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For years, we’ve been taught that zebras’ stripes protect the animals from predators—lions are unable to see individual zebras moving in a large herd because the stripes provide camouflage. But UC Davis researchers wanted to see the evidence.

Charles Darwin and Alfred Russel Wallace may have been some of the first scientists to wonder about zebras’ unusual coloring and pattern adaptation 120 years ago. Over the years, five major hypotheses were posed: that zebras’ stripes exist to provide camouflage; disrupt predatory attack by visually confusing carnivores; act as a mechanism of heat management; serve a social function; and finally, to avoid ectoparasite attack, such as from biting flies.

Tim Caro, a UC Davis professor of wildlife biology, and his colleagues decided to try all of the hypotheses out, and not just on the three species of African striped zebras, but also on their unstriped Asian relatives and the closely-related African wild ass.

The team mapped the geographic distributions of the seven different species, noting the thickness, locations, and intensity of their stripes on several parts of their bodies. Next, they compared these animals’ geographic ranges with different variables, including woodland areas, ranges of large predators, temperature, and the geographic distribution of tsetse flies and tabanid biting flies. Finally, they examined where the striped animals and these variables overlapped.

After analyzing the five hypotheses, the scientists ruled out all but one: avoiding the blood-sucking flies. “I was amazed by our results," says Caro. “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.”

Why would zebras evolve to have stripes whereas other hooved mammals did not? The team discovered that, unlike other African hooved mammals living in the same areas as zebras, zebra hair is shorter than the mouthpart length of biting flies, so zebras may be particularly susceptible to annoyance by biting flies.

“No one knew why zebras have such striking coloration,” Caro says. “But solving evolutionary conundrums increases our knowledge of the natural world and may spark greater commitment to conserving it.”

The study is published this week in Nature Communications.

Image: Caro et al.

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