“Wet Mammals Shake at Tuned Frequencies to Dry”, 2012-08-17 ():
In cold wet weather, mammals face hypothermia if they cannot dry themselves. By rapidly oscillating their bodies, through a process similar to shivering, furry mammals can dry themselves within seconds.
We use high-speed videography and fur particle tracking to characterize the shakes of 33 animals (16 animals species and 5 dog breeds [mouse weanling, adult mouse, rat, squirrel, guinea pig, chihuahua dog, cat, otter, poodle dog, small Husky dog, chow dog, kangaroo, large Husky dog, Labrador, goat, pig, sheep, black bear, lion, tiger, panda, brown bear]), ranging over 4 orders of magnitude in mass from mice to bears.
We here report 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 [−3⁄16] based upon the balance of centrifugal and capillary forces. We also observe a novel role for loose mammalian dermal tissue: by whipping around the body, it increases the speed of drops leaving the animal and the ensuing dryness relative to tight dermal tissue…Among these animals, we observe a clear dependency of shaking frequency on body size: mice must shake at 30 Hz, dogs at 4.5–8 Hz and bears at 4 Hz.
…Shaking water from an animal surface reduces the combined energetic costs of carrying this water and evaporating it. Small animals may trap substantial volumes of water in their fur for their size[12–14]: emerging from a bath, a human carries 1 pound of water, a rat 5% its mass and an ant 3× its mass. Wet fur is a poor insulator because water’s conductivity of 0.6 Wm−1 K−1 is 25× greater than that of air and 12× greater than that of dry fur,15 causing a wet animal to lose heat very quickly. Evaporation of the entrapped water from an animal’s fur may sap a substantial portion of the animal’s energy reserves. The specific energy required16 is e = 0.6 λ, where the heat of vaporization of water λ = 2257 kJ kg−1. Consequently, a wet 60-pound dog, with one pound of water in its fur, would use 20% of its daily caloric intake simply to air-dry. It is thus a matter of survival that terrestrial animals remain dry in cold weather.17