by Greg Mayer (update below)
Pedro Reis of MIT and colleagues are about to publish a paper in Science analyzing high speed films of cats drinking. A preprint is already available. I’m not sure there’s really anything interesting here, other than cool video. You can tell that only the tip of the cat’s tongue contacts the milk, so the ‘brushy’ part of the cat’s tongue is not involved in getting milk into the mouth, which is not what I would have expected. (The brush has a large surface area, which I thought would be involved in liquid adhesion, but it’s not.)
There are more videos at Reis’s website, here and here. Ed Yong at Not Exactly Rocket Science has more background on the work, and notes that MIT’s Harold ‘Doc’ Edgerton (of Loch Ness monster fame) made high speed film of cat lapping in 1940. The cat is at about 4:41.
Edgerton (or at least his film’s narrator) thought the back of the tongue cupped the milk, but Reis’s better films show the milk is on the front of the tongue, not the back, curled part of the tongue. The BBC also has a story up.
P.M. Reis, S. Jung, J. Aristoff and R. Stocker. 2010. How Cats Lap: Water uptake by Felis catus. Science in press.
Update. MIT has a press release with more background available here. I mentioned that cats don’t ladle the milk, nor is it the brushy part, and I’ve been queried about what it is they do do. Here’s how the MIT press release puts it:
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 release goes on to mention that the cat instinctively knows when to close its mouth, but the paper did not in fact address that question, and cats may well learn the timing (much as first basemen learn the perfect time to snap their glove shut). The NY Times has a story up now too. Besides the cool video, the most interesting thing to me about the paper is that they were able to predict a relationship between lapping frequency and body mass, and show that their prediction roughly held (see Fig 4c of the paper; the authors make only a glancing reference in the text).