How cats drink: A lesson in fluid mechanics
In the annals of animals who have contributed to science, there are Laika the Soviet space dog; Koko, the gorilla who is said to use sign language; and Lancelot, the blind dog who regained some vision after gene therapy at the University of Pennsylvania.
And now, Cutta Cutta, the cat?
The feline did not exactly provide a cure for cancer, but he has made something of a ripple in the field of fluid mechanics with a mundane body part: his tongue.
Scientists used high-speed cameras to study how Cutta Cutta and other cats lap up water, and the answer, reported online Thursday in the journal Science, was a surprise:
A cat's tongue scarcely brushes the surface - not scooping up liquid like a dog, but pulling it up from above, in a manner that seems to defy gravity.
More explanation later, but briefly: With each darting flick of the tongue, the animal relies on the property of adhesion and Bernoulli's principle - the same phenomenon involved in making baseballs curve and airplanes fly. Lions and other larger cats do it, too, though their larger tongues lead them to do it fewer times per second, at a speed the researchers were able to predict with a mathematical formula.
The study's authors, who hail from the Massachusetts Institute of Technology, Virginia Tech, and Princeton University, pursued the topic without funding, largely out of intellectual curiosity. Yet in their paper, they wrote that the findings could lead to research in soft robotics - a growing field that derives inspiration from other flexible structures in the animal kingdom, such as elephant trunks and octopus arms.
Still, they are prepared for bemused reactions from those who may not immediately perceive the beauty in commonplace things.
Just ask Princeton's Jeffrey M. Aristoff, one of the authors. At a physics conference later this month, he will be presenting research on another everyday phenomenon, the aerodynamics of jumping rope.
Frame by frame
"It's something you see every day," Aristoff said of things such as the cat's tongue or the jump rope, "but you never really think to ask: 'How does it work? What is it useful for?' "
The cat study began when Roman Stocker, Cutta Cutta's owner and an associate professor of civil and environmental engineering at MIT, puzzled over how the feline was getting milk or water into his mouth. The tongue moved too fast for Stocker to see clearly, so he enlisted colleagues to help him capture it on film, then advanced the pictures frame by frame.
The images revealed that Cutta Cutta's tongue curled underneath until the top of it was facing downward, toward the liquid, in a sort of J shape. When the tongue was retracted, a column of water rose up with it for just a split second.
The animal's mouth snapped shut at the exact moment the column was at its highest, and the process began anew, the whole thing taking place four times a second.
Here's why it works: A certain amount of liquid adheres to the tongue just as if you dipped your finger into some water. But because the cat's tongue is raised so quickly, its velocity creates a region of lower pressure that, because of Bernoulli's principle, sucks more water from below, Stocker said. The higher a fluid's speed, the lower its pressure.
The same thing occurs with a shower curtain, Aristoff said. Warm air inside the shower rises, causing the air pressure to drop relative to the air pressure outside the curtain. The higher pressure on the outside pushes the curtain in.
With a lapping cat, at a certain point the rising column becomes big enough that its inertia is overcome by gravity. But at just that moment, the cat snaps its mouth shut.
"We find that it is a very elegant, refined mechanism," Stocker said. "Cats seem to be very clever about their fluid mechanics."
Though the researchers came from the fields of physics, mathematics, and engineering, they have managed to challenge an assumption from the field of biology.
The conventional wisdom was that cats used their tongues more like a scoop or ladle, like dogs, said John Lewis, assistant professor of dentistry and oral surgery at the Matthew J. Ryan Veterinary Hospital at the University of Pennsylvania.
"It's one of those gee-whiz articles, but in some ways it may have some clinical applications as well," he said, such as when a vet treats a cat with an injured tongue.
Stocker, who used to live in Australia, named his pet after that country's Cutta Cutta caves, known for their dramatic limestone formations. The name means "many stars."
The team studied the lapping frequency of larger cats from YouTube videos and by filming them at two zoos operated by Zoo New England: Boston's Franklin Park Zoo and the Stone Zoo in Stoneham, Mass.
As the scientists predicted, animals with bigger tongues created a larger column when dipping into the water, and thus dipped them into the fluid less frequently. Tigers and lions lap about twice a second compared with the house cat's four times, Stocker said.
Just why felines use this odd mechanism remains unclear.
It's just speculation, but the authors note that a cat's whiskers are delicate sensory tools, which might not work as well when wet. The whiskers of a lapping cat stay dry, said Princeton's Aristoff.
"If they were to drink like a dog," he said, "they'd splash water everywhere."
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