How The Zebra Got Its Stripes

10 Feb

I’ve long been fascinated with zebras. They don’t sound quite like you’d expect and their elegant black and white stripes make individuals difficult to distinguish among a group.

Now scientists in Hungary and Sweden have come up with a theory that those stripes evolved to stave off blood-sucking horseflies that carry disease and distract animals from feeding.

Gábor Horváth and his colleagues have published new research in the Journal of Experimental Biology suggesting why:

[T]hese insects are attracted to horizontally polarized light because reflections from water are horizontally polarized and aquatic insects use this phenomenon to identify stretches of water where they can mate and lay eggs. However, blood-sucking female tabanids are also guided to victims by linearly polarized light reflected from their hides. Explaining that horseflies are more attracted to dark horses than to white horses, the team also points out that developing zebra embryos start out with a dark skin, but go on to develop white stripes before birth. The team wondered whether the zebra’s stripy hide might have evolved to disrupt their attractive dark skins and make them less appealing to voracious bloodsuckers, such as tabanids.

At a horsefly-infested horse farm near Budapest, they tested their idea using black and white striped patterns that varied by width, density, angle of the stripes, and direction of polarization of the light reflected. They found that patterns attracted fewer flies, which became most evident as stripes became narrower. On top of that, the stripe polarization patterns of reflected light from real zebra hides correlated well with the patterns least attractive to horseflies.

While it may sound like one of Rudyard Kipling’s Just So Stories, perhaps the zebra got its stripes to fool the horsefly.

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