Zebra stripes may "dazzle" blood-sucking flies as they are coming into land, new research has shown.
Scientists conducted an unusual experiment involving zebras and horses dressed in black and white striped coats.
They found that horse flies gathered around domestic horses and zebras at a similar rate – but landed on zebras a quarter as often.
When uniformly coloured horses were dressed in "zebra coats" the flies made far fewer landings on the striped areas but were not kept away from the uncovered head.
Video footage showed that the flies made uncontrolled approaches when faced with a striped landing strip.
Typically they came in too fast, often crashing into their prey or aborting the landing altogether.
Study leader Professor Tim Caro, from the University of California at Davis, US, said: "Once they get close to the zebras... they tend to fly past or bump into them. This indicates that stripes may disrupt the flies' abilities to have a controlled landing."
Dr Martin How, a member of the team from the University of Bristol, said: "Stripes may dazzle the flies in some way once they are close enough to see them with their low-resolution eyes."
In a similar way, human pilots can be dazzled when attempting to land into the sun.
The study took place on a UK horse farm in Somerset that keeps both domestic horses and zebras.
The purpose of zebra stripes has long been a mystery. Theories about their function have included camouflage, a means of confusing predators, a method of signalling other zebras, and a system of heat control.
More recent research has suggested that somehow the stripes reduce the chances of a zebra being bitten by flies.
As additional protection, zebras swish their tails almost continuously to keep flies off, the study found. If the flies are particularly persistent, they will stop feeding or attempt to flee from them.
In contrast domestic horses chiefly twitch to flick away flies and only occasionally swish their tails.
The research is reported in the journal Public Library of Science ONE.