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Why Do Zebras Have Stripes?

AILSA CHANG, HOST:

And now a question that has puzzled biologists since the days of Darwin - why do zebras have stripes?

LEILA FADEL, HOST:

Martin How of the University of Bristol says there have been quite a few theories - camouflage, for one.

MARTIN HOW: Doesn't really make sense, though. They look so conspicuous.

CHANG: Another idea - comfort.

HOW: Maybe they're striving to generate convection currents on the skins and make them sort of cool down in that - in the hot weather.

FADEL: But How's team had a different idea after observing horses and captive zebras on a farm in the U.K. They saw that lots of flies were bothering the horses but leaving the zebras alone. And thus an experiment was born.

HOW: We then decided to essentially dress horses up as zebras and then look at the different effects that these had on the amount of biting flies coming to land.

CHANG: The conclusion, which they published last year - horses decked out in striped coats kept flies away, too, suggesting that the stripes themselves deter flies, not just a zebra's stench, for example.

FADEL: And repelling biting flies is no easy feat. Erica McAlister, curator of flies at the Natural History Museum in London, says female flies can be quite tenacious.

ERICA MCALISTER: Most of the times, the bloodmeal is not for them. It's for the development of their offspring. So these are mothers hell-bent on getting a proper start in life for all these eggs you see.

FADEL: Which raises another question - how do stripes keep these hell-bent mothers away?

CHANG: Well, Martin How has a few hypotheses for that.

HOW: One of which is that these stripes generate an optical illusion in the eyes of the flies.

CHANG: You see, he says stripes might create an illusion that, to a fly's eyes, looks something like a rotating barbershop pole, and that confuses them.

FADEL: Now in a new study, they tested that idea by dressing horses in striped coats or checkered coats, which wouldn't generate that illusion.

CHANG: And it turns out checkered patterns repel flies, too, putting to rest the barber pole idea. The work appears in the Proceedings of the Royal Society B.

FADEL: McAlister says the study makes clear just how little we know about fly perception.

MCALISTER: It's thrown out more questions than it has resolved any ancient riddle.

CHANG: But the work does have one huge implication.

HOW: Our new findings suggest that zebras could equally be checkered.

CHANG: Which makes it all the more impressive that zebras earn their stripes. Transcript provided by NPR, Copyright NPR.