New high-speed videos show us what we already knew: that dogs make a giant mess at the water bowl.

But the clips above and below, filmed in X-ray and visible light, challenge assertions that canines drink by scooping up fluid with a backwards-curled tongue. Instead, dogs pull up a column of liquid and chomp it — just like cats.

“It only looks like dogs scoop with back of their tongue. They drink the same way as cats, just sloppier,” said evolutionary biologist Alfred Crompton of Harvard University.

Crompton leads a study of dog drinking published May 25 in the Journal of the Royal Society Biology Letters. The new work follows research on cat-lapping mechanics published last fall by a group at MIT. In that study, researchers deconstructed how cats drink, and suggested dogs drink differently.

“We didn’t use X-ray video like Crompton. When we saw their clip, we were like, ‘Wow, it is the same!’” said physicist and mechanical engineer Pedro Reis of MIT, a co-author of last year’s cat research.

Cats and dogs last shared a common ancestor some 43 million years ago, but neither evolved thick cheeks now present in many other animals, including humans. Such cheeks form a tight seal that both retains liquid and allows suction-powered drinking. Without them, cats and dogs needed to develop a different way to drink.

Reis’ MIT group used high-speed videos to show a cat begins to drink by flattening its tongue and barely touching the surface of a liquid. As the tongue pulls back, liquid sticks to its underside, and a column is pulled into the air. Just before the column drops, the cat closes its mouth and squeezes the drink down.

But whereas Reis originally thought dogs drank in a different fashion, Crompton suspected the truth was simply losy amidst the mess. To test the proposition, he enlisted Matilda, his 10-year-old Portuguese water dog.

When Matilda lapped beef broth in front of high-speed X-ray and visible light cameras, she demonstrated the same column-lifting mechanics of cats. The clips do show Matilda scooping up fluid by curling the back of her tongue, but it doesn’t go anywhere useful.

“It sprays everywhere and drops out of the mouth. Without cheeks, there’s no mechanism to get it on top of the tongue,” Crompton said.

Once columns of liquid land on a dog’s tongue, they’re pushed against the roof of the dog’s mouth, where ridges called rugae stop liquid from rolling out. Crompton said it takes three or four laps to get a drink into the mouth, along the tongue, and down into the throat.

Crompton’s team hopes to investigate how other animals drink. Civil engineer Roman Stocker of MIT, who with Reis led the cat-lapping study, suggested bird drinking as ripe for more research.

“Nature’s plentiful solutions to a given problem suggest that many surprises still await us when it comes to animal drinking,” Stocker said.

Videos: AW Crompton, Catherine Musinsky/Journal of the Royal Society Biology Letters

Citation: “How dogs lap: ingestion and intraoral transport in Canis familiaris.” A.W. Crompton and Catherine Musinsky. Journal of the Royal Society Biology Letters, published online May 25, 2011. DOI: 10.1098/rsbl.2011.0336

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