A team of scientists led by Dr Sunghwan Jung of Virginia Tech has identified and modeled the fluid dynamics at play when dogs (Canis lupus familiaris) drink water.
Dogs and cats need to move fluid against gravity into the body by means other than suction; they do this by lapping fluid with their tongue. When a dog drinks, it curls its tongue posteriorly while plunging it into the fluid and then quickly withdraws its tongue back into the mouth. During this fast retraction fluid sticks to the ventral part of the curled tongue and is drawn into the mouth due to inertia. The scientists studied several variations of this drinking behavior among many dog breeds, specifically, the relationship between tongue dynamics and geometry, lapping frequency, and dog weight; and also compared the results with the physical experiment of a rounded rod impact onto a fluid surface. Image credit: Emmanuel Keller aka Tambako The Jaguar / CC BY-ND 2.0.
If you’ve ever watched a dog drinks water, you know that it can be a spilly, splashy affair.
Behind all of the happy, wet messes, however, lies the mechanical logic of carnivorous compensation – dogs splash when they drink because they have the cheeks of a predatory quadruped.
As members of the order Carnivora, dogs and cats have incomplete cheeks, which allow them to open their mouths wide to deliver killing blows. But what makes pack hunting possible also makes suction drinking impossible.
Unable to seal their cheeks completely, there is no way for a dog to suck up water. Conversely, humans have ‘complete’ cheeks, and drink by creating negative pressure, allowing them to suck water into the mouths and down the throats.
Cats, too, lack suction, and they compensate by drinking via a two-part ‘water entry-and-exit’ process. This consists of a plunging and a pulling phase, in which a cat gently places its tongue on the water’s surface and then rapidly withdraws it, creating a column of water underneath the cat’s retracting tongue.
“We thought that dogs drink similarly to cats. But it turns out that it’s different, because dogs smash their tongues on the water surface – they make lots of splashing – but a cat never does that,” Dr Jung said.
When dogs withdraw their tongue from water, they create a significant amount of acceleration – roughly 5 times that of gravity – that creates the water columns, which feed up into their mouths.
To model this, Dr Jung and his collaborators placed cameras under the surface of a water trough to map the total surface area of the dogs’ tongues that splashed down when drinking.
They found that heavier dogs drink water with the larger wetted area of the tongue. This indicates that an allometric relationship exists between water contact area of the dog’s tongue and body weight – thus the volume of water a dog’s tongue can move increases exponentially relative to their body size.
In order to better understand how the physiology works, the scientists could only go so far by watching dogs drink.
They had to have the ability to alter the parameters and see how they affected this ability, and since they could not actually alter a dog in any way, they turned to models of the dog’s tongue and mouth.
For their model, the researchers used glass tubes to simulate a dog’s tongue. This allowed them to mimic the acceleration and column formation during the exit process. They then measured the volume of water withdrawn.
They found that the column of water pinches off and detaches from the water bath primarily due to gravity.
Dogs are smart enough to close their mouth just before the water column collapses back to the bath.
The scientists reported their results November 25, 2014, at the American Physical Society’s Division of Fluid Dynamics Meeting in San Francisco, California.
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