Wet-dog physics

Wet, hairy animals shake at just the right speeds to get rid of water

Mice do it. Chihuahuas do it. So do tigers, rats and pandas. These animals really know how to shake.

Researchers at the Georgia Institute of Technology in Atlanta recently recorded video of 40 different animals, representing 15 different species. The scientists wanted to see how wet, hairy mammals shake off water after they get drenched. What the videos revealed is that animals’ shaking behavior could be described by physics — the science of matter, energy and motion. (Watch a video.)

Dogs (such as this Labrador retriever) and other hairy mammals shake at just the right frequencies to remove water most efficiently from their bodies — and directly on to any unfortunate humans nearby. Andrew Dickerson et al./Georgia Institute of Technology
The scientists say the animals oscillate at just the right frequencies to lose water droplets as efficiently as possible. Oscillate means to move back and forth, and frequency is the number of cycles — movements back and forth — per second.

“I think it’s pretty amazing they can do that,” David Hu told Science News. Hu is an engineer, but he’s also a mathematician and does research in biology. At his laboratory at the Georgia Institute of Technology, Hu studies the physics of fluids, which means he wants to know how fluids move and react to forces. He is particularly interested in how animals interact with water.

This study was led by Andrew Dickerson, a graduate student in Hu’s lab. His team calls this get-dry shake “nature’s analogy to the spin cycle of a washing machine.” Both the washing machine and shaking animals can get rid of water quickly — but animals are much more efficient than washing machines. For animals, this process helps them control heat in their bodies.

“If a dog couldn’t dry itself, we calculated that it would have to use 25 percent of its daily calories to heat its body to get rid of the water,” Hu told Science News. “Every time they got wet they would get hypothermia and die.”

The bigger the animal, the slower it shakes, according to Dickerson and his team. A mouse moves its body back and forth 27 times per second, but a grizzly bear shakes only four times per second.

Animals’ skin also helps them get rid of water. When the scientists placed a fluorescent straw on the back of a dog and watched the dog wiggle, they observed that the skin can move halfway down the dog’s side in either direction. Loose skin lets the dog lose more water than if the skin were tighter.

The scientists are continuing to look at how animals interact with water in the natural world. In particular, the scientists want to know how water droplets interact with hair — which means investigating animals that have adapted to life in the water, like beavers and otters.

Stephen Ornes lives in Nashville, Tenn., and his family has two rabbits, six chickens and a cat. He has written for Science News Explores since 2008 on topics including lightning, feral pigs, big bubbles and space junk.

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