Zebras’ bold striped patterns have puzzled scientists for nearly 150 years. Researchers have offered a lengthy list of possible explanations, from confusing predators by creating a distracting dazzle when a herd gallops away, to helping the animals avoid biting flies. Support for the dazzler hypothesis comes from computer tests using people, who have trouble tracking striped, moving objects on a computer; while other studies have shown that the flies prefer to land on uniformly colored, not striped, surfaces. Now, a team of scientists reports online today in Nature Communications that it has tested these hypotheses—as well as suggestions that the stripes might cool zebras down or make them more attractive to mates—to see which one makes the most ecological sense. The winner: those pesky, blood-sucking, disease-carrying (such as parasitic trypanosomiasis) biting flies. The team discovered that the ranges of the horse fly and tsetse fly species and the three most distinctively striped zebra species (Equus burchelli, E. zebra, and E. grevyi) overlap to a remarkable degree. They did not find a similar ecological match for any of the other hypotheses, not even those involving predators. Instead, the researchers argue that biting flies are the most likely reason that zebras, such as those shown above grazing in Tanzania’s Katavi National Park, evolved their distinctive ornamentation. The insects, they note, harass the equids almost year-round, and are known to torment domesticated horses in these areas. The zebras’ black-and-white patterns, which others have shown seem to interfere with the flies’ vision, at least give them a bit of a break. Why equids are so susceptible to the flies’ attacks remains mysterious, but, as the researchers found, the zebras’ short coats may make them particularly vulnerable, and the diseases the flies carry are often fatal.