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Why Do Zebras Have Stripes? Scientists Camouflaged Horses to Find Out
If you spend time around horses or flies, you might want to invest in some zebra print.
By JoAnna Klein
What’s black, white and striped all over — except for its head?
Horses wearing zebra coats on a farm in Britain.
The animals weren’t attending a masquerade. They were dressed for studies investigating a mystery that has puzzled scientists for more than a century.
With solid coats of brown or gray, “most mammals are pretty boring,” said Tim Caro, who studies animal coloration at the University of California, Davis, and a co-author of a study published on Wednesday in PLOS One. “So when you see these bold patterns like on a giraffe or zebra, as a biologist you say, Why?”
At least since the days when Charles Darwin and Alfred Russel Wallace were theorizing about evolution, scientists have debated the function of this sassy animal print. It’s been called camouflage to confuse big predators, an identity signal to other zebras and a kind of wearable air conditioner. Now most scientists agree that the function of a zebra’s stripes is to ward off biting flies that can carry deadly diseases.
But what exactly is it about a zebra’s wardrobe that flies don’t like?
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The answer to that question has been hard to find. Zebras in the wild are not easy to get close to. So Dr. Caro and a colleague, Martin How, went to Hill Livery, a horse farm moonlighting as an orphanage and a conservation hub for captive zebras near the University of Bristol in Britain. With their students, they observed and filmed horse flies trying to bite zebras. They also dressed some horses in zebra print to see if it helped them avoid fly bites.
The flies pestered all of the horses and the zebras in the paddocks equally. But once they got close, the zebra stripes seemed to dazzle the flies so much that they couldn’t manage a controlled landing. Flies zoomed in too fast and either veered off just in time — or simply bumped into the zebra and bounced off. The flies didn’t seem to like the zebra coats on horses, either, but their bare heads were fair game.
“Something is stopping the fly from realizing that it’s close to making a landing,” Dr. Caro said. “We don’t know what that is, but stripes are exerting an effect to the very last second.”
The only thing they can say for certain is that the high contrast between black and white most likely tricks the fly’s low-resolution vision, which relies on sensing movement.
“It’s probably just blowing the fly’s vision away,” Dr. How said.
In an optical illusion called the barber pole, diagonal stripes appear to move up or down, depending on which way the pole is rotating. Something similar could be happening as flies approach zebra stripes. From afar, the fly may interpret the object as gray, but as it moves closer, the zebra’s diagonal stripes may appear to be moving in false directions. As a result, a fly may think it’s headed toward open space instead of landing. Or perhaps the sudden appearance of stripes may overload the fly’s vision and startle it into a buzzing stupor.
The researchers are now conducting tests with coats of different patterns, contrasts and thickness, to see just what it is about the stripes that stops the flies. “By playing around with those variables, we’ll be able to get inside the head of the fly, or the eye of the fly, to work out what’s sort of confusing to it,” said Dr. Caro.
In the meantime, people planning on being around horses or horse flies may want to consider wearing zebra print rather than a solid to avoid being bitten. You should probably make your horse a twinsie, too.
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