Watch a fly land on the kitchen table, and the first thing it does is clean itself, very, very carefully. Although we can’t see it, the animal’s surface is covered with dust, pollen and even insidious mites that could burrow into its body if not removed.
Staying clean can be a matter of life and death. All animals, including us human beings, take cleaning just as seriously. Each year, we spend an entire day bathing, and another two weeks cleaning our houses. Cleaning may be as fundamental to life as eating, breathing and mating.
Yet somehow, cleaning has gotten little attention.
In our new review article in the Journal of Experimental Biology, we discuss how cleaning happens in nature and whether animals indeed have principles for getting clean. We looked at microscope images to count the number and sizes of hairs across hundreds of animals. We read nearly a hundred articles on cleaning in nature, trying to put numbers onto the cleaning process.
Extrapolating principles is an important step for science, and even more necessary for engineering. Learning better ways to clean will not just allow us to understand the humble fly, but also to build new kinds of devices that stay clean longer.
Hair vastly amplifies a body’s surface area
To understand how animals get clean, one must first understand how they get dirty. Dirt accumulates on an animal’s exterior just as a consequence of living life. The surface area of an animal is not as easy to figure out as measuring the dimensions of a cardboard box. Most animals – from mosquitoes to elephants – are hairy. Beyond the exterior of a creature’s skin, hairs provide further surface area where dirt can accumulate.
We found that on average, hair increases an animal’s apparent surface area by a factor of a hundred. Thus, a cat has a surface area of a ping pong table. (This explains why its so hard to get pets clean.) A chinchilla has the surface area of an SUV. And a sea otter has the surface area of a hockey rink.
We people have about 100,000 hairs on our head. The number of hairs on other animals is comparatively staggering. A butterfly has 100 billion hairs, more than 10 times that of a beaver. The bee has 3 million hairs, the same number as a squirrel.
Moreover, on animals there are as many types of hairs as we have types of hairdos. Animals have trichia, spines, macrotrichia, setae, scales, hairs of all shapes and sizes. One thing is clear: hair increases the surface area of the body, and so makes the problem of cleaning much worse. Which would you rather clean, a linoleum floor or a shag carpet?