
    November 14, 2010

    I agree with Bryan. After reading the paragraph with the inertia explanation, I furrowed my brow in dissatisfaction. The first two pictures in the figure really really look like the tongue, as you point out, isn’t scooping liquid, it’s just tapping the surface and jerking upwards. So, I would think that surface tension and fluid pressure should be what gives the liquid its impulse.

    But then I read the paper, and they completely ignore the creation of the liquid column. To a suspicious degree, I think, but maybe that’s just not what they focused on for this paper. They do focus on the column dynamics, such as its diameter and height before it’s pinched off. So: it’s ok, I guess. It just seems incomplete.
    Disclaimer: I’m in physics, but I don’t regularly do fluid dynamics, so my word isn’t final.

    side note: I enjoyed the part of the paper where they model the cat’s tongue’s height over time. It describes an inverted Gaussian, and the tip of the liquid column follows the tongue back up, roughly making it an error function. This seems weird to me!
    Ok, so I understand why this isn’t intriguing to anyone else.

  18. Matt McIrvin
    November 16, 2010

    I have a cat who likes to kick the bowl around to get the water sloshing (often out onto the floor). My guess has always been that he does it so he can see that there’s water in the bowl, and see where the surface is. Cats’ vision is not very acute, but they’re good at seeing motion, so it may very well be his only way of seeing the surface of the water.

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