Amazing Physics At Work When Cats Drink
Scientists at MIT, Virginia Tech and Princeton University studied the way domestic and wild cats drink and found an exquisite balance of two forces at work.
The way cats drink was first described in 1940, whenMIT engineer Doc Edgerton used strobe lights to make a stop-action film of a domestic cat lapping milk. This footage showed that cats extend their tongues straight down toward the bowl with the tip of the tongue curled like a capital "J" to form a ladle, so that the top surface of the tongue actually touches the liquid first.
But Edgerton's film had only captured part of the beauty of the cat's drinking mechanism.
Scientists had assumed that cats, like dogs, use their tongues like a ladle to scoop up whatever liquid they're drinking. However, the high-speed videos made by the research team showed the truth:Cats don't dip their tongues into the liquid and scoop it up after all.
Instead, when a cat laps, the tip of its tongue barely brushes the surface and then retracts. As it does, a column of milk forms between the moving tongue and the liquid's surface. Then the cat closes its mouth and pinches off the top of the column, keeping its chin dry while enjoying a drink.
How did this study even begin? MIT professor Roman Stocker, a fluid mechanics expert, was watching his cat, Cutta Cutta, drinking at breakfast time. He became fascinated by the motion and decided to find out more.
Stocker assembled a team including engineers, physicists and mathematicians to investigate the mystery of how cat lapping works. Unlike most scientific research, this study did not receive outside funding and did not involve the work of graduate assistants.
The researchers analyzed video footage of domestic cats and wild cats and found that they all drank in the same way: the cat's tongue is extended, it curls under, and the top surface of the tip of the tongue makes contact with the liquid but doesn't break the surface of the fluid. Some liquid then sticks to the tongue and rises in a column as the cat pulls its tongue back.
The team then built a robotic model of a cat's tongue that could move up and down over a bowl of water, allowing them to explore the mathematical and physical mechanisms underpinning lapping.
The liquid column, it turns out, is created by a delicate balance between gravity, which pulls the liquid back to the bowl, and inertia, which in physics refers to the tendency of the liquid or any matter to continue moving in a direction unless another force interferes.
The cat instinctively knows just how quickly to lap in order to balance these two forces, and just when to close its mouth. If it waited even another tiny fraction of a second, the force of gravity will overtake inertia, causing the column to break, the liquid to fall back into the bowl, and the cat's tongue to come up empty.
The scientists' video revealed that domestic cats make about four laps per second, and each of those laps brings up 0.1 milliliters of liquid into the mouth. Bigger cats can draw up bigger columns of liquid and therefore can lap more slowly.
"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. Our research suggests that the cat chooses the speed in order to maximize the amount of liquid ingested per lap," said Jeffrey Aristoff, a co-author on the study. "Cats are smarter than people think, at least when it comes to hydrodynamics."
Their article was published in the most recent online edition of Science.