# The physics of the wet dog shake

Anyone who has a dog as a pet knows to avoid getting close to them when they are wet because they have a remarkable ability to shake themselves in such a way that the water flies off and drenches everything nearby. The interesting questions are why they do that and how they do that.

However much we may suspect that they do it because they find it fun to soak those around them, it turns out that it may be a survival necessity, that a lot of water clinging to their fur may cause hypothermia and so they have to get rid of as much as possible and soon as possible. It turns out that the way they shake can get rid of about 70% of the water in as little as four seconds.

Researchers have found that animals tune the frequency of their shakes according to a power law depending of the body size of the animal in order to generate centripetal forces ranging from 10 to 70 times that of gravity.

Here’s a great video from Nature that explains the phenomenon, accompanied by slow motion footage of various animals shaking off water.

You can read more about it here and the abstract of the paper on this work by David Hu and colleagues at the University of Georgia can be read here.

Science is so cool.

1. Onamission5 says

I love mostly that someone thought to study this at length. Who says science isn’t fun, or funny?

2. says

3. Paul Hunter says

Thanks, science is always interesting even about things we thought we already knew

4. Mano Singham says

They improved the mathematical modeling and now can have better idea of the g-forces that are generated by the gyrations.

5. says

I’m trying to keep track. I think that’s way #27,873,942 how dogs are awesome!! (But I may have counted the ball-chasing twice)

6. says

Thanks! I could have compared the abstract to the paper from a few years ago but it would have been taken a me while to figure out how to find that paper*.

* Unlike this paper, I do have access to the paper from a few years back.

7. KG says

A related question: how far is the shaking under voluntary control? Our dog tends to wait until she’s indoors to shake; I’ve tried to train her to shake on command, but at most she does a half-hearted shake that barely goes below the neck.

8. Timberwoof says

I was disappointed that the video did not tell what the power law is: given the weight of the animal, how fast does it shakes itself dry? What other variables affect the law?

Aha! The Google Fu is strong with this one.
http://rsif.royalsocietypublishing.org/content/early/2012/08/16/rsif.2012.0429.abstract

“…the power law relationship between shaking frequency f and body mass M to be f ∼ M^−0.22, which is close to our prediction of f ∼ M^−0.19 based upon the balance of centrifugal and capillary forces.” But no unit constants.

9. Crudely Wrott says

The interesting questions are why they do that . . .

I’d guess it is for the same reason that you or I or anyone else shakes their head when coming to the surface after being underwater. Water running into eyes and ears is not only annoying, it interferes with the function of those senses. Survival instincts. If you can’t see or hear well you are more likely to get eaten.

This isn’t the time for a funny anecdote about my swimming in the Gulf of Mexico at the time that the movie “Jaws” came out. Suffice to say that I got very afraid . . .

10. Fascinating, ne?