The Wet Dog Shake: Scientists Uncover Secret

Oct. 22, 2010— -- Call it Fido physics. Intrigued by the infamous wet-dog shake, scientists at the Georgia Institute of Technology have figured how fast a sopping wet pooch must shimmy to dry himself off.

How did they do it?

Instead of running for the hills when the waterlogged animals got ready to shake, the researchers risked getting wet themselves. They got up close and personal with a slow-motion video camera, and then analyzed the shaking.

They discovered that the smaller the animals, the faster they must generally shake to dry themselves off.

David Hu, assistant professor of mechanical engineering at the Georgia Institute of Technology, said the research is interesting because drying wet fur is critical to how mammals regulate heat.

"A lot of animals have developed fur mostly for thermal insulation purposes and they need special mechanisms to basically get the water off," he said.

If the standard 60-pound dog had to rely on evaporation alone to dry off, he would have to spend a quarter of his daily calories to get rid of the water, Hu said. Smaller animals would have to use even more of their daily energy to do the job.

Researchers Studied Shaking Mice, Dogs, Grizzly Bears, Panda and More

"To evaporate the water is very costly to them," he said. "Physiologists call this the wet-dog shake. That's why this thing evolved, and it's basically a slight variation of shivering."

Hu said scientists have studied the phenomenon biologically, but haven't taken a look before at how it works and how efficient it is.

Hu said he and the researchers, including graduate student Andrew Dickerson, videotaped 16 different species shaking off water. They studied various breeds of dogs, including the Chihuahua, Labrador, poodle and husky, as well as mice, rats, guinea pigs, grizzly bears and the panda.

Some of the researchers got a little wet, they said, but it was worth it: They found a mathematical relationship between the radius of the animal and the frequency with which it had to shake in order to dry off.

The smaller the animal, the more rotations per second it would have to make in order to dry off. For example, while a mouse with a radius of 2 cm (less than an inch) would have to shake with a frequency of 27 hz (shakes per second), a dog with a radius of 20 cm would only have to shake with a frequency of 4.7 hz to get comparably dry.

Larger Animals Shake More Than They Need to When Wet

The reason the larger animals don't have to shake as quickly has to do with surface tension, the researchers said.

Hu said the droplets stick to the animals because of surface tension between the water and the fur. To repel the liquid, the animal must create enough centripetal force to overcome the surface tension.

"If a dog shakes at a sufficiently high frequency, that centripetal force can overcome the surface tension and pull it off," he said. "All the animals have to reach the same speed [which is equal to frequency times radius], but because the larger animals have a larger radius, they can move at a slower frequency."

But Hu said that while their mathematical model works for the most part, it doesn't apply as directly to larger animals.

"Basically the larger animals shake faster than they need to and we don't understand why," he said. "We predict that as they get bigger and bigger and bigger they don't have to shake that fast."

The grizzly, the panda and the Sumatran tiger are all among these over achievers.

But Hu said they think the answer might lie in the animals' hair.

"The analysis we have is assuming that the fur is not playing a role in helping them shed [water], but larger animals might have some property," he said. "there's something else going on besides just physics in figuring out what frequency they shake at."