How do stripes stop flies? Horses dress as zebras to find out

Why do zebras, a species of wild horse, have striped coats when other equines don’t?

Experts have theorized that the stripes serve as camouflage; that they disorient predators, or that they communicate signals to other zebras. Some have posed that the stripes help keep the animals cool under a hot sun. The latest research, however, suggests that the black-and-white stripes serve as a natural insect repellant, perhaps keeping off flies that can spread deadly diseases that tend to be more prevalent in Africa than other places where horses roam.

To figure out exactly how stripes accomplish this, a team of researchers traveled to the United Kingdom and borrowed three zebras and nine horses from England’s Hill Livery. They then spent about 16 hours watching flies land on them.

It wasn’t as boring as that sounds, however. There was fashion involved. Some of the horses and zebras were observed while they were wearing black, white or black-and-white striped coats, to see if the coats played a role in fly behavior.

Although flies circled and touched horses and zebras at similar rates, they completely landed on zebras/stripe-coated horses less than 25% as much as coatless or single-color coat-wearing horses. Similarly, flies barely landed on striped coat-wearing horses’ covered bodies — but they did land frequently on those horses’ uncovered heads.

The researchers, watching recorded video of their study closely, observed that flies slowed down just before landing on uncovered horses, but they approached zebras at full throttle, impeding their ability to land.

“Once they get close to the zebras, however, they tend to fly past or bump into them,” said a University of California Davis professor involved in the study, Tim Caro.

The conclusion: Flies aren’t deterred from attempting to land on a striped coat (whether it be a garment or all-natural fur), but they are somehow prevented from successfully landing with ease. This reduces the flies’ ability to bite the animals.

“Stripes may dazzle flies in some way once they are close enough to see them with their low-resolution eyes,” said University of Bristol professor Martin How.

Zebras were also extra protected by their own efforts. The researchers saw that zebras take greater steps to repel flies than horses do, by swishing their tails and running away. Horses, on the other hand, took smaller measures like twitches and occasional tail swishes.

Sounds like horses could stand to learn a few things from their stripe-y pals. But could we do the same with striped t-shirts? Caro, who plans to study fly landing patterns further by testing different stripe configurations and patterns, answered this question for The Atlantic.

“I’ve been very cautious about saying that until we got these results, but now I’m not so sure,” he said. “I think that a striped t-shirt might work very nicely.”

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