Why do zebras have stripes?

Several hypotheses have competed for over a century: camouflage in tall grass, confusing predators through motion dazzle, social recognition among individuals, and thermoregulation. Systematic testing has eroded most of them. Lions and hyenas see poorly enough at the distances involved that stripes provide little concealment, and studies of predator behavior have not found the expected effect. Motion dazzle experiments have produced mixed and largely unsupportive results. Thermoregulation studies have not found consistent temperature differences.

The fly hypothesis has accumulated the strongest evidence. Field experiments in which horses were dressed in striped coats found that horseflies approached at normal rates but failed to decelerate properly on final approach, colliding with the animal or veering away rather than landing, apparently because the stripes disrupt the optical flow cues the insects use to judge distance. Comparative work has also found that stripe patterns across equid species and populations correlate with the distribution and seasonal activity of biting flies more strongly than with predator density, temperature, or habitat type.

Why this matters more than it sounds: biting flies are not merely irritating in Africa but transmit trypanosomiasis and other diseases lethal to equids, so a defense against them is a substantial fitness advantage. Zebras also have shorter hair than many other African mammals, which may make them more vulnerable to fly bites and increase the value of the defense. The finding has been applied practically, with striped fly rugs marketed for horses. It is worth noting that the explanations are not mutually exclusive, and secondary benefits from social recognition or predator confusion may exist alongside the primary function without being what drove the trait's evolution.