How zebras earned their stripes

Zebras evolved from all black to striped in order to repel insects that distract them from feeding, a new study contends.

Researchers from Hungary and Sweden said that zebras' black and white stripes are the least-attractive hide pattern to disease-carrying bloodsuckers known as tabanids or more commonly, horseflies.

The researchers found that horseflies are drawn to horizontally polarised light resembling reflections from water. They explained that this is how insects find areas of water where they can lay their eggs. Female horseflies also use the light reflected from animals' hides, particularly black hides, to detect their victims, the study noted.

Developing the white stripes

The study, published online in an issue of the Journal of Experimental Biology, pointed out that as embryos, zebras start out with dark skin. They go on to develop their white stripes before birth. The researchers suggested this was an evolutionary adaptation to help them avoid bug bites.

The researchers tested their theory at a horse farm near Budapest that was infested with horse flies. They altered the width, angle and density of the stripes and changed the direction of polarisation of the light they reflected. Using oil and glue, they trapped the insects to reveal which pattern attracted the most horse flies.

The study showed the narrower the stripes, the fewer flies they attracted. The researchers also tested the attractiveness of a white, dark and striped horse. The study found the striped horse drew the least flies.

"We conclude that zebras have evolved a coat pattern in which the stripes are narrow enough to ensure minimum attractiveness to tabanid flies," the researchers wrote in a journal news release.

(HealthDay News, February 2012)

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