Why do zebras have stripes? Scientists have camouflaged horses to find it

What's black, white and scratched everywhere – except his head?

Horses wearing zebra coats on a farm in Britain.

The animals did not participate in a masquerade. They were dressed for studies on a mystery that has puzzled scientists for over a century.

Tim Caro, who studies animal staining at the University of California at Davis, and co-author of a study published Wednesday in PLOS One, said: "Most mammals are rather boring." "So when you see these bold patterns like on a giraffe or a zebra, as a biologist, you say, Why?"

At least since the time Charles Darwin and Alfred Russel Wallace theorized about evolution, scientists debated the function of this sassy animal print. This is called camouflage to confuse big predators, an identity signal for other zebras and a kind of portable air conditioner. Today, most scientists agree that zebra's function is to protect biting flies that can cause deadly diseases.

The flies harassed all the horses and zebras in the pens. But once they got closer, the welts seemed to dazzle the flies to the point that they could no longer land. The flies zoomed too fast and deflected just in time – or simply clashed with the zebra and bounced back. The flies did not seem to like the zebra coats on the horses either, but their bare heads were a fair game.

"Something is preventing the fly from realizing that it is about to make a landing," said Dr. Caro. "We do not know what it is, but the stripes exert an effect until the very last minute."

The only thing they can say is that the high contrast between black and white is probably detrimental to the fly's low-resolution vision, which is based on motion detection.

"It's probably the vision of the fly is swept away," said Dr. How.

In an optical illusion called the barber pole, diagonal stripes seem to go up or down depending on the direction of rotation of the pole. Something similar could happen when flies approach welts. From afar, the fly can interpret the object in gray, but as it gets closer, the diagonal stripes of the zebra may seem to move in wrong directions. As a result, a fly may think that it is heading towards an open space instead of landing. Or maybe the sudden appearance of rays can overload the vision of the fly and make it jump.

The researchers are currently testing with layers of patterns, contrasts and thicknesses to see what streaks stop flies. "By playing with these variables, we will be able to get into the head of the fly or into its eye to determine what may be confusing," said Dr. Caro.

In the meantime, people planning to mix with horses or horse flies may consider zebra prints rather than solids to avoid stings. You should probably make your horse a couple, too.

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