Scientists Dressed Horses Like Zebras to Figure Out Why They Have Stripes

The experiment aimed to resolve the longstanding debate over the evolutionary purpose of zebra stripes.

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Feb 20 2019, 10:14pm

Horse in zebra print. Image: T. Caro (2019)

Why do zebras have stripes?

Evolutionary biologists have proposed many possible theories, such as camouflage or vision aids for recognizing individual zebras. But in recent years, pest control has emerged as the leading explanation for zebra stripes.

Researchers led by Tim Caro, an evolutionary ecologist at UC Davis, set out to test this idea in the field. The results, published Wednesday in PLOS ONE, reveal that stripes are a powerful deterrent to horse flies, a common nuisance that suck blood and bite flesh.

The experiment managed to find the most delightful way to help explain these uniquely patterned coats—by getting horses to cosplay as zebras.

Caro’s team recorded close-up video footage of three captive zebras and nine horses at the Hill Livery, a horse farm in Dundry, United Kingdom. The horses had coats with uniform colors (grey, black, or brown), with no stripes or markings.

The researchers counted the number of horse flies that landed on the horses or zebras, and assessed if there was any difference in the way the pests approached the animals. They also examined the responses of the horses and zebras to flies, such as tail-flicking and skin-twitching.

While the flies circled the horses and zebras at roughly the same rates, they landed on the zebras about a quarter as often.

The flies that landed on the zebras often failed to properly slow down for their approach, perhaps due to optical confusion caused by the stripes. This prevented the flies from achieving clean landings, causing some of them to rapidly rebound into the air.

Zebras were also more likely to use their tails to bat approaching flies away, while the horses were more likely to twitch their skin to get rid of flies that made contact with them.

Read More: The Reason Zebras Have Stripes Isn't What You Think

On top of those trials, Caro’s team dressed the horses up in zebra-print coats to see if the flies would behave differently with faux-zebras. The horses were also outfitted in other solid-colored coats of different colors for the experiment.

Again, the flies avoided touching down on the striped pattern compared to the uniform-colored pattern. When the horses wore stripes, flies landed as often on their uncovered heads as when the horses were not wearing the disguises.

The team said that the experiment corroborated an “emerging consensus among biologists that the primary function of contrasting black and white stripes on the three species of zebras is to thwart attack from flies,” according to the paper.

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