"The Secret of Zebra Stripes Revealed: Scientists Disguised Horses to Uncover the Truth" (Published 2019)

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If you're someone who spends time around horses and flies, you might want to invest in some zebra stripes. It turns out that those black and white markings might be nature's most effective insect repellent.

Horses dressed in zebra coats might seem like a fun masquerade, but they're actually part of a study investigating a mystery that has puzzled scientists for more than a century. As Tim Caro, who studies animal coloration at the University of California, Davis explains, "most mammals are pretty boring," with solid coats of brown or gray. So when scientists see bold patterns like those on a giraffe or zebra, they can't help but wonder why.

Since the days of Darwin and Wallace, scientists have debated the function of those sassy zebra stripes. Theories have ranged from camouflage to confuse big predators and an identity signal to other zebras, but most scientists now agree that the stripes ward off biting flies that can carry deadly diseases.

In a recent study published in PLOS One, researchers dressed horses in zebra coats to investigate fly behavior. The flies pestered both horses and zebras equally, but the stripes seemed to dazzle the flies so much that they couldn't manage a controlled landing. Flies either veered off just in time or simply bumped into the zebra and bounced off. Even the horses dressed in zebra coats were less attractive to the pesky insects, but their bare heads were still fair game.

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According to Dr. Caro, something is preventing flies from landing, and while the reason is unknown, it seems that stripes are responsible for this dilemma, exerting an effect until the very last moment. The high contrast between black and white stripes is believed to deceive the fly's low-resolution vision, which detects movement.

Dr. How suggests that the vision distraction may just eradicate the fly's ability to see. As flies approach zebra stripes, an optical illusion similar to the barber pole may be at work, causing diagonal stripes to appear as if they are moving in different directions, confusing the fly, which thinks it is heading towards open space instead of a surface that can support landing. Alternately, sudden stripes may overwhelm a fly's senses, resulting in a state of electric shock.

The researchers are now testing coats with patterns, thicknesses, and contrasts to determine what causes flies to avoid stripes. Dr. Caro believes that experimenting with these variables will reveal what confounds them.

Meanwhile, people who plan to be around horses or horse flies may want to consider dressing in zebra print to avoid being bitten. Equally, ensuring that your horse is a twinsie can also reduce the risk of fly bites.

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