Dogs and Fluid Dynamics

If you have a dog, or have simply watched one drink from a bowl, stream or lake, you know how messy that process is.

Dogs quickly plunge their tongue into the water and then pull it back up into their mouth, repeatedly, slurping some water with each flick before eventually swallowing. Depending on the dog, if they’re inside your house drinking from a bowl, it often seems like half of the water ends up on the floor.

I can tell which of my two dogs is drinking from their water bowl in another room by the sound and speed of their lapping. It’s quite individual. And seemingly inefficient, although Finn’s fairly neat compared to all of the water Conall “spills” onto the floor.

Early yesterday morning my dogs and I were running trails in the forest. Toward the end of our run, we stopped where the trail crosses a small stream so the boys could drink and cool their toes. The sun had risen high enough that a small rapid in the water’s flow was bright with sunlight. It caught my eye.

While the boys waded and drank, I decided to see what slow motion video of that small rapid might look like.

Instead, I got videobombed by Conall, and I’m so glad I did! When I got home and watched the video on my computer screen I was delighted to see, in beautiful, slow-motion detail, exactly how Conall laps water. Curling the tip of his tongue backward as it enters the water, he opens his jaws wide and pulls his tongue – along with a small column of water – up toward his mouth, although not all of the water reaches its intended destination.

Because it all happens so fast in real time, I had never noticed just how wide Conall’s jaw opens to accept the water following his tongue into his mouth.

Conall also sometimes bites the water, something one of my earlier Malamutes did. I have no idea if that method is as effective as the curled tongue – I doubt it – but it must work on some level, forcing some volume of water to splash up into his mouth. I’ve never seen Finn bite water as a drinking method, so maybe it’s just a Malamute thing. (Toward the end of the video, Finn is seen searching for grass to eat; his stomach was upset that morning. Eating grass is a natural way for dogs to settle their stomachs. Or make them throw up any bile in their stomach, which has the same end result of helping them feeling better.)

The physics of how dogs managed to drink wasn’t understood until recently. Dogs (and cats) don’t have the cheek muscles that allow them to suck, like elephants and humans, so how did they manage to get enough water into their mouths using just their flat, smooth tongues?

A few years ago, some scientists set out to answer that question, using…slow motion video! They used 19 volunteer dogs of various breeds and sizes and attached two Go-Pro video cameras to their clear water bowls – one on the side, one underneath. In short, the experiment revealed that the back curl of the tongue, and the speed of lapping, created a column of water that followed the tongue from the water’s surface up into the dog’s mouth.

The results of this study help to explain the qualitative observations of many dog owners: Dogs tend to be messy drinkers and splash water on themselves and the floor. This phenomenon may be a by-product of their lapping mechanism. The large effective area of a dog’s tongue that impacts the liquid surface, the penetration of the tongue below the liquid surface, and the high acceleration of the tongue as it is raised out of the liquid bath all contribute to increasing the volume of fluid extracted per lap. However, not all of the fluid displaced by the curled tongue enters the vertical column, and some is splashed laterally. In addition, when the tongue is accelerated upward, the water in the ladle is generally tossed to either side of the dog’s mouth. Although dogs do not use their tongue to actively scoop water into their mouth, it is possible that the scooped liquid has some positive effect on the water column dynamics below the tongue. In particular, the scooped liquid could spill around the tongue and feed into the water column below, extending the formation time and increasing the volume of the column. This hypothesized role of scooping in the fluid column dynamics should be investigated further.

Sean Gart, John J. Socha, Pavlos P. Vlachos, and Sunghwan Jung, Dogs lap using acceleration-driven open pumping; Proceedings of the National Academy of Sciences (PNAS), December 29, 2015 112 (52) 15798-15802.

Cats use a different, less messy method to drink, but it also involves physics and fluid dynamics.

The same group of researches noted that, unlike dogs, cats flick their tongues onto the water. Just the top part of their tongue touches the surface, never breaking the surface tension. Cats pull water from the surface into their mouths at a rate of four times per second, preventing gravity from pulling the water back into the bowl.

Now you know! Blame fluid dynamics for the watery mess on your floors, not your dog, who is, after all, doing the best s/he can with the design they were born with.

Feature photo: Finn and Conall lapping water from a different stream during a run on the same mountain two days earlier.