Why zebras have black and white stripes is a question that has intrigued¹ scientists and spectators for centuries. A research team led by the University of California, Davis, has now examined this riddle² systematically³. Their answer is published April 1 in the online journal Nature Communications. The scientists found that biting flies, including horseflies and tsetse flies, are the evolutionary⁴ driver for zebra's stripes. Experimental work had previously⁵ shown that such flies tend to avoid black-and-white striped surfaces, but many other hypotheses for zebra stripes have been proposed since Alfred Russel Wallace and Charles Darwin debated the problem 120 years ago.

 

These include: A form of camouflage⁶ Disrupting predatory attack by visually confusing carnivores A mechanism⁷ of heat management Having a social function Avoiding ectoparasite（皮外寄生物） attack, such as from biting flies 

 

The team mapped the geographic⁸ distributions of the seven different species of zebras, horses and asses⁹, and of their subspecies, noting the thickness, locations, and intensity¹⁰ of their stripes on several parts of their bodies. Their next step was to compare these animals' geographic ranges with different variables, including woodland areas, ranges of large predators¹¹, temperature, and the geographic distribution of glossinid (tsetse flies) and tabanid (horseflies) biting flies. They then examined where the striped animals and these variables overlapped¹².

 

After analyzing¹³ the five hypotheses, the scientists ruled out all but one: avoiding blood-sucking flies.

 

"I was amazed by our results," said lead author Tim Caro, a UC Davis professor of wildlife biology. "Again and again, there was greater striping on areas of the body in those parts of the world where there was more annoyance¹⁴ from biting flies."

 

While the distribution of tsetse flies in Africa is well known, the researchers did not have maps of tabanids (horseflies, deer flies). Instead, they mapped locations of the best breeding conditions for tabanids（虻）, creating an environmental proxy¹⁵ for their distributions. They found that striping is highly associated with several consecutive¹⁶ months of ideal conditions for tabanid reproduction.

 

Why would zebras evolve to have stripes whereas other hooved mammals did not? The study found 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 susceptible¹⁷ to annoyance by biting flies.

 

"No one knew why zebras have such striking coloration," Caro said. "But solving evolutionary conundrums¹⁸（难题） increases our knowledge of the natural world and may spark greater commitment to conserving¹⁹ it."

 

Yet in science, one solved riddle begets²⁰ another: Why do biting flies avoid striped surfaces? Caro said that now that his study has provided ecological²¹ validity to the biting fly hypothesis, the evolutionary debate can move from why zebras have stripes to what prevents biting flies from seeing striped surfaces as potential prey²², and why zebras are so susceptible to biting fly annoyance.

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