Cats have up to 130,000 hairs per square inch. This keeps them warm and fuzzy, but it means lots of grooming, too — about 40 percent of each day for a typical cat.
All that fur also means cats are larger than they look. Researchers from the Georgia Institute of Technology went to the trouble of calculating a cat's total surface area, along with that of 27 other furry animals, as part of a new study aimed at helping human engineers borrow cleanliness secrets from the natural world.
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It turns out cats have about the same surface area as a Ping-Pong table. A sea otter, which has up to 1 million hairs per square inch, is roughly equivalent to a professional hockey rink. And although a honeybee has 3 million hairs, the same number as a squirrel, that only gives it the surface area of a slice of toast.
This is an interesting way to look at animals, but what does it have to do with engineering? By studying the surface area of mammals and insects, and how each species uses hair to stay clean, the researchers hope to inspire new biomimicry strategies for keeping filth off man-made devices like sensors and drones.
"Animals likely evolved with hair in order to stay warm. But it also brings a burden," says Georgia Tech professor David Hu — who co-authored the new study with Ph.D. student Guillermo Amador — in a statement about the research. "More hair means more surface area that can trap dirt, dust and pollen."
Yet as Hu and Amador point out in the study, even that burden may be a blessing in disguise, since trapping dirt is the first step in cleaning it up. "[Hair] increases the area on which particles can land by a factor of 100," the researchers write, "but also suspends particles above the body, reducing their adhesion and facilitating removal."
The study divided animals into two basic types of hairy hygiene. Some use non-renewable methods, which rely on their own energy for active efforts such as grooming, body shaking or chemical secretions. "Dogs shake water off their backs, just like a washing machine," Amador explains. "Bees use bristled appendages to brush pollen off their eyes and bodies. Fruit flies use hairs on their head and thorax to catapult dust off of them at accelerations of up to 500 times Earth's gravity."
Other animals use more efficient, renewable tactics, in which hair itself does most of the work. This includes eyelashes, for example, which redirect wind — and any airborne particles — away from mammals' eyes. And cicadas have evolved tiny spikes on their wings, which are sharp enough to puncture and kill bacterial cells.
Neither approach is necessarily better; they've just evolved to meet different biological needs. But for the purposes of biomimicry, Hu and Amador say passive methods like the antibacterial cicada wings are most intriguing. By examining how hair affects an animal's overall surface area and cleanliness, the goal is to spur more efficient defenses against dirt in a wide range of technologies.
"Drones and other autonomous rovers, including our machines on Mars, are susceptible to failure because of the accumulation of airborne particles," Hu says. "Understanding how biological systems, like eyelashes, prevent soiling by interacting with the environment can help inspire low-energy solutions for keeping sensitive equipment free from dust and dirt."
Does that mean future Mars rovers might be furry? We can only hope. For now, it's at least nice to know there's a scientific silver lining to all those hairballs around the house. It's nice to be reminded of our pets' natural talents, too, whether it's dirt-trapping fur or lightning-fast reflexes. Hair may help cats cover a Ping-Pong table in theory, but agility can help them cover it in real life — as this 1951 video illustrates: