November 12, 2010
How Cats Lap: The Biomechanics of Feline Liquid Uptake
Above, how a cat drinks, recorded at 120 fps and slowed down 4x.
Below, the physics behind the physiology, slowed down 67x.
Below, Nicholas Wade's story in today's New York Times.
It has taken four highly qualified engineers and a bunch of integral equations to figure it out, but we now know how cats drink. The answer is: very elegantly, and not at all the way you might suppose.
Cats lap water so fast that the human eye cannot follow what is happening, which is why the trick had apparently escaped attention until now. With the use of high-speed photography, the neatness of the feline solution has been captured.
The act of drinking may seem like no big deal for anyone who can fully close his mouth to create suction, as people can. But the various species that cannot do so — and that includes most adult carnivores — must resort to some other mechanism.
Dog owners are familiar with the unseemly lapping noises that ensue when their thirsty pet meets a bowl of water. The dog is thrusting its tongue into the water, forming a crude cup with it and hauling the liquid back into the muzzle.
Cats, both big and little, are so much classier, according to new research by Pedro M. Reis and Roman Stocker of the Massachusetts Institute of Technology, joined by Sunghwan Jung of the Virginia Polytechnic Institute and Jeffrey M. Aristoff of Princeton.
Writing in the Thursday issue of Science, the four engineers report that the cat’s lapping method depends on its instinctive ability to calculate the point at which gravitational force would overcome inertia and cause the water to fall.
What happens is that the cat darts its tongue, curving the upper side downward so that the tip lightly touches the surface of the water.
The tongue is then pulled upward at high speed, drawing a column of water behind it.
Just at the moment that gravity finally overcomes the rush of the water and starts to pull the column down — snap! The cat’s jaws have closed over the jet of water and swallowed it.
The cat laps four times a second — too fast for the human eye to see anything but a blur — and its tongue moves at a speed of one meter per second.
Being engineers, the cat-lapping team next tested its findings with a machine that mimicked a cat’s tongue, using a glass disk at the end of a piston to serve as the tip. After calculating things like the Froude number and the aspect ratio, they were able to figure out how fast a cat should lap to get the greatest amount of water into its mouth. The cats, it turns out, were way ahead of them — they lap at just that speed.
To the scientific mind, the next obvious question is whether bigger cats should lap at different speeds.
The engineers worked out a formula: the lapping frequency should be the weight of the cat species, raised to the power of minus one-sixth and multiplied by 4.6. They then made friends with a curator at Zoo New England, the nonprofit group that operates the Franklin Park Zoo in Boston and the Stone Zoo in Stoneham, Mass., who let them videotape his big cats. Lions, leopards, jaguars and ocelots turned out to lap at the speeds predicted by the engineers.
The animal who inspired this exercise of the engineer’s art is a black cat named Cutta Cutta, who belongs to Dr. Stocker and his family. Cutta Cutta’s name comes from the word for “many stars” in Jawoyn, a language of the Australian aborigines.
Dr. Stocker’s day job at M.I.T. is applying physics to biological problems, like how plankton move in the ocean. “Three and a half years ago, I was watching Cutta Cutta lap over breakfast,” Dr. Stocker said. Naturally, he wondered what hydrodynamic problems the cat might be solving. He consulted Dr. Reis, an expert in fluid mechanics, and the study was under way.
At first, Dr. Stocker and his colleagues assumed that the raspy hairs on a cat’s tongue, so useful for grooming, must also be involved in drawing water into its mouth. But the tip of the tongue, which is smooth, turned out to be all that was needed.
The project required no financing. The robot that mimicked the cat’s tongue was built for an experiment on the International Space Station, and the engineers simply borrowed it from a neighboring lab.
another video, 5x slower than at the top of this post.
Here is a video interview with two of the scientists involved in the study.
The abstract of the Science paper follows.
How Cats Lap: Water Uptake by Felis catus
Animals have developed a range of drinking strategies depending on physiological and environmental constraints. Vertebrates with incomplete cheeks use their tongue to drink; the most common example is the lapping of cats and dogs. We show that the domestic cat (Felis catus) laps by a subtle mechanism based on water adhesion to the dorsal side of the tongue. A combined experimental and theoretical analysis reveals that Felis catus exploits fluid inertia to defeat gravity and pull liquid into the mouth. This competition between inertia and gravity sets the lapping frequency and yields a prediction for the dependence of frequency on animal mass. Measurements of lapping frequency across the family Felidae support this prediction, which suggests that the lapping mechanism is conserved among felines.
Read the Science paper in its entirety, including figures, here.
Full disclosure: from time to time I turn off all ambient sound so I can listen to Gray Cat lap up her water.
I like to watch, too.
November 12, 2010 at 05:01 PM | Permalink
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How interesting and revealing this study is. Mechanically speaking, there is no reason that a cat couldn't ingest water the same way that dogs do. That the mechanism is so different and is distributed across the entire family Felidae clearly demonstrates that the mechanism gave the early Felidae an evolutionary advantage over Felidae that used other mechanisms to ingest water.
Posted by: 6.02*10^23 | Nov 13, 2010 11:23:13 AM
But did you ever get a collar on her?
Posted by: Carol | Nov 13, 2010 2:50:51 AM
I guess that this means IV hydration if we decide to take our cats into microgravity environments.
Posted by: Lawlibrarian | Nov 13, 2010 1:58:17 AM
Finally - something really important!
Posted by: Flautist | Nov 12, 2010 10:02:22 PM
i just love this post.
thanks for the fascinating content, as always, joe.
Posted by: jo Novelli | Nov 12, 2010 5:16:21 PM
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