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Everyone loves a slow motion video of a dog shaking to dry off, but what is the science behind it?

Host: Hank Green

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Sources:
http://www.nature.com/news/scientists-do-the-wet-dog-shake-1.11177
http://www.npr.org/sections/thetwo-way/2010/10/22/130758623/scientists-reveal-how-fast-dogs-must-shake-to-dry-their-fur
http://rsif.royalsocietypublishing.org/content/9/77/3208
Who doesn’t love a slow motion video of a dog shaking to dry off, skin folds flapping every which way as it showers everyone nearby?

But did you ever wonder about the science behind the wet dog shake? Getting dry quickly can be a life-or-death problem, because an animal’s fur only works as insulation when it’s dry and able to trap a layer of air next to the animal’s body.

So being wet in cold weather can lead to hypothermia. And it turns out evolution has honed that wild shake into the perfect quick-dry method for furry animals. For a study published in 2012, scientists filmed 16 mammal species at Zoo Atlanta shaking themselves dry, using a high-speed video camera to get slow-motion footage.

The animals they looked at included mice, goats, tigers, and bears, as well as five dog breeds in all shapes and sizes. Analyzing their videos, the researchers counted each species’ skin oscillations per second to see how they compared. They also built a robotic wet dog simulator in a lab to study how water droplets are thrown off a shaking animal in detail.

Or, I guess, it was more of a wet deer simulator: picture a rotating panel with white-tailed deer fur glued to it that the scientists could get wet and spin at different speeds. Not as cute as the real thing. The study found that each particular species shakes at a particular frequency, to balance how dry it gets with how much energy it’s using.

Shaking animals need to generate enough force to overcome surface tension, a property that’s a result of water molecules sticking together, and to surfaces like fur. And force just depends on how much mass something has, as well as its acceleration. If you don’t have as much mass, that means you need more acceleration to generate the same amount of force, so small animals need to shake the fastest.

While mice shake as many as 27 times a second, larger animals like giant pandas can get just as dry with a slower shake, doing a lazy 4 or 5 oscillations each second. And all that loose, flapping skin plays a role too. The way it whips around helps throw the water off quickly, sometimes generating so much force that animals instinctively close their eyes while they shake to protect them.

The scientists behind this project think that understanding the wet dog shake could help us figure out how to quickly shed water from sensitive equipment, or even build better washers and dryers. But why don’t humans shake when we get out of the shower? Well, biologists think we never evolved to use that quick-dry technique because we don’t have a layer of fur that we need to keep dry.

Our skin doesn’t trap nearly as much water as fur does, so it doesn’t take as much body heat to evaporate it. And other hairless mammals don’t shake themselves dry, either: the researchers filmed hairless guinea pigs along with all those dogs and mice, but the wet guinea pigs didn’t shake — they only shivered. Luckily for us, what we lack in fur, we make up for with towels.

Thanks to our Patreon patrons for asking! If you want more videos explaining cute doggo behavior, check out our video where we explain why pups tilt their heads.