Cat's tongue a lapping wonder

We always knew about the cat's whiskers, but U.S. engineers have discovered it is really the cat's tongue we should be raving about.

Cats have a unique lapping technique a cat's tongue moves as fast as one metre per second while drinking, the researchers reported in Friday's Science journal.

And while it might be an interesting addition to feline facts, the researchers also believe this insight may benefit the design of soft robotics to handle liquids.

Co-author Dr. Roman Stocker, of the department of civil and environmental engineering, at the Massachusetts Institute of Technology, says the inspiration for the work came from his cat, Cutta Cutta, named after Cutta Cutta National Park in the Northern Territory, which he visited while working in Perth from 2000-02.

"Three years ago, I was watching Cutta Cutta lap during breakfast and realized there was an interesting biomechanics problem behind this simple action," he says.

"Initial high-speed videos revealed the elegance of the lapping action, which the naked eye misses entirely."

Vertebrates are divided into two classes of animals, says Stocker. There are those with complete cheeks, for example, humans, sheep, horses and pigs, and those with incomplete cheeks, for example most carnivores.

He says animals with incomplete cheeks have evolved to open their jaws extensively to catch prey, but cannot generate suction with their mouths, so they have to use different strategies to drink.

Dogs drink by extending their tongue out from the mouth, morphing it into a cup. As their tongue enters the water, the 'cup' fills with liquid and they then bring it to the mouth.

"In our study we show that cats drink using a very different mechanism that exploits a detailed balance between gravity and inertia," says Stocker.

Elegant action

The videos show that, unlike their canine counterpart, cats do not scoop up the liquid.

Rather they bend the upper side of their tongue under so that the top surface lightly touches the liquid.

The cat then raises its tongue rapidly, drawing the water up in a liquid column and closes its jaw to capture the fluid before gravity breaks up this column.

The researchers are unsure why this difference has evolved, but suggest it may be connected with how "messy" the dog's action is in terms of water splashes and spray.

"We can speculate it may be related to keeping their whiskers dry, since these play a very important sensory function," says Stocker.

This elegant action is often missed by cat owners, however, as it is done too fast for the human eye to see the cat laps four times a second and its tongue moves at a speed of one metre per second.

Itseems the cat has a good grasp of physics, says co-author Dr Jeffrey Aristoff, of Princeton University.

The researchers predicted and came up with a formula that showed cat size would affect lapping speed, with larger cats lapping more slowly.

They then tested the theory by video recording eight species of large cat at the New England Zoo and found the felines were well ahead of them.

"Cats appear to take advantage of the physics [hydrodynamics] of lapping by adjusting their lapping frequency to its optimum value. The optimum frequency depends on the size of the feline's tongue: the larger the feline, the slower it should lap.

"Large cats lap more slowly because they have larger tongues."

Robotic design prospects

Aristoff says the work could help in the design of soft robots that interact with liquid surfaces.

He says complex movement by the tongue in the absence of a skeletal structure, as with the elephant's trunk and octopus arms, is inspirational because it shows a soft robot without a rigid structure could perform many useful tasks.

"A fundamental understanding of their functionality can lead to new design concepts and is essential to inform biomechanical models," the researchers write.

"In particular, a soft robot could respond to its environment by changing its shape. For example, it could squeeze through narrow gaps (like an octopus does), or be transported in a small pipe and then expand once it reaches the opposite end," says Aristoff.

"With the recent invention of new materials that mimic soft biological structures, the possibilities are endless."