Rapid Oscillation Sheds Water

Biological Strategy

Mammals

Mary Hoff

Image: Jörg Künkel / Pixabay / Free non-commercial use

Modify Material Characteristics

The materials found in living systems are variable, yet often made from the same basic building blocks. For example, all insect exoskeletons consist of a material called chitin. Because material resources are limited, each material within or used by a given living system must frequently serve multiple purposes. Therefore, living systems have strategies to modify materials’ softness, flexibility, and other characteristics. To ensure survival, the benefits of these modifications must outweigh the living system’s energy and material expenditure to generate them. For example, spiders store the liquid components of spider silk in a gland, converting them into silk thread when needed. Some threads have different characteristics, such as elasticity and UV reflectance, than others.

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Protect From Temperature

Many living systems function best within specific temperature ranges. Temperatures higher or lower than that range can negatively impact a living system’s physiological or chemical processes, and damage its exterior or interior. Living systems must manage high or low temperatures using minimal energy, which often requires controlling responses along incremental temperature changes. To do so, living systems use a variety of strategies, such as avoiding high or low temperatures, removing excess heat, and holding heat in. Insulation is a well-known example of managing low temperatures by retaining heat using thick layers of hair, fur, or feathers to hold warm air next to the skin.

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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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Maintain Homeostasis

When a living system is in homeostasis, it means that internal conditions are stable and relatively constant. For example, a human’s internal temperature is approximately 37 degrees Celsius (98.6 degrees Fahrenheit) unless there’s an illness. The human body maintains this temperature despite external ambient temperature. However, as with all physiological processes, maintaining homeostasis requires communication and coordination. So living systems have ways to detect changes from the norm, mechanisms to cause an adjustment, and negative feedback connections between the two. A desert lizard called the Gila monster offers a good example of maintaining homeostasis. The lizard goes from eating large meals to fasting for extended time periods. To maintain its blood sugar levels at a steady level, when food is scarce, its endocrine system releases a hormone that raises its blood sugar levels.

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Expel Liquids

There are various reasons living systems discharge liquids, such as to defend themselves, to avoid liquid buildup that can cause decay, to eliminate waste, or to dry off. Because liquids cannot be effectively moved through pushing, a different kind of force is needed to expel them. Creating that force requires energy, so living systems must have efficient strategies worth the energy investment or use an outside force (such as gravity). This typically entails strategies that build up pressure or use other forces to propel liquids away. An example of a living system that expels liquids is the scallop. This creature moves through water by clapping its shells together, generating a water jet that propels it in the opposite direction.

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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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Quick movement dries mammalian fur by ejecting droplets using centripetal force.

Introduction

Take your dog to play in the water, and what’s the first thing she’ll do when she gets out? Likely shake her head and shoulders in a semicircular motion, flinging fat water droplets onto everything in the vicinity.

This act may not seem particularly strategic, but from the standpoint of energy efficiency, it very much is. In fact, many species of mammals do the same thing—and the magic of the movement all comes down to physics.

The Strategy

Drying off quickly is important for mammals because water mats their fur, dramatically reducing its insulating value. In cool weather, removing the water and regaining the insulation value of the coat can be literally a matter of life and death. And even though it takes energy to rapidly rotate the head and shoulders, it would take orders of magnitude more energy to heat the body enough to evaporate the large amount of water a fur coat can take on.

The efficiency of the shoulder-shake technique comes in at the molecular level. Surface tension and hold the moisture to the hair. A shaking animal counters those forces by using a circular motion to generate centripetal force—the same action that spins water out of your clothes in a washing machine.

Image: VasilySmirnov / iStock by Getty Images /

Rotational shaking deploys centripetal force to counter the surface tension that holds water to hair.

Image: S. Carter / Flickr / CC BY SA - Creative Commons Attribution + ShareAlike

Rotational shaking deploys centripetal force to counter the surface tension that holds water to hair.

How rapidly an animal shakes depends on its size. Tiny mice rotate their heads and shoulders an incredible 27 times per second, while big bears do so only around 4 times per second. Loose skin on the upper half works like a soft, circular version of a spear-thrower. It swings forward farther than the muscles themselves move, and it snaps back when the shake reverses direction, providing more force from the amount of muscle energy expended than would be the case with tighter skin.

The speed of the shake relative to body mass is fast enough to overcome the surface tension holding the water to the hair, but not so fast that it causes the hair clumps to split into individual hairs, which are more resistant to shedding water. Scientists have found that a centrifugal force of 10–70 times the force of gravity can remove 70% of the water in far less time than evaporation would take.

Watch a dog shake water from its fur in slow motion.

The Potential

Lessons learned from the design of fur and loose skin, and the rate at which mammals shake to expel water from their fur can apply to optimizing human activities as well. For example, they can help us set ideal speeds and drum sizes for washing machines and centrifuges or to design entirely new versions of them. They also can guide the design of self-drying or self-cleaning equipment, such as cameras on remote vehicles exploring other planets.

Related Strategies

Biological Strategy

Fur Keeps Heat in and Cold Water Out

Otters

Long scales on the underhair of otters and fur seals trap air to retain heat and prevent water penetration.

Biological Strategy

Ultra-absorbent Feather Structure Carries Water

Sandgrouse

Helically-structured barbules within the abdominal feathers of male sandgrouse reversibly absorb water.

Last Updated March 29, 2022

References

The increase in shaking speed for smaller animals is important in compensating for their smaller radius. This tuning of shaking frequency with body size is necessary to generate the large centrifugal forces required to shed drops, Rv2/g . 10–70 gravities, for the animals listed in table 1 . If, for example, all animals shook at the frequency of a dog, the smallest animals would have insufficient force

to remove drops: for example, a mouse shaking at 4 rather than 30 Hz would generate only 1 gravity of centrifugal acceleration, and would remain just as wet. (Dickerson et al. 2012:3215)

Journal article

Wet mammals shake at tuned frequencies to dry

Journal of the Royal Society: Interface | August 17, 2012 | A.K.Dickerson, Z.G. Mills, D.L. Hu

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Reference

When the dog shakes, centripetal forces pull the water away. So for the water to be ejected from the fur, the centripetal force has to exceed the surface tension. This model leads to an interesting prediction. If the animal has a radius R, the shaking frequency must scale with R^0.5. That makes sense, smaller animals will need to oscillate faster to generate forces large enough to dry themselves. (Emerging Technology from the arXiv 2010:1)

Magazine article

Physicists discover universal “wet-dog shake” rule

MIT Technology Review | October 20, 2010 | Emerging Technology from the arXiv

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