David Hu* and his colleagues at Georgia Tech have deconstructed the motion dogs make to shake the water off their fur! The scientific explanation - the dry graph above to illustrate the wet action (video here) - applies across animal species. The shake is a very efficient tactic for animals to dry themselves off, but is dictated by the animal's size and the surface tension and density of the water. The research team used high-speed video and fur particle tracking to understand the common physical characteristics that unite the wet-shakes of 33 mammals from 16 species. The animals (dogs of varying sizes, pigs, goats, kangaroos, rats, mice, and others) oscillate their bodies at a frequency that removes as much water as possible with minimal physical effort. Larger animals require lower frequencies to generate the centrifugal force necessary to overcome the strength of the surface tension that keeps water attached to their fur. Small animals, on the other hand, need to oscillate at a high frequency or they will remain just as wet. Allowing the animals to achieve the necessary centrifugal force when whipping around their bodies is loose skin that allows some to generate forces up to 70 times that of gravity. The result of their fine-tuning is that a large, soggy dog can shed as much as 70% of the water in its fur in just 4 seconds!
*Dr. Hu made the New York Times in June after determining that raindrops do not pose a flight risk to mosquitoes, a finding - also based on observations of high-speed video - that has implications for development of robotic airborne search-and-rescue vehicles.