Why do zebras have stripes? Scientists have asked this question for more than 100 years. Today, the best answer is that stripes keep away flies that bite and drink blood. These flies often cannot land well on striped bodies.

In Japan, scientists painted black and white stripes on cows. Fewer biting flies stayed on the striped cows. This study won an Ig Nobel Prize in 2025.

Scientists have offered many explanations for zebra stripes, such as hiding from lions or keeping cool. In 2014, Tim Caro and his team compared five ideas at once and found that stripes were most closely linked to areas where biting flies are active. In 2019, they filmed flies around zebras and horses in Britain.

The flies approached both animals in the same way, but near zebras they often failed to slow down and land. To test this idea, a Japanese team painted stripes on six black cattle. Cows with black and white stripes had clearly fewer biting flies than cows with only black stripes or no paint. This shows that the pattern, not the smell of the paint, made the difference.

The function of zebra stripes has puzzled naturalists since the time of Darwin and Wallace. Proposed explanations include camouflage, confusing predators, temperature control, and social signaling. In 2014, Tim Caro and colleagues tested five hypotheses together across zebras, horses, and their relatives and found that striping was most strongly associated with regions of intense biting-fly activity; none of the other ideas received consistent support.

A 2019 field study in Britain clarified the mechanism. Horseflies approached zebras and horses alike from a distance, but near zebras they frequently failed to decelerate and either bumped into the animal or touched it briefly without landing. Horses wearing striped coats were similarly protected, except on their uncovered heads.

In another study, a Japanese team painted six Japanese Black cattle and compared black-and-white stripes, black stripes only, and no paint. The black-and-white cows carried markedly fewer biting flies and performed fewer fly-repelling movements, and the work later received a 2025 Ig Nobel Prize. However, the 2014 analysis was correlational, and the cattle trial covered only six animals over a brief period, so recommending stripes for livestock awaits larger, longer studies.