Ever since Charles Darwin and Sir Alfred Russel Wallace developed the theory of evolution, naturalists have been hard at work figuring out why various animals evolved as they did, including why zebras are striped.

Over the last century, various theories have included:

• The stripes camouflage the zebras by confusing predators,
• The stripes lower body temperature or
• They aid in socialization.

But none of those theories have been proven scientifically.

The real answer is that the stripes protect zebras against biting flies. The stripes are protective because they interfere with the flies’ depth perception and interfere with their ability to land on the zebras’ bodies. It’s like the striped pattern creates an optical illusion from the flies’ perspective.

In the original research paper on this topic from 2014, scientists at the University of California at Davis discovered that striped equid species (the taxonomic family of horses) are found in geographies with high tabanid (biting fly) activity and non-striped equids live where there are few or no biting flies. In fact, the researchers found that “body stripe presence was ‘perfectly’ associated with the presence of [biting] fly distribution.”

Here’s a cool figure from the paper showing how the presence of biting flies and striped equids line up nearly perfectly (here’s a link to the full-sized image if you want to see it better).

The same University of California researchers performed a new series of experiments five years later by observing horses and zebras on a farm in Britain and how often flies bit them. They found that fewer flies landed on the zebras. Then, they draped horses in both striped and plain coverings and found that the striped coats prevented flies while the plain coats didn’t. Wow. But there’s more . . .

Building on the University of California studies, researchers in Japan studied cows and biting fly incidence. The researchers painted some cows so they were striped like zebras, painted some black (to make sure the paint wasn’t a fly deterrent), and left some unpainted. Their results? The cows painted with stripes experienced fewer fly bites. The researchers noted, “The apparent effects of the stripes were remarkable. The number of biting flies observed on zebra-striped cows was less than half the number seen on unpainted cows and far less than cows painted with black stripes.” Here’s a chart of their findings (CONT is no paint, B&W is striped, and B is cows painted black):

And, for fun, here’s a photo from the study of a cow painted with stripes:

Are biting flies that big of a deal? Yes. According to the University of California researchers, “in Africa where zebras live, tabanids carry diseases fatal to zebras including trypanosomiasis, equine infectious anemia, African horse sickness and equine influenza and zebras are particularly susceptible to infection because their thin pelage allows biting flies to probe successfully with their mouthparts.”

Of course, the biting fly theory may not be the only reason zebras have stripes. While there is some evidence that the theory that stripes act as camouflage is not correct, scientists are still researching that possibility as well as whether the stripes provide thermoregulation benefits.