Several hypotheses have competed for over a century: crypsis in tall grass, dazzle confusion for pursuing lions, social recognition, and thermoregulation. The evidence has converged on biting-fly avoidance. Comparative analysis found that striping intensity across equid species and subspecies tracks the distribution and seasonal activity of tabanid flies more closely than it tracks predator density or temperature.
Direct experiments then tested the mechanism. When horses were dressed in striped coats and filmed, flies circled at the same rate as around plain coats but decelerated poorly on approach, colliding with the surface or veering away instead of settling. The disruption appears to occur in the final approach, where the striped pattern interferes with the visual cues the fly uses to judge distance.
The biting-fly cost is not trivial. Equids have thin summer coats, and tabanids transmit trypanosomes and other pathogens. What is still unclear is whether stripes deliver a secondary thermoregulatory benefit, since results on skin temperature over striped surfaces have been inconsistent.