Why study the proverbial wet-dog shake? Shaking when wet is universal among many furry mammals. Such a study provides insight into mammal survival, adaptation, and fluid mechanical phenomena which links numerous species.
Drop ejection from spinning disks is well-studied, but are the first to look at dripping from long fibers under accelerations higher than that of gravity.
Why do mammals shake when wet? The simple answer, as you may guess, is to dry. Perhaps a better question is why an animal wants to be dry. According to our calculations, a wet animal could spend 20% or more of it’s daily food energy to evaporate water in its fur, if it cannot shake.
In our study we found the largest mammals such as bears, tigers, and large dogs shake about 4 times per second while small mice shake at more than 30 times per second!
Shaking mammals generate high centrifugal accelerations when spinning. A drop residing in a mammals fur will experience an acceleration many times gravity when ejected. We found that mammals can generate accelerations 10-70 times gravity in their fur when shaking.
Smaller animals shake more quickly than larger animals. A certain spinning speed is required to remove water from fur. Larger animals have a size advantage and do not need to shake as vigorously to get their skin whipping fast.
Plot of shaking frequency versus animal mass. The solid line was the best fit to our data, while the dotted line is the fit expected via mathematics.
A scaling law was fit to the data to provide a relationship between animal size (mass) and the frequency they shake when wet. The best fit to our data yielded a scaling relationship f ~ M-0.22. This relationship is close to the one predicted mathematically, f ~ M-0.19.
Mammals shaking is very effective. A shaking mammal can remove about 70% of the water trapped in its fur in few seconds, when fully wet. The remaining moisture content (RMC) in the fur is about 30%. When comparing the accelerations generated by animals to data gathered on our “wet-dog simulator” (see Methods below), we note that animals shake in the region where drying tapers off.
We found shaking longer or faster does not contribute to further drying. Therefore, mammals “tune” their shaking to achieve maximal dryness with the least effort.
Plot of remaining moisture content (RMC) versus shaking vigor, measured in number of gravities. We found that RMC begins to taper off at 10 gravities, the minimum number generated by any animal we observed.
The kangaroo does not display the ability to shake the entire body. We believe this occurs due to the largest of the hind quarters and shape of the spine. Additionally, kangaroos reside in hot and dry climates where shaking is of less priority.
NO animals were harmed in this study. We employed a portable high-speed camera for filming all animals and drop release sequences on our “wet-dog simulator.”
The wet-dog simulator allowed us to spin small brushes, acting as fur mimics, at high speeds and film their ejections. The simulator uses constant rotation, but generates centripetal forces similar to those of animals. We used Matlab software to determine drop volume at various speeds.