Fur Dries by Shaking

white polar bear standing on brown rock shaking off water

Biological Strategy

Mammals

Eleanor Banwell

Image: Karen Blaha / Flickr / Some rights reserved

Protect From Excess Liquids

While water is essential to life, too much water or other liquids can overwhelm living systems. Excess liquids can, for example, decrease a living system’s access to oxygen, promote excessive bacterial or fungal growth, or strip away soil and nutrients. To prevent the accumulation of excess liquids, living systems must control the movement of liquids across their boundaries or surfaces. They do so using waterproofing materials or structures, slowing flow, and/or facilitating flow to move the liquid away. Plant leaves, for example, commonly have waxy surfaces comprised of water-repelling chemicals to keep water from engorging the leaves or facilitating bacterial and fungal growth.

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Mammals

Class Mammalia (“breast”): Bats, cats, whales, horses, humans

Mammals make up less than 1% of all animals on earth, but they include some of the most well-known species. We know first-hand some of the characteristics that make mammals unique, like having hair, being able to sweat, and producing milk through mammary glands. Another critical shared feature is a set of highly-specialized teeth. Unlike sharks or alligators, for example, whose teeth are generally all the same size and shape, mammals have differently shaped teeth in different areas of the jaws to target specific foods or foraging strategies.

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Fur of mammals is optimally dried by changing shaking frequency.

Getting wet is a fact of life for furry mammals, but it can be very dangerous to stay that way. Wet fur conducts heat, increasing the risk of hypothermia and the problem exists for animals of all sizes. Mice are small and have a relatively high surface area to volume ratio, meaning they lose heat very quickly, while for larger animals like polar bears their hunting habits and regional climate make getting dry quickly very important. A medium-sized dog, like a labrador, would use 1/5th of its daily calorie intake to offset the energy lost through air drying. To get around this problem, furry mammals shake.

Shaking is a surprisingly finely tuned process. Because shaking costs energy, mammals shake at exactly the frequency that achieves the maximum return on their energy investment. Small water droplets stick to fur and are very difficult to dislodge, while large droplets fall off easily. Animals shake at a speed calibrated to remove the larger droplets, getting rid of more than 70% of the water on their fur in seconds. In this way, they do not waste energy attempting to dislodge tiny water droplets that will evaporate quickly anyway.

Shaking is a rotation around the central axis of the body that works like a centrifuge to fling water away from the fur. Because the fur travels in an arc, the distance travelled and the amount of force generated during one shake depends on the size of the animal. Larger animals move their fur further and generate more force than smaller animals. To compensate, small animals shake at a higher frequency.

Furry mammals tend to have loose skin. At the end of one shake, when the animal stops and begins to twist in the opposite direction, the skin continues moving. Once it reaches its full extent, the skin is snapped back by the body that is already moving very quickly back the other way. This snap generates very high forces on the water droplets that make shaking as much as three times more energy efficient than with tight skin.

Adapted from Dickerson AK; Mills ZG; Hu DL. 2012. Wet mammals shake at tuned frequencies to dry. J. R. Soc. Interface. 9, 3208, by permission of the Royal Society.

Last Updated April 19, 2018

References

“By shaking at such frequencies, furry animals act like a high-pass filter, causing drops above a critical size m to eject.” (Dickerson et al. 2012:3215)

“In tests with animal hair, we found the observed frequencies were capable of drying the animal appreciably, as defined by removing 70 per cent of its accumulated water. Furthermore, we found that increased speeds imparted to the fur would achieve diminishing return. Thus, the animal obtains a reasonable amount of dryness without expending an excess of energy.” (Dickerson et al. 2012:3216)

“Previously, we found loose dermal tissue increases the shake amplitude by threefold, and thus the speed of shaking by ninefold. If dermal tissue were tight rather than loose (as on humans), animal shaking frequencies would cause RMC values to remain close to 1, indicating the animal would remain wet. Thus, an important role of the loose dermal tissue is to increase the efficacy of the shake, as shown by the sensitivity of the RMC to changes in skin speed.” (Dickerson et al. 2012:3216)

Journal article