We Finally Know Why Zebras Have Stripes

Why does the zebra have stripes? It's not a riddle — scientists say they've actually found an answer. And no, it's not camouflage for hiding under a slatted roof. The stripes actually serve as an insect repellent.

The study, published today in Nature Communications by a group from the University of California Davis led by Tim Caro, takes a look at the different major hypotheses proposed by biologists to explain the zebra's fashion-forward style, including camouflage, to confuse predators, to keep cool in the heat, social reasons, or pest control. But it was only for the final one, stripes as bug repellent, that they found a correlation.

To test for the correlation, researchers mapped out the location of zebras, horses, and asses along with a look at the type of stripes (if they had them at all) that were prevalent, including data on the number of stripes, their relative thickness, and where on the animal they were located.

Then, they ran that data against maps looking at factors including temperature variations, the location of biting flies, and predators. When they ran an analysis, they found that striping became heavier in areas where biting flies were more of a concern.

This isn't the first time that scientists have floated the stripes as pest control theory, as Alan Boyle at NBC notes :

A couple of years ago, the bug-repellent idea got a boost when researchers built horse mannequins, painted them in a variety of patterns, coated them with sticky stuff, and found that horseflies seemed to avoid landing on the fake horses that were painted with black and white stripes.

This wasn't an April Fool's joke, either.

The proposed explanation was that the flies preferred to land on dark surfaces. Such surfaces reflect the kind of polarized light that reminds the flies of the water or mud where they breed. Light surfaces aren't as attractive, but dark-and-light patterns are even worse — perhaps because such patterns confuse the flies' navigational sense.

The question of just what it is about stripes that makes biting flies turn away is still open.

You can check out the full paper at Nature Communications.

Image: Zebra in South Africa / Jack Zalium