Researchers at the University of Bristol have found why zebra fur is thinly striped and sharply outlined.

Their findings, published Feb. 17 in the Journal of Experimental Biology, reveal that stark black-white distinctions and small dark patches are particularly effective in thwarting horsefly attack. These characteristics specifically eliminate the outline of large monochrome dark patches that are attractive to horseflies at close distances.

The team theorizes that the thin back stripes serve to minimize the size of local features on a zebra that are appealing to the biting flies.

The research was led by Professor Tim Caro and Dr. Martin How, both from the University of Bristol’s School of Biological Sciences.

Prof. Caro explained, “We knew that horseflies are averse to landing on striped objects—a number of studies have now shown this, but it is not clear which aspects of stripes they find aversive.

“Is it the thinness of the stripes? The contrast of black and white? The polarized signal that can be given off objects? So we set out to explore these issues using different patterned cloths draped over horses and filmed incoming horseflies.”