Scientists solve the 'annoying wet dog shake' mystery

Washington, Nov 13: Seeing a dog shake and shimmy after a bath is a common sight, and now scientists at the Georgia Institute of Technology have figured how fast a sopping wet pooch must shimmy to dry himself off.

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

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," Hu, assistant professor of mechanical engineering 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.

"Physiologists call this the wet-dog shake. That's why this thing evolved, and it's basically a slight variation of shivering," he said.

Results of the study revealed that 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.

"If a dog shakes at a sufficiently high frequency, that centripetal force can overcome the surface tension and pull it off. 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," Hu said.


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