When a dog gets wet, what does it do? Answer: it shakes, and very efficiently — a large, soggy dog can shed as much as 70% of the water in its fur in just four seconds.

But mammals of all shapes and sizes employ the quick-shake tactic to dry themselves. Now, a team of researchers has used high-speed video and fur particle tracking to understand the common physical characteristics that unite the wet-shakes of 33 animals from 16 species.

The researchers, led by Georgia Institue of Technology biologist David Hu, demonstrate that whether it's a mouse, a goat, a dog or a bear, the shake of a wet mammal is anything but random. On the contrary, the researchers write that the frequency of an animal's shake — i.e., the number of times it oscillates its body per second — is tuned to "(i) the animal's size and (ii) the properties of water, namely surface tension and density," in order to remove as much water as possible with minimal physical effort.


Small animals, like mice and rats, tune their shakes to a high frequency, oscillating fast in order to generate the centrifugal force necessary to overcome the strength of the surface tension that keeps water attached to their fur. Larger animals, by comparison, can tune their shakes to much lower frequencies to generate the same force. (The video up top, compiled by the Nature news team, illustrates various animals shaking to their tuned frequencies.)


The relation between shaking frequency f and animal radius R, Hu et al.

"If... all animals shook at the frequency of a dog," explain the researchers, "the smallest animals would have insufficient force to remove drops: for example, a mouse shaking at 4 rather than 30 Hz would generate only one g of centrifugal force, and would remain just as wet."

Loose skin also factors into the equation. "By whipping around the body," loose skin allows some mammals to generate centrifugal forces as great as seventy times that of gravity, increasing "the speed of drops leaving the animal and the ensuing dryness relative to tight dermal tissue."

The researchers' findings are published in the latest issue of the Journal of the Royal Society Interface.