Who knew that a cat’s drinking strategy, when studied by Virginia Tech, the Massachusetts Institute of Technology (MIT), and Princeton engineers, would show a way to defeat gravity, and pull liquid into the feline’s mouth. 

The subtle biological trait has implications in bioengineering, and is featured in an article "How cats lap: Water uptake by Felis catus" in the Nov. 11 publication of Science released today.

The “lapping of domestic cats is governed by the competition between inertia and gravity,” wrote Sunghwan “Sunny” Jung, assistant professor of engineering science and mechanics at Virginia Tech, and his co-authors Pedro Reis and Roman Stocker, engineering faculty at the Massachusetts Institute of Technology, and Jeffrey M. Aristoff of mechanical and aerospace engineering at Princeton University. 

Their paper describes how they used high-speed photography to reveal the cat’s unique lapping ability. A cat uses the tongue to drink, and the photography illustrated that the animal laps the fluid by a subtle mechanism based on water adhesion to the dorsal side of the tongue.

Thus, a cat is able to exploit fluid inertia to defeat gravity and pull liquid into its mouth. “An interesting implication of this competition between inertia and gravity is that this sets the lapping frequency that depends on the animal’s mass,” they wrote.   

“There are significant implications of this for the development of novel microfluidic devices,” said Ishwar Puri, professor and head of the engineering science and mechanics department at Virginia Tech. Microfluidics is the behavior of fluids at the microscale level. A relatively new technology, it has already shown promise in revolutionizing certain procedures in molecular biology and in proteomics, among other fields.

Microfluidic devices are structures that carry a fluid, and applications for this delivery method in the medical field are prolific.

In their Science paper, Jung and his colleagues said that “casual observation hardly captures the elegance and simplicity” of the domestic cat’s lapping of milk or water. “The tongue’s motion is too fast to be resolved by the naked eye.”

Their high-speed imaging captured both the tongue and the fluid during the lapping motion. While the cat’s face was oriented downwards, its tongue extends from the jaw, and the tip curls sharply caudally. The lowest point of the tongue only rests on the liquid’s surface, and when the tongue is lifted back into the mouth, liquid adheres to the upper side of the tip, is drawn upwards, and forms a column as it enters the body. It is captured upon the closing of the jaw.

The process is unlike a dog’s drinking mechanism. A canine scoops water into the mouth by penetrating the liquid surface. A cat’s tongue never dips into the liquid, Jung explained. Jung has plans to collaborate with Jake Socha, also of engineering science and mechanics, and Pavlos Vlachos of mechanical engineering, both at Virginia Tech on additional investigations into the lapping traits of dogs.

Jung and his colleagues differentiate cats from other vertebrates for their experimental purposes saying that cats are among the various species that have “incomplete cheeks.” Most carnivores have this characteristic, and are “unable after weaning to seal their mouth cavity to generate suction, and must rely on the tongue to move water into the mouth,” Jung, who directs the Biologically Inspired Fluid Laboratory, said. 

By contrast, vertebrates with complete cheeks, such as pigs, sheep, and horses, employ suction to draw liquid upwards and use their tongues to transport the fluid into their bodies, similar to how a person uses a straw to drink water.

As engineers, the four collaborators used their movies to “quantify the lapping kinematics.” They further described “how the tongue accelerates as it leaves the water’s surface, attaining a remarkable maximum speed, then decelerates as it enters the mouth.”

The fluid dynamics of lapping, as the engineers wrote, “is governed by inertia and gravity, whereas viscous and capillary forces are negligible. Inertial entrainment draws liquid upwards into a column, while gravity acts to collapse it. Ultimately gravity prevails and the column pinches off.”

In layman’s terms, this means that there was a competition going on between the inertia and gravity, and the latter won.

“The lapping of felis catus is part of a wider class of problems in biology involving gravity and inertia, sometimes referred to as Froude number mechanisms,” the researchers said. The Froude number, the ratio of a characteristic velocity to a gravitational wave velocity, sets the maximum practical swimming speed in ducks and controls the walking speed of legged animals.

The engineers suggested that the complex movement of the tongue is “remarkable” given its lack of skeletal support, and should “continue to inspire the design of soft robots,” as well as lead to new engineering design concepts that mimic biology.