Any ailurophile who’s at all internet literate knows that four scientists have just solved the age-old problem of “how do cats drink”? This is one of those cool cases where a simple problem has been ignored because it’s not considered scientifically sophisticated—even if that problem has a relatively simple solution. The solution is published in this week’s Science, which has a nice cover.
The title of the papers is succinct: “How cats lap: water uptake by Felis catus.” (I would have used just the first three words; the second part is redundant.)
Like most other people, I’d always thought that cats curled their tongues backwards when they drink, forming a little cup to convey liquid to the mouth. The new work shows that this is wrong. Here’s the abstract with the TRUTH:
Abstract. 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.
Translation: the cat curls the tip of its tongue “downward” a bit, touches it to the surface of the liquid, then quickly draws its tongue upward. The inertia produces a column of liquid which the cat then “bites off” and holds it in cavities in its mouth, until it swallows the liquid—every 3 to 17 laps, according to the authors.
And I want to put in the first paragraph because it’s informative and also a pretty good model of clear scientific writing:
Terrestrial animals have evolved diverse means to acquire water, including absorption through the skin (1) or extraction of moisture from food (2), but most rely on drinking (3–12). Drinking presents a challenge to land vertebrates, because fresh water occurs mainly as horizontal liquid surfaces, such as puddles, ponds, lakes, or streams, and animals must displace water upward against gravity to drink it. Crucial in the drinking process is the role of the tongue, which in vertebrates is used in two distinctly different ways. Vertebrates with complete cheeks, such as pigs, sheep, and horses, use suction to draw liquid upward and use their tongue to transport it intraorally (13, 14). In contrast, vertebrates with incomplete cheeks, including most carnivores, are unable (after weaning) to seal their mouth cavity to generate suction and must rely on their tongue to move water into the mouth (13). When the tongue sweeps the bottom of a shallow puddle, the process is called licking (4). When the puddle is deeper than the tongue excursion into the liquid, it is called lapping (15). Here, we report on the lapping mechanism of the domestic cat (Felis catus).
Ed Yong’s take is at Not Exactly Rocket Science, and there’s no need for me to repeat what he said.
Here are some photos from the paper showing how the tongue forms the liquid column (below is the caption from the paper):
The lapping process. (A to F [“C” is below]) Snapshots showing the movement of the tongue of F. catus and the dynamics of the liquid column during a lapping cycle. Lapping occurs by fluid adhesion to the dorsal part of the tongue’s tip and by lifting a liquid column through the tongue’s upward motion, before jaw closure. Time elapsed from the first frame is given in the top left corner of each frame.
Another photo from the paper. Note that the part of the tongue that the cat uses for lapping (the front part) is free from papillae. Presumably these would somehow inhibit lapping, but the rougher parts of the tongue are of course necessary for grooming and are also used for eating (bigger cats use the papillae to scrape flesh from bones).
(G) Photograph of the dorsal side of the tongue of F. catus, acquired under anesthesia (16). Only the smooth tip is used in lapping.
Here are three short supplemental movies from the paper, two showing an “artificial tongue” used to calculate the physics of the process (captions taken from the paper).
Cat lapping. Three lapping events of an adult domestic cat, during its normal drinking process. The video was recorded with a Sony HDR-SR5 camera operated at 120 frames/s and is here shown at 30 fps (i.e., slowed down four times). As the tip of the tongue comes in contact with the liquid surface, water adheres to the dorsal side of the tongue’s tip. A liquid column forms when the tongue is rapidly lifted. The liquid column grows by inertia, until gravity induces its break-up through pinch-off. Jaw closure results in the capture and ingestion of part of this column. The lapping frequency can be calculated from the number of laps in the video.
Artificial cat tongue 1. Physical model of a cat lapping. The physical experiments consisted of lifting a glass disk from the surface of water. The disk was initially in contact with the free surface of a water bath and was moved upward by a motorized linear stage, FiSER (Filament Stretching Rheometer). As the disk moved upward, the liquid column was imaged from the side with a high-speed digital camera (Phantom V5) at 1000 frames/s and is here shown at 15 frames/s (i.e., slowed down 67 times). In Movie S2, The physical parameters were R =12.7 mm, H = 30 mm, UMAX = 50 cm/s, where R is the radius of the disk, H is the maximum height of travel, and UMAX is the maximum speed attained. In Movie S3, the physical parameters closely match those of the domestic cat: R = 5 mm, H = 30 mm, and UMAX = 74 cm/s.
Artificial cat tongue 2. (see description just above).
Reis, P. M., S. Jung, J. Aristoff, and R. Stocker, 2010. How cats lap: water uptake by Felis catus. Science 330:1231-1234.