I had always assumed that dogs shake vigorously after a bath or a swim in order to share massive quantities of water with all people in the immediate vicinity. That hypothesis fits in with my philosophical view that dogs have a generosity of spirit that knows no bounds and that they love us very much. It also matches my personal experience as a dog groomer and as a dog guardian.
A recent study takes a far more scientific approach to this behavior. Mammals that are wet suffer the risk of hypothermia, so water removal is a serious issue. Animals who remain wet can use 20 percent of their daily energy staying warm and generating the heat necessary for evaporation of that water. If an animal can quickly and efficiently remove excess water, they will dry faster, suffering less risk from the cold and saving energy.
In “Wet mammals shake at tuned frequencies to dry”, Andrew Dickerson, Zachary Mills, and David Hu enlighten us about the specifics of shaking. They studied the water removal affects of shaking behavior in animals ranging in size from mice to bears, and including dogs.
These researchers observed mammals shaking themselves when wet and came to several conclusions. One is that there is a mathematical relationship between the size of the animal and the frequency with which they shake. The smaller an animal is, the more water they take on relative to body weight, and the faster they shake to remove that water. Smaller animals shake faster (at a rate of 29 oscillations per second for mice) while larger animals shake more slowly (4 oscillations per second for bears.) As animals in the middle of the size range studied, dogs had an intermediate rate of 5-7 oscillations per second depending on the size of the dog. The dogs in the study included members of various breeds: poodle, Labrador retriever, chow, Siberian Husky and Chihuahua.
Another conclusion of the researchers is that the ability of mammals to remove water relates to a property of their skin—its looseness. The amplitude of the shake is increased by loose skin. Though the rotation of the spinal column only reaches 30 degrees, the skin movement allows a total rotation of up to 90 degrees. Loose skin in mammals has previously been hypothesized to help with limb movement but this study suggests another function—extra movement in shaking that allows additional water removal.
If you’ve ever been near a wet dog shaking, it will come as no surprise at that dogs, as well as other mammals, can remove about 70 percent of the water from their fur with just a few seconds of shaking. The effectiveness of shaking behavior is extraordinary, though I must confess that I rarely appreciate it when I experience it at close range.