One night, Roman Stocker sat at home and watched his cat, Cutta Cutta, lap milk from a bowl.
The MIT engineering professor, who studies the locomotion of microorganisms, began to wonder: How, exactly, did the milk travel from Cutta Cutta's bowl to his mouth?
The answer, based on extensive research published online Thursday in the journal Science, came as a surprise.
Unlike dogs — who use their tongues like ladles, scooping water into their mouths in a characteristically straightforward manner — cats apply an instinctive understanding of fluid mechanics to take the biggest sips.
"Cats know just when to close their jaw to get the most water," said Pedro Reis, a fluid mechanics expert at MIT who collaborated with Stocker. In fact, he said it's as if "they're doing the equations in their heads."
The unusual experiments involved four MIT scientists, 10 domestic cats, one "robotic cat" forged from a prototype originally built for the International Space Station, four zoo animals and six videos uploaded to YouTube by people who filmed large cats in zoos, on safaris and elsewhere. The research took 3 1/2 years to complete.
Stanford marine biologist Mark Denny praised the work, although he recognized that it lacks much immediate practical relevance.
"There's still a part of science that is just plain fun," said Denny, who wasn't involved in the research. "Everyone gives lip service to the scientific method," he added, but "this is a great example of it."
To figure out the feline fluid dynamics, the MIT team first made "detailed observations in real cats," Reis said. They filmed Cutta Cutta with a high-speed video camera, spending "many failed evenings" at Stocker's house in Cambridge, Mass., waiting for the cat to drink. They also filmed nine cats at a nearby shelter run by the Massachusetts Society for the Prevention of Cruelty to Animals.
Watching the videos, the team saw that the drinking cats' tongues extended downward toward their bowls in a "J" shape. At first, the scientists thought the animals might use their tongues the way dogs do, Reis said.
In fact, only the very tips of the cats' tongues touched the water. Their tongues then moved upward at the blazing-fast speed of more than three feet per second, generating a column of liquid. The cats quickly closed their mouths to drink a portion of those columns, the study reported.
The team theorized that the cats maximized the amount of water they consumed in each sip by lapping at a precise speed that balanced two forces: the inertia pulling water upward and the gravity pulling it back down.
They tested their hypothesis with their robotic cat, which dipped a glass disk — a proxy for the cats' tongues — in and out of water at rapid speeds.
They also tested the theory at local zoos. The fluid dynamics equations suggested that a larger cat with a larger tongue would lap less frequently to achieve the needed balance between inertia and gravity. Sure enough, the animals they filmed at the zoos — a lion, a tiger, a jaguar and an ocelot — drank at the slower rates the researchers had predicted.
So did the large felines in the YouTube videos: a cheetah, a bobcat, two lions, a tiger and a leopard.
The research marked the first time YouTube videos were mined for data in a paper published in Science, Reis said.
He suggested that the results could help scientists design "soft robots" — which are less rigid than traditional models — or figure out new ways to manipulate liquids in the laboratory. But mostly, he said, the results were valuable because they were interesting.
"Not all studies need to have a clear application," Reis said.