Scientists Dressed Horses Up Like Zebras to Determine the Purpose of Stripes

A new study supports the theory that zebras’ distinctive coats repel flies

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The coat is functional AND stylish. School of Biological Sciences, University of Bristol

It’s one of nature’s more intriguing and enduring mysteries: Why do zebras have stripes? Over the years, researchers have proposed a number of theories. Perhaps the animals’ signature coats help them camouflage, facilitate social signalling, or keep zebras cool. Today, many scientists believe that the black-and-white stripes actually function as a fly repellant, but because it’s difficult to get close to wild zebras, it hasn’t been clear how the pattern might deter the pesky critters from landing on the animals and taking a bite.

So, as JoAnna Klein reports for the New York Times, a group of researchers headed to a farm in Britain where domestic horses are kept alongside zebras that were born in captivity. Hoping to find out how flies interacted with the different species, the team conducted careful observations of the animals—and dressed the horses up in snazzy, zebra-print coats.

The experiment involved three zebras and nine horses with uniformly white, black, grey or brown coats. Researchers both watched the animals and filmed them, recording the number of horseflies—which bite animals to obtain blood—that hovered nearby. Flies approached the zebras and horses at equal rates, the researchers found, which isn’t surprising because flies are thought to use smell, rather than sight, to locate their victims from a distance. But once the flies got closer to the zebras, stripes did seem to interfere with their ability to hone in on the source of their intended snack.

The insects didn’t slow down as they approached the zebras, “suggesting they did not see the target, or did not regard the striped surface as an appropriate place to land, or were confused somehow by the stripe pattern,” the researchers write in a new study published in PLOS One. The flies tended to fly over or glance off the zebras; when it came to horses, many more flies were able to stick the landing.

“Just like when you’re flying on an airplane, a controlled landing is extremely important for flies,” Tim Caro, lead study author and behavioral ecologist at UC Davis, tells Popular Science’s Jessica Boddy. “They don’t want to break a leg or damage an eye. So when a fly comes in looking for a blood meal they need to slow down. Somehow stripes are preventing that from happening.”

To confirm that it was indeed coat pattern that was thwarting the flies’ precision, the researchers kitted some of the horses out in three cloth jackets: one white, one black and one zebra-striped. Fewer flies landed on the striped jacket, compared to the black and white ones. But the stylish zebra-striped attire did not stop flies from landing on the horses’ unadorned heads. In other words, there seem to be “enormous benefits to having a striped coat for a horse,” Caro tells Ed Yong of the Atlantic.

Why, then, aren’t all equids adorned with this dazzling pattern? Zebras, according to the study authors, are particularly susceptible to dangerous fly bites. In Africa, where wild zebras roam, flies carry a number of diseases that are fatal to the striped creatures, and their thin coats make them especially easy to bite. Stripes may therefore offer zebras vital protection, though the researchers aren’t entirely sure why the pattern seems to confuse flies. In their study, they write that the contrasting stripes might disrupt the insects’ optic flow, or their sense of motion of the objects around them.

Stripes may dazzle flies in some way once they are close enough to see them with their low-resolution eyes,” says study co-author Martin How.

Moving forward, the researchers plan to tease out how different variables—like coat thickness and subtle differences in pattern—might impact hungry flies. Speaking to Yong, however, Caro stop short of recommending that horse owners outfit their animals in head-to-hoof zebra suits. “I wouldn’t want to suggest that horse-wear companies sell striped livery for their riders yet,” he explains. “We need to do the work first.”

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