Pedro M. Reis, the lead author of the study, was watching his cat (Cutta Cutta) drink one day. He knew that like dogs, cats cannot use their cheeks to suck in liquid. Thus, they must pull liquid into their mouths using their tongues. As he watched his cat, he wondered what physical mechanisms were at work. He thought surely someone had studied how cats drink water before, but the best thing he could find was a 1940 film called Quicker ‘n a Wink, which featured MIT professor Harold “Doc” Edgerton.1 While it had some nice high-speed photography of a cat drinking, it didn’t really explore what was going on in terms of the physics involved. As a result, his team decided they would find out for themselves.
Using high-speed imaging, Reis and his colleagues studied Cutta Cutta’s drinking habits in a very detailed way. They found that while a dog forms a cup-like shape with its tongue and simply scoops water into its mouth, a cat’s way of drinking is significantly more elegant. As they say in their article:
Almost everyone has observed a domestic cat lap milk or water. Yet casual observation hardly captures the elegance and complexity of this act, as the tongue’s motion is too fast to be resolved by the naked eye.2
Essentially, the cat quickly sticks out its tongue, forming it into the shape of a “J.” The tip of the tongue (which is curled under) simply touches the water, and then the cat pulls its tongue quickly back into its mouth. Because water molecules are attracted to the tongue, some water comes back up with the tongue. But because water molecules are also strongly attracted to one another, an entire column of water (or milk) follows. Just before gravity can overcome the liquid’s motion and destroy the column, the cat catches the column in its mouth.
In order to fully test how this elegant mechanism works, the team actually built an “artificial lapper,” in which a disk touches the surface of the water and is then rapidly pulled upwards, dragging a column of water with it. One interesting result from this phase of the study was that a smooth disk worked better than a rough one. Niavely, I would think that the roughness of a cat’s tongue would help in this process, but a smooth disk made the most efficient artificial lapper.
Well…guess what? While most of a cat’s tongue is incredibly rough, there is one part that is quite smooth…the tip of the tongue that touches the water during the lapping process! In other words, the cat’s tongue is perfectly designed to exploit the fluid dynamics of water and other aqueous mixtures, such as milk. Of course, that’s not surprising, since the original cats were designed by the One who determined the rules of fluid dynamics to begin with!
Interestingly enough, when the researchers examined videos of other species of cats (like lions and tigers) drinking, they found that each species has its own pace at which it laps, and that pace is set to maximize the amount of water that the cat can drink in a given amount of time.
So why bother to study how a cat laps liquids? As the authors state near the end of their paper:
The subtle use of the tongue in the drinking process of F. catus is remarkable, given the tongue’s lack of skeletal support. Complex movement in the absence of rigid components is a common feature of muscular hydrostats, which in addition to tongues include elephant trunks and octopus arms. The functional diversity and high compliance of these structures continue to inspire the design of soft robots, and a fundamental understanding of their functionality can lead to new design concepts… (emphasis mine)
In other words, when we learn how the Ultimate Engineer designed things, we can make better designs ourselves.
PLEASE NOTE: I have written an update to this article, which grudgingly admits that cats aren’t more elegant than dogs…at least when it comes to drinking.
1. Susan Milius, “Cats lap liquids with a flick of the tongue and fluid dynamics,” Science News 178(12):5-6, 2010.
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2. Pedro M. Reis, Sunghwan Jung, Jeffrey M. Aristoff, and Roman Stocker, “How Cats Lap: Water Uptake by Felis catus,” Science 330:1231-1234, 2010.
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