How did zebras get their stripes?

In our world where so much is manufactured – or created by film magic – it’s hard to remember how astonishing nature has been in the creation of so many of her creatures. The exquisite stripes on zebras, for example, must have seemed miraculous to the first Europeans who saw them. Now some European scientists have presented a mundane explanation for this miracle of nature, saying that zebras’ stripes exist to stave off blood-sucking horseflies. This explanation, as it turns out, is one of many possibilities.

Ádám Egri, Gábor Horváth, Susanne Åkesson and colleagues from Hungary and Sweden present their discovery that zebra stripes are the least attractive hide pattern for voracious horseflies in the Journal of Experimental Biology, in the March 1, 2012 issue.

These scientists point out that horseflies (tabanids) “deliver nasty bites, carry disease and distract grazing animals from feeding.” The team asked themselves how the zebra’s stripes might make them less appealing to bloodsuckers, such as horseflies. The team traveled to what they said was a “horsefly-infested horse farm near Budapest” to test how attractive horseflies found black-and-white stripes.

In their experiments, they varied the width, density and angle of a number of stripes, as well as the direction of polarization of the light that the striped patterns reflected (polarization describes the way the light waves are oriented). To count the number of horseflies that were attracted, the scientists trapped the insects with oil and glue.

The team found that the striped patterns attracted fewer flies as the stripes became narrower, with the narrowest stripes attracting the fewest horseflies. There was more to the study, which you can read here. But, overall, the conclusion was that:

… zebras have evolved a coat pattern in which the stripes are narrow enough to ensure minimum attractiveness to [horseflies].

Meanwhile, many other explanations for zebras’ stripes remain. It is likely that, with so much in nature, a zebras’ stripes evolved in response to multiple factors.

There is the obvious reason: camouflage. A zebra’s stripes might help to hide it in the tall grass or dead wood of an African savanna. Lions, the zebras’ primary predator, might be colorblind, which would reinforce this idea.

A related idea is that, since zebras are herd animals, the stripes might help confuse predators. A lion might have trouble picking out a single zebra from a herd of zebras fleeing in front of it, as the great never-ending game of hunter versus hunted plays out on the savanna.

Or consider that zebras, like snowflakes, each have their own individual pattern. Can zebras recognize each other from their stripes, and is this recognition among fellow zebras one reason the stripes evolved? Personally, I like this explanation best.

By the way, scientists used to think that zebras were white animals with black stripes, because the underbellies of some zebras are white. But it turns out that zebra embryos start out with a dark skin, and go on to develop white stripes before birth. So they are, essentially, dark creatures.

Bottom line: There are many explanations for a zebra’s stripes. A recent one comes from scientists in Hungary and Sweden. Ádám Egri, Gábor Horváth, Susanne Åkesson and colleagues say that that zebra stripes are the least attractive hide pattern for voracious horseflies in the Journal of Experimental Biology, in the March 1, 2012 issue.

August 23, 2012

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Deborah Byrd

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