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Drinking dogs are just as dainty as cats

Posted on behalf of Marian Turner.

Cats and dogs are different, right? Dogs nag you to go for a walk, cats sit curled in the sun. Dogs run in circles in excitement when you come home, cats look up disdainfully from the windowsill. Dogs slurp up their water, splashing everywhere, cats daintily lap. Wrong, according to the authors of a study published in Royal Society Biology Letters today.

Alfred Crompton and Catherine Musinsky from the Museum of Comparative Zoology at Harvard University filmed a dog lapping milk from a bowl and found that the way dogs transport liquids is actually very similar to cats.

Crompton and Musinsky decided to look at dog lapping after a paper was published in Science last year which analysed the technique used by cats (which we covered here). In that study, Roman Stocker and his engineering team at the Massachusetts Institute of Technology showed that a cat’s tongue touches just the surface of a liquid, and the liquid sticks to the upper side of the tongue. The cat rapidly pulls its tongue back in, so that a column of liquid forms. When the cat then closes its mouth, the top part of this column gets trapped inside. And the cat gets the cream.

Stocker’s team watched a Discovery Channel video of dogs lapping and came to the conclusion that dogs do it differently. It looked like the dogs dip their tongues deeper into the water and curl their tongue to make a scoop in the under side of their tongues.

This is indeed how it looked when Crompton and Musinsky filmed Crompton’s dog, Mathilda, using normal film:

Dog lapping broth, four cycles from AW Crompton on Vimeo.

But Crompton and Musinsky wondered how the dogs got the liquid from the underside of the tongue to the back of their mouths to swallow it. They filmed Mathilda lapping milk with barium in it, so they could take X-ray videos. They did see the tongue’s underside making a scoop and picking up the milk, but the milk fell out of the scoop as the dog withdrew her tongue. During this process, though, the milk formed a column which adhered to the upper side of Mathilda’s tongue, just like in cats. And also just like in cats, it was the liquid at the top of the column that stayed inside the dog’s mouth when she closed her jaw. So the formation of a liquid column adhering to the tongue is the key mechanism for moving liquid in both cats and dogs, say the authors.

X-ray video of lapping in a dog from AW Crompton on Vimeo.

And while dogs might look messy when they’re drinking from a bowl, their technique is actually highly dextrous, says Crompton. Mathilda used three lap cycles to move liquid from the bowl to the back of her mouth. Crompton and Musinsky’s video shows how the dog traps the liquid between the roof of her mouth and the upper side of her tongue to stop liquid being lost while the protruding end of the tongue goes back down for more.

Pedro Reis from MIT, lead author on the cat paper, says “it’s great to see these more sophisticated videos showing that dogs actually use the same mechanism.”

And the moral of the story is: not all is as it seems on the Discovery Channel.


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