The team measured stripe widths from different body parts on all three zebra species, and used published data to estimate the acuity of lion and hyena eyes. They then calculated how good those predators are at resolving zebra stripes at different distances and light levels.

They found that in daylight, humans with 20/20 vision can resolve zebra flank stripes from around 180 meters away. By contrast, lions can only do so at 80 meters, and hyenas at 48 meters. Those values get much worse for Grevy's zebra (the species with the thinnest stripes), for leg stripes (which are also thinner), and at darker times of day. At dawn and dusk, lions, and hyenas can only resolve zebra stripes at 46 meters and 26 meters respectively.

“At most distances, the zebras are going to look to a lion like a gray waterbuck,” says Caro. “Those stripes are going to fuse together and be indistinguishable.”

That rules out both the blends-among-trees idea and the breaks-up-outline one—neither can possibly be true if the predators can't see the stripes. “If the stripes are doing something exciting, they’ll be doing it close up, by which point the predators have probably realized the zebra is there, because they can smell or hear it,” says Caro. Zebras, being very noisy browsers, are hardly stealthy.

“It’s the first proper test of a very longstanding and prominent idea,” says Martin Stevens from the University of Exeter, who studies camouflage. Its only flaw is that the team didn't specifically measure how closely a zebra matches its background environment, in either color or brightness. Still, “I very much doubt zebra stripes do work in concealment,” adds Stevens.

So, if not camouflage, then what?

Caro, who has been studying zebras for a decade and has written a forthcoming book about their stripes, thinks he knows the answer. “I’ve come to the conclusion that really, it just has to be biting flies,” he says.

In Africa, horses are plagued by horseflies and tsetse flies. Collectively, these insects can drink up to half a liter of blood a day, and they spread deadly diseases like sleeping sickness, equine influenza, and African horse sickness. And for some reason, these blood-sucking insects don't like to land on black-and-white stripes. Gabor Horvath from Eotvos University showed that through several experiments, including one where his team stuck several painted horse models in a fly-infested field.

Caro found more evidence to support this idea. First, he mapped the geographic ranges of all seven species of wild horse—the three striped zebras, the African wild ass with thin stripes on its legs, and the uniformly colored Asiatic wild ass, Przewalski's horse, and kiang. Then, he compared these ranges to other factors, including lion and hyena distributions, habitat, temperature, herd size, and the presence of biting flies.