Study reveals the subtle dynamics underpinning how felines drink
By Denise Brehm
Civil & Environmental Engineering
Cat fanciers around the world appreciate the gravity-defying grace and exquisite balance of their feline friends. But do they know those traits extend even to the way cats lap milk?
Researchers at MIT, Virginia Tech and Princeton University analyzed the way domestic and big cats lap and found that felines of all sizes take advantage of a perfect balance between two physical forces. The results were published in the Nov. 11 online edition of the journal Science.
It was known that when cats lap, they extend their tongues straight down toward the bowl with the tip of the tongue curled backwards like a capital “J” to form a ladle, so that the top of the tongue touches the liquid first. That insight came from a 1940 film of a cat lapping milk, made by the renowned Harold “Doc” Edgerton, who first used strobe lights in photography to stop action.
But recent high-speed videos made by this team clearly reveal that the top of the cat’s tongue is the only surface to touch the liquid. Cats, unlike dogs, aren’t dipping their tongues into the liquid like ladles after all. Instead, 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 rapidly draws its tongue back up. As it does so, a column of milk 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 unusual lapping mechanism begins when the cat’s tongue touches the liquid surface and some water sticks to it through liquid adhesion, much as water adheres to a human palm when it touches the surface of a pool. But in this case, 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 of power will change, and it closes its mouth in the instant before gravity overtakes inertia. If the cat waited, the column would break, 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, the 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.
Analyzing the mechanics
In this research, Roman Stocker of CEE, Pedro Reis of CEE and the Department of Mechanical Engineering, Sunghwan Jung of Virginia Tech and Jeffrey Aristoff of Princeton analyzed high-speed digital videos of domestic cats, including Stocker’s family cat, and a range of big cats (a tiger, a lion and a jaguar), thanks to a collaboration with Zoo New England’s mammal curator John Piazza and assistant curator Pearl Yusuf. And, in what could be a first for a paper published in Science, the researchers also gathered additional data by analyzing existing YouTube.com videos of big cats lapping.
With these videos slowed way down, the researchers established the speed of the tongue’s movement and the frequency of lapping. Knowing the size and speed of the tongue, the researchers then developed a mathematical model involving the Froude number, a dimensionless number that characterizes the ratio between gravity and inertia. For cats of all sizes, that number is almost exactly one, indicating a perfect balance.
To better understand the subtle dynamics of lapping, they also created a robotic version of a cat’s tongue that moves up and down over a dish of water, enabling the researchers to systematically explore different aspects of lapping, and ultimately, to identify the mechanism underpinning it.
“The amount of liquid available for the cat to capture each time it closes its mouth depends on the size and speed of the tongue,” said Aristoff, a mathematician who studies liquid surfaces. “Our research — the experimental measurements and theoretical predictions — suggests that the cat chooses the speed in order to maximize the amount of liquid ingested per lap. This suggests that cats are smarter than many people think, at least when it comes to hydrodynamics.”
Aristoff said the team benefitted from the diverse scientific backgrounds of its members: engineering, physics and mathematics.
“This work is as splendid a case as I can recall of things looked at … but seen in a way that no one else has seen,” said Professor Steven Vogel of Duke University, a biomechanics researcher who was not involved in this project. “Now that I’ve been clued in, I can report that what these people describe and explain agrees entirely with my own casual observations of the lapping action of the feline in charge of this establishment.”
“In the beginning of the project, we weren’t fully confident that fluid mechanics played a role in cat’s drinking. But as the project went on, we were surprised and amused by the beauty of the fluid mechanics involved in this system,” said Jung, an engineer whose research focuses on soft bodies, like fish, and the fluids surrounding them.
The work began three-and-a-half years ago when Stocker, who studies the biophysics of the movements of ocean microbes, was watching his cat lap milk. That cat, eight-year-old Cutta Cutta, stars in the researchers’ best videos and still pictures. And like many movie stars (Cutta Cutta means “stars stars” in an Australian aboriginal language), he doesn’t mind making people wait. With their cameras trained on Cutta Cutta’s bowl, Stocker and Reis said they spent hours at the Stocker home waiting on Cutta Cutta … to drink, that is.
“Science allows us to look at natural processes with a different eye and to understand how things work, even if that’s figuring out how my cat laps his breakfast,” Stocker said. “It’s a job, but also a passion, and this project for me was a high point in teamwork and creativity. We did it without any funding, without any graduate students, without much of the usual apparatus that science is done with nowadays.”
“Our process in this work was typical, archetypal really, of any new scientific study of a natural phenomenon. You begin with an observation and a broad question, ‘How does the cat drink?’ and then try to answer it through careful experimentation and mathematical modeling,” said Reis, a physicist who works on the mechanics of soft solids. “To us, this study provides further confirmation of how exciting it is to explore the scientific unknown, especially when this unknown is something that’s part of our everyday experiences.”
Besides their obvious enthusiasm for the work itself, the researchers are also delighted that it builds on Edgerton’s 1940 film of the cat lapping. That film appeared as part of an MGM-released movie called “Quicker’n a Wink,” which won an Academy Award in 1941. Reis and Stocker say they’re moving on to other collaborations closer to their usual areas of research. But their feline friend Cutta Cutta might have Oscar hopes.
Editor’s note: Subsequent research by A.W. Crompton and Catherine Musinsky of Harvard University published in Biology Letters May 25, 2011, shows that dogs rely on a similar mechanism to cats when drinking.