Like a lot of other people, I assumed that when dogs and cats lap water out of a bowl they scoop the water up with their tongues. Some recent research in the field of fluid dynamics that makes use of high-speed photography and some neatly designed physical modeling has shown that isn’t what’s happening at all. Dogs and cats have to solve an interesting problem when they drink and they solve this problem in different but related ways.
First, the problem. Like most vertebrates (humans being an exception) cats and dogs usually have to drink from water that lies in a horizontal plane that is lower than their head which means they have to find a way to defeat gravity and raise the water up to their mouth. Like most carnivores, cats and dogs have what are called incomplete cheeks which allow them to open their mouths really wide to get a grip on prey. Incomplete cheeks pose a problem for drinking, however, because they do not allow the animal to close their mouth fully enough to create suction. In other words, cats and dogs can’t suck the water up from the bowl into their mouth. Humans have complete cheeks and are able to create suction when closing the mouth. People suck, their pets don’t.
If dogs and cats solved this problem by scooping the water up into their mouths, you would expect them to curl their tongue upward (like people who can curl their tongues do) to make the scoop. But that’s not what they do. When they drink both cats and dogs curl their tongue back and up so that it forms something like a “J” shape which you can see in the picture of the dog’s tongue on the right.
The hook in the “J” looks something like a ladle which suggests that cats and dogs bring the water into their mouths in the cup of the ladle. There may be a small amount of water brought into the mouth this way (for dogs, not so much for cats as we will see) but most of the water is brought into the mouth by a different mechanism. After their tongue is in contact with the water both dogs and cats pull their tongue back into their mouth at a very high speed. The small volume of water that is in contact with the tongue is drawn into the mouth and the surface tension of the water causes a much larger column of water to rise up into the mouth following the tongue. This rising jet of water is caught in a battle between inertia from the upward motion caused by the speed of the tongue and gravity that pulls the water back down into the bowl. At the moment when gravity overcomes inertia and the column of water begins to fall, the animal closes its mouth and swallows the water.
Dogs and cats both make use of this basic method to get water into their mouths. They differ, however, in how they bring their tongues into contact with the water. Dogs form a deeper J-shape when they curl their tongue and splash their tongue into the water. The deep J brings more of the tongue into contact with the water which results in a larger column of water rising under the tongue into the mouth. You can see this process in action in this slo-mo video.
The splash when the tongue enters the water combined with the large quantity of water brought into the dog’s mouth (which can be more than the dog’s incomplete cheeks can contain) explains why dogs tend to be sloppy drinkers. Also, the volume of water raised by the fast tongue pull is exponentially related to the area of the tongue that is brought into contact with the water which is why large dogs tend to be sloppier drinkers than small dogs.
Cats also produce a rising column of water under the tongue when they drink but they don’t splash their tongue into the water like dogs do. Cats form a shallow J-shaped curl and bring the bottom of the curled portion of the tongue lightly and gently into contact with the surface of the water. Then, like dogs, they pull the tongue back into their mouth very quickly and close their mouth over the column of water just as gravity overcomes the inertia from the fast pull. You can just see the shallow J-shaped curve at the tip of the cat’s tongue in the photo above.
Here’s a slo-mo video of a cat drinking milk. Notice how much smaller the column of fluid is compared to the dog. The cat’s cheeks are better able to handle this smaller volume of water which is why cats tend to be neat or dainty drinkers.