Ever wonder why zebras have stripes? Scientists have too, and now they have an answer. Researchers going as far back as Charles Darwin have offered a number of theories about how stripes might benefit zebras. Did they develop their unusual multi-hued coats as camouflage to help deter predators? To keep cool beneath the harsh African sun? Do their stripes help them identify each other? A new study topples all of those theories, leaving just one still standing. As it turns out, stripes are an excellent bug repellent---at least for zebras.
Researchers from the University of California at Davis knew that certain flies avoid black and white surfaces, so they wondered: Could zebra stripes have evolved to keep the animals free from suffering the bites of those very same flies, which can carry fatal diseases? To tackle that question, the researchers examined the distribution of zebras and the locations of the best breeding grounds for the stripe-averse flies. Sure enough, they found that they overlap. The same was true for other animals in the horse family that had stripes on various parts of their bodies. "I was amazed by our results," said lead author Tim Caro, a wildlife biologist at UC-Davis. "Again and again, there was greater striping on areas of the body in those parts of the world where there was more annoyance from biting flies." The study, published yesterday in Nature Communications, builds on a 2012 study by researchers at Sweden's Lund University that used horse models painted black and white to test whether flies would avoid stripes.
But why are stripes anathema to flies? It turns out that flies find stretches of water to mate and lay their eggs in by looking for the horizontally polarized light it reflects. Zebra stripes, however, are vertical and reflect polarized light differently, which makes them unappealing to horseflies. Unlike other African mammals, zebras have shorter hair that flies may be able to penetrate more easily to bite into the skin. Researchers have yet to test their bug repellent theory in the wild, however. Zebras have striped insect armor, but they also emit odors that may attract flies, potentially canceling out the benefits of their striped coats.
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