• How do cats lap? Cutta Cutta is about to take a drink.

    Photo: Pedro Reis, Micaela Pilotto and Roman Stocker

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The surprising physics of cats’ drinking

A new study reveals that even the way cats lap up liquid displays the perfect balance for which they’re known.


Cat fanciers appreciate the gravity-defying grace and exquisite balance of their feline friends. But do they know that those traits extend even to the way cats lap milk?

Researchers analyzed the way domestic and big cats lap and found that felines of all sizes take advantage of a perfect balance between two physical forces. The results were published in the Nov. 11 online edition of the journal Science.

It was known that when cats lap, they extend their tongues straight down toward the bowl with the tip of the tongue curled backwards, so that the top of the tongue touches the liquid first. That insight came from a 1940 film of a cat lapping milk, made by Harold “Doc” Edgerton, the MIT electrical engineering professor who first used strobe lights in photography to stop action.

But recent high-speed videos made by MIT, Virginia Tech and Princeton researchers reveal that the top of the cat’s tongue is the only surface to touch the liquid. Cats, unlike dogs, don’t dip their tongues into the liquid like ladles. The cat’s lapping mechanism is far more subtle and elegant. The smooth tip of the tongue barely touches the surface of the liquid before the cat draws its tongue back up. As it does so, a column of liquid forms between the moving tongue and the liquid’s surface. The cat then closes its mouth, pinching off the top of the column for a nice drink, while keeping its chin dry.

When the cat’s tongue touches the liquid surface, some of the liquid sticks to it through liquid adhesion, much as water adheres to a human palm when it touches the surface of a pool. But the cat draws its tongue back up so rapidly that for a fraction of a second, inertia — the tendency of the moving liquid to continue following the tongue — overcomes gravity, which is pulling the liquid back down toward the bowl. The cat instinctively knows just when this delicate balance will change, and it closes its mouth in the instant before gravity overtakes inertia. If the cat hesitated, the column would break, the liquid would fall back into the bowl, and the tongue would come up empty.

While the domestic cat averages about four laps per second, the big cats, such as tigers, know to slow down. Because their tongues are larger, they lap more slowly to achieve the same balance of gravity and inertia.

Ready for my close-up

Roman Stocker of MIT’s Department of Civil and Environmental Engineering (CEE), Pedro Reis of CEE and the Department of Mechanical Engineering, Sunghwan Jung of Virginia Tech and Jeffrey Aristoff of Princeton analyzed high-speed digital video of domestic cats, including Stocker’s family cat and a range of big cats (a tiger, a lion and a jaguar), thanks to a collaboration with Zoo New England’s mammal curator John Piazza and assistant curator Pearl Yusuf. And, in what could be a first for a paper published in Science, the researchers gathered additional data by analyzing existing YouTube.com videos of big cats lapping.

With these videos slowed way down, the researchers established the speed of the tongues’ movements and the frequency of lapping. Knowing the size and speed of the tongue, the researchers then developed a mathematical model of lapping, which involves the ratio between gravity and inertia. For cats of all sizes, that number is almost exactly one, indicating a perfect balance.

Cutta Cutta drinking
Video: Pedro M. Reis, Sunghwan Jung, Jeffrey M. Aristoff and Roman Stocker

To better understand the subtle dynamics of lapping, they also created a robotic version of a cat’s tongue that moves up and down over a dish of water, allowing them to systematically explore different aspects of lapping and, ultimately, identify the mechanism underpinning it.

“This work is as splendid a case as I can recall of things looked at but seen in a way that no one else has seen,” says Professor Steven Vogel of Duke University, a biomechanics researcher who was not involved in the project. “Now that I’ve been clued in, I can report that what these people describe and explain agrees entirely with my own casual observations of the lapping action of the feline in charge of this establishment.”

