Why dogs are such messy drinkers: Canines 'slap' their tongues on water to create liquid columns because they can't suck


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If you have ever watched a dog drink water, you will know that it can be a splashy and messy affair.

But until now scientists have not been exactly sure why, assuming that canines use their tongues like an inefficient ladle.

High-speed cameras have now revealed that dogs 'smash' their tongues against the water to create water columns, which feed into their mouths.

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High-speed cameras have revealed that dogs 'smash' their tongues against the water to create water columns, which feed into their mouths (pictured)

High-speed cameras have revealed that dogs 'smash' their tongues against the water to create water columns, which feed into their mouths (pictured)

Scientists at Virginia Tech and Purdue University modelled the fluid dynamics at play when dogs of different sizes drink water.

Dogs and cats have large jaws and 'incomplete cheeks' typical of a predatory quadruped, allowing to open their mouths wide to deliver killer blows.

But what makes hunting possible also makes drinking using suction impossible.

Unable to seal their cheeks completely, there is no way for a dog to suck up water, unlike humans, who have 'complete' cheeks, which allow us to create negative pressure and suck water into their mouths.

HOW DO DOGS DRINK? 

Dogs can't such liquid due to their 'incomplete cheeks'.

They smash their tongues on the water's surface to create splashes.

When a dog withdraws its tongue from water, it creates a significant amount of acceleration - roughly five times that of gravity - that creates the water columns that feed up into its mouth.

The columns of water 'pinch off' and detach from the water bath primarily due to gravity.

Dogs are smart enough to close their mouths just before the water column collapses back to the bath, allowing them to drink.

While cats lack suction too, they drink using a two part 'water entry and exit' process by placing the tip of their tongue on the water's surface and rapidly withdrawing it to generate a column of water under their retracting tongue.

Sunny Jung, an assistant professor at Virginia Tech, said: 'When we started this project, 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.'

When a dog withdraws its tongue from water, it creates a significant amount of acceleration - roughly five times that of gravity - that creates the water columns that feed up into its mouth.

To model this, Professor Jung placed cameras under the surface of a water trough to map the total surface area of the dogs' tongues that splashed down when drinking.

The researchers found that heavier dogs drink water with the larger wetted area of the tongue. 

When dogs withdraw their tongue from water, they create a significant amount of acceleration - roughly five times that of gravity - that creates the water columns that feed up into their mouths. Canines are smart enough to close their mouths just before the water column collapses back again

When dogs withdraw their tongue from water, they create a significant amount of acceleration - roughly five times that of gravity - that creates the water columns that feed up into their mouths. Canines are smart enough to close their mouths just before the water column collapses back again

This finding suggests that there's a relationship between the water contact area of a dog's tongue and its body weight, so the volume of water a dog's tongue can move increases relative to its body size.

In order to better understand how the physiology works, Professor Jung and his colleagues made models of different sized dogs' tongues and mouths using glass tubes.

This allowed them to mimic the acceleration and water column formation created when a dog splashes its tongue one water's surface. They then measured the volume of water withdrawn.

The scientists found that the column of water 'pinches off' and detaches from the water bath primarily due to gravity.

This means that dogs are smart enough to close their mouths just before the water column collapses back to the bath - just like cats.

They will today describe the findings at the American Physical Society's Division of Fluid Dynamics (DFD) meeting in San Francisco, California.

Dogs and cats have large jaws and 'incomplete cheeks' typical of a predatory quadruped, allowing to open their mouths wide to deliver killer blows. But cats drinks differently to dogs and are more delicate

Dogs and cats have large jaws and 'incomplete cheeks' typical of a predatory quadruped, allowing to open their mouths wide to deliver killer blows. But cats drinks differently to dogs and are more delicate

CATS DRINK MORE DELICATELY THAN DOGS - BUT HOW? 

In 1940, Harold Edgerton discovered that when cats lap, they extend their tongues straight down toward the bowl with the tip of the tongue curled backwards, so that the top of the tongue touches the liquid first.

Three years ago, a team from MIT, Virginia Tech and Princeton built upon his study by using high speed cameras to discover that the top of the cat's tongue is the only surface to touch liquid.

Cats, unlike dogs, don't dip their tongues into the liquid, so the cat's lapping mechanism is far more subtle and elegant.

The smooth tip of the tongue barely touches the surface of the liquid before the cat draws its tongue back up. As it does so, a column of liquid forms between the moving tongue and the liquid's surface.

The cat then closes its mouth, pinching off the top of the column for a nice drink, while keeping its chin dry. This part of the process is similar to how a dog drinks.

Three years ago, a team from MIT, Virginia Tech and Princeton built upon his study by using high speed cameras to discover that the top of the cat's tongue is the only surface to touch liquid. A video referenced in the study is shown

When the cat's tongue touches the liquid surface, some of the liquid sticks to it through liquid adhesion, much as water adheres to a human palm when it touches the surface of a pool.

But the cat draws its tongue back up so rapidly that for a fraction of a second, inertia - the tendency of the moving liquid to continue following the tongue - overcomes gravity, which is pulling the liquid back down toward the bowl.

The cat instinctively knows just when this delicate balance will change and it closes its mouth in the instant before gravity overtakes inertia.

If the cat hesitates, the column will break, so that the liquid would fall back into the bowl and the tongue would come up empty.

While the domestic cat averages about four laps per second, big cats, such as tigers, know to slow down. Because their tongues are larger, they lap more slowly to achieve the same balance of gravity and inertia.



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