Cats drink using lap-and-gulp trick

Imbibing a delicate interplay between inertia and gravity

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NO SCOOP The cat that inspired a new study of how felines drink starts to curl its tongue tip back to form a J shape that will touch the liquid surface and then retract. This motion allows the cat to drink without scooping up the liquid. Micaela Pilotto, Roman Stocker, Pedro Reis

Sorry, Fido. A paper in the journal Science has just ascribed “elegance and complexity” to the way cats drink.

A dog drinks by forming its tongue into a little cup that merely ladles liquid into its mouth, says coauthor Pedro Reis of MIT. “Cats are much more sophisticated in the knowledge of fluid dynamics,” he deadpans.

Instead of scooping, cats use what the researchers describe as a “subtle mechanism” in which water sticks to the tip of the tongue and is pulled up into the mouth, taking advantage of the water’s inertia. Though cats have been lapping liquids in public for millennia and early high-speed photography showed some of the basic motions of their drinking, Reis says he knows of no detailed analysis of the phenomenon preceding the one he and his colleagues published online in Science November 11.

“It’s amazing how you look at something and think, somebody must have studied that before. But as happens with many things in everyday life, that is not the case,” Reis says. “That’s one of the excitements of science.”

The gaps in lapping analysis do not surprise functional morphologist Rebecca Z. German of Johns Hopkins School of Medicine in Baltimore. “What we know about mammalian feeding is woefully incomplete,” she says. It’s hard to observe, and scientists have studied other feats such as locomotion in much more detail.

Just figuring out the basic motion of cat lapping took high-speed photography, as the project’s lapper-in-residence typically managed 3.5 sips per second. What the camera revealed is that a cat shoots its tongue down in a J shape with the tongue tip curled under to touch the liquid’s surface, explains coauthor Roman Stocker, also of MIT. The tip does not scoop into the liquid but pulls directly back into the mouth, reaching peak speeds of 78 centimeters per second.

Liquid adheres to the tip of the tongue and rises in a column as the tip retracts. Just before gravity can overcome the rising liquid’s inertia and collapse the column in a splash, the cat catches the top portion of the liquid column in its mouth.

Along with watching movies of cats drinking, researchers tested their ideas about the process with a device that tapped a disc against a liquid and pulled upward at whatever speed the researchers wanted. With this device and a lot of calculation, the researchers were able to confirm or collapse various hypotheses.

At first researchers thought that the distinctive roughness of a cat’s tongue would play a role in pulling up the liquid column, Stocker says. Wrong. Cat tongues have smooth tips, and as it turns out a smooth, wet surface works well for lapping. To get similar wetting properties in their experimental setup, the researchers used a glass disc in their test device.

The viscosity of the liquid did not make a difference in the process, at least within the range of water or milk or anything else a cat would likely lap, the researchers say. Instead the main factors were the inertia of the rising liquid and gravity.

Looking at videos of other lapping felines, including a fine selection available on YouTube, the researchers noted that frequencies changed as would be expected if cats of various sizes optimized lapping frequencies to catch a good volume of liquid before gravity claims the liquid column.

Studies like the cat analysis offer insight into basic puzzles of the evolution of mammalian feeding. “All infant mammals suckle by very similar mechanisms, yet there is tremendous variation in adult drinking mechanisms,” German says. Also understanding nature’s diverse drinking strategies may inspire help for the many people who struggle to swallow.

And then there’s the inherent cool factor, which German rates high.

FAST LAPS from Science News on Vimeo.

Demonstrating mastery of fluid dynamics, the feline companion to MIT’s Roman Stocker shows how tapping just the surface of a liquid and letting a column follow the tongue back into the mouth permits drinking without suction or scoops. Researchers shot the film at 120 frames per second and here slowed it down four times.

Credit: Roman Stocker, Sunghwan Jung, Jeffrey M. Aristoff and Pedro M. Reis

TONGUE SUBSTITUTE from Science News on Vimeo.

A glass disk that touches and retreats from liquid allows researchers to determine that cap lapping depends on the interplay of inertia and gravity. Researchers shot the film at 1,000 frames per second but slowed it down to 15/sec.

Video courtesy of Micaela Pilotto, Roman Stocker and Pedro Reis, narrated by Nadia Ramlagan of AAAS

Back Story – Stopping Time

Muybridge horse over Stubb horse paintingCredit: Wikimedia Commons;

Researchers have used high-speed photography to investigate the details of animal motion since the 1870s, when photographer Eadweard Muybridge teamed with industrialist and Stanford University founder Leland Stanford to demonstrate that galloping horses lift all four hooves off the ground. Muybridge set up a series of cameras to photograph a horse at 0.04-second intervals as it ran along a track, showing that all four hooves do in fact leave the ground simultaneously (top) — but not as many artists had imagined. A painting (bottom) by the English artist George Stubbs (1724 – 1806) depicts a horse with its legs splayed out in a configuration that would be possible only if the animal hopped along like a rabbit.
In the 20th century, photographic studies by researchers such as MIT professor Harold Edgerton revealed the mechanics of human motion and the flight of bats and birds. More recent studies have harnessed high-speed cameras and pulsing lasers to show how hummingbirds can hover over an open flower by using a few aerodynamic tricks also employed by insects.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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