Zebras are black and white to help protect them from insect bites, scientists say.
Researchers from the University of California, Davis, believe zebras' unusual monochrome markings evolved in order to repel biting insects, such as horseflies and tsetse flies, which tend to avoid striped surfaces," said Tim Caro, lead author and professor of wildlife biology at the university.
"The extent of polarised light reflected also depends on the nature of the surface – think of gloss and matte paint – and hairs probably reflect polarised light in different ways.
"So it may be that the different hairs of the zebra's pelt are important in preventing flies from landing on them."
Varying explanations for zebra stripes, which have been proposed since Alfred Russel Wallace and Charles Darwin debated the problem 120 years ago, have included a form of camouflage, assisting escape from predators by visually confusing them, heat management or some kind of social function.
The study, published in the journal Nature Communications, mapped the geographic spread of seven different species of zebras, horses and asses and their subspecies and recorded the thickness, location and intensity of their stripes on several parts of the body.
It compared the animals' geographic reach with variables such as woodland habitats, the range of predators, temperatures and the numbers of ectoparasites such as tsetse flies.
After examining where the striped animals and variables overlapped, the scientists ruled out all but one of the existing explanations, that of avoiding bloodsucking flies.
"I was amazed by our results," said Professor 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."
While the distribution of tsetse flies in Africa is well known, the researchers did not have maps of tabanids, which include horseflies and deer flies, so they mapped locations of the best breeding conditions for these insects.
They found zebra striping was highly associated with several consecutive months of ideal conditions for tabanid reproduction.
Unlike other African hoofed mammals living in the same areas as zebras, zebra hair is shorter than the mouthpart length of biting flies, making them particularly susceptible to these insects, the team found.
"No one knew why zebras have such striking colouration," Professor Caro said, adding that it was not yet known why biting flies avoided striped surfaces.
"But solving evolutionary conundrums increases our knowledge of the natural world and may spark greater commitment to conserving it."
The scientists said the findings could also help protect humans from insect bites, although they cautioned that the type of surface and material could alter the stripes' effectiveness.
"A T-shirt may help somewhat but it might not be the whole story. Certainly if you are going to buy a T-shirt make sure the stripes are thin," said Professor Caro.
"Don't buy a striped jumper too quickly."