The lapping research began three years ago, when Stocker, who studies the biophysics of the movements of ocean microbes, was watching his cat drink. That cat, 8-year-old Cutta Cutta, stars in the researchers’ videos and still pictures. And like a lot of movie stars (Cutta Cutta means “stars stars” in an Australian aboriginal language), he doesn’t mind making people wait. Stocker and Reis spent hours at the Stocker home with their cameras trained on Cutta Cutta’s bowl, waiting for him to drink.

“Science allows us to look at natural processes with a different eye and to understand how things work, even if that’s figuring out how my cat laps his breakfast,” Stocker says. “This project for me was a high point in teamwork and creativity. We did it without any funding, without any graduate students, without much of the usual apparatus that science is done with nowadays.”

“Our process in this work was typical — archetypal, really — of any new scientific study of a natural phenomenon. You begin with an observation and a broad question — ‘How does the cat drink?’ — and then try to answer it through careful experimentation and mathematical modeling,” says Reis, a physicist who works on the mechanics of soft solids. “To us, this study provides further confirmation of how exciting it is to explore the scientific unknown, especially when this unknown is something that’s part of our everyday experiences.”

Besides their obvious enthusiasm for the work itself, the researchers are also delighted that it builds on Edgerton’s 1940 film of the cat lapping. That film appeared as part of an MGM-released movie called Quicker’n a Wink, which won an Academy Award in 1941. Reis and Stocker say they’re moving on to other collaborations closer to their usual areas of research. But their feline friend Cutta Cutta might have Oscar hopes.


Topics: Civil and environmental engineering, Collaboration, Mathematics, Mechanical engineering, Physics, Research

Comments

I don't know if it is legal for this to be up on YouTube, I didn't post it, but since it's there - here's the Edgerton film mentioned above: http://www.youtube.com/watch?v=jUZwFrGzQGw
This is my cat doing just that - you can see it does not break the surface: http://www.youtube.com/watch?v=B6XKzUFWNA4
The surface of a cat's tongue is much rougher than sandpaper - it is like the fine knurled surface on a metal file. The surface creates a strong pulling force, almost suction, when it slides against bare skin. I'd bet such a surface texture inherently pulls up more water (due to wetting and surface tension) than our tongues, and it is the adhered/wicked water on the cat's tongue, not the water column adjacent it, that satiates the cat.
This story was featured on the 11-12 edition of NBC Nightly News. A star is born!
Very interesting study! It would also be interesting to find out why such a drinking mechanism evolved. After all, it's much slower than a more "normal" mechanism. (Thus, one would expect it to have some benefit(s) to outweigh that cost/risk.) For example, does this approach result in the intake of fewer particulates from the water? Or, is there a survival benefit to not getting the face of the cat wet?
Actually, if you examine the cat's tongue (not easy) the roughness does not extend to the tip, and the part that turns under in the video to meet the liquid is actually pretty smooth.
We were taking 8.701 (I think), the senior physics course in theoretical physics (1963): the lecturer, P.A.M Lemmer, visiting from South Africa, came to the topic of "canonical coordinates", which don't appear in the LaGrangian for the system under consideration. The associated kind of momentum (linear, angular) in the system is conserved. As an example, Lemmer proposed, "Consider the Cat: we hold it upside down and then drop it. What happens? The Cat lands on its feet! Why (the Cat is in free fall, unconstrained by anything but gravity)? Well, it turns out that there is a coupling between the Cat's front end and its…, its… (at which point the student audience erupts) No, really! there's a coupling between the Cat's forequarters and its,…, its…" Again the student audience erupts in laughter! Lemmer realizes what's going on; puts his hands on the lecture table and lets us see the redness creep up his bald pate. Finally, he wrote on the blackboard a reference to a paper, "Paris: Comtes Rend…(classmates help here!) and left the lecture room 20 minutes early. That was Wednesday: On Friday, when Lemmer returned to do his lecture, what should appear on the demonstration table but a cat?
I think that all the small bristles of the tongue have a higher "water-catching" performance. Otherwise, why does my cat drink in this manner?
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