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Why do zebras have stripes? New research offers answer.

Zebras' black and white stripes keep biting flies away, according to a new research.

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A zebra races with wildebeest across the tall grasses of the Masai Mara National Reserve in the southwestern corner of Kenya in this picture taken in 2003

Andy Nelson / The Christian Science Monitor

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The stripes on zebras are not just there for looks; they are an effective defense mechanism against blood-sucking flies, according to a new study.

The riddle of zebras' stripes has intrigued researchers for many years. Since the time of Alfred Russel Wallace and Charles Darwin, various hypotheses have been floated. Some scientists suggested that the stripes helped cool the zebras. Other suggested that the stripes were a form of camouflage, or a way of confusing predators. Others proposed that the stripes served a social function.

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And according to another theory, the stripes were effective in deterring ectoparasites such as biting flies. Early tests of this hypothesis employed striped cardboard pieces with glue stuck on them, to check if flies were attracted to them or not, says lead author Tim Caro, a UC Davis professor of wildlife biology.

To solve the puzzle, Dr. Caro and his team mapped out geographic distributions of the seven different living species of zebras, horses, and asses, including their 20 subspecies found in the Old World, and tested them against all the previous hypotheses.

The team first looked at the variation in striping patterns in different parts of the body – from their neck to face and so on, says Caro.

They then mapped their geographic distribution in the Old World. They also mapped distribution of their predators, such as lions and hyenas, along with the distribution of flies, forests, and temperatures, and other factors that might have brought about the evolution of the stripes.

Both the mapping distributions were compared to each other and fed into a statistical model to examine the "overlaps."

"We observed that stripped species have a great degree of geographical range overlap with biting flies," Cato told the Monitor.

And "conversely, there is no consistent support for camouflage, predator avoidance, heat management or social interaction hypotheses," according to the findings published in a paper titled "The function of zebra stripes" in the online journal Nature Communications on April 1.

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But to map distribution of biting flies such as, tabanids (horseflies, deer flies), the scientists had to create an "environmental proxy" for their distribution. Unlike tsetse flies, which can be found only in Africa, tabanids such as horseflies and deer flies live all over the world, except at the poles and Oceana.

But the hot and humid conditions in Africa are most conducive to large tabanid populations. After analyzing the distribution and mapping, the team noted significant overlap between tabanids and striping.

"I was amazed by our results," said 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."

Further research in this area will explore why such patterns dissuade biting flies. It could have to do with the way flies see things, says Caro. Also, susceptibility to ectoparasite attack could be due to short coat hair of zebras, blood loss, or disease transmission. But exactly how the flies harm zebras needs to be explored further, say researchers.