Why is the sky blue? Why is the grass green? Why do zebras have stripes? A team of scientists at the University of California, Davis, has only recently discovered the answer to the last question. Lead author Tim Caro, a professor of wildlife biology, has solved the conundrum, which has caused debate amongst luminaries like Charles Darwin for over 100 years. The team’s findings concluded that the purpose of the stripes is most likely to protect zebras from harmful biting flies. Caro explains why there’s more to the mystery than meets the eye.
Was it easy to dismiss the common theory that the stripes are for camouflage?
I went in agnostic about it. I didn’t know what was going to come out of the analysis. When we saw the results of the statistical model we found that biting flies were the factor that was associated with intense striping across the different species of horses, asses and zebras. It was easy to dismiss everything except these biting flies.
The stripes are more densely packed on a zebra’s head. Why is this?
My hunch is that most biting flies cruise between 30 centimeters to a meter — around the legs or belly of domestic livestock or about waist level for us. We know from experimental studies that when a horse or zebra has its head down grazing, these flies find it harder to land on thin stripes rather than thicker stripes.
In a wider sense, why is this finding significant?
The discovery is interesting from a biological standpoint because the issue has been talked about for about 100 years. Also, as a child, I was introduced to natural history by being told the colors of giant pandas, giraffes and zebras but never told why they were colored the way they are. For me, that was the missing piece. If we can make story richer for school kids it might attract more of them to be thinking about nature.
Why do insects avoid stripes?
We don’t know the answer to that. There’s something in the visual system of the flies that is befuddling to these eyes. That’s the next sort of area of research that our study has set up.
What’s the next discovery?
We’ve moved the debate from why a zebra has black and white stripes but we don’t know why equids (horse family) are so susceptible to biting flies. It could be because of the diseases that are carried or the potential for extensive blood loss if they’re plagued by hundreds of flies in Africa. We measured the thickness of coats of zebra and other species of antelope that live in the same places as zebras in African National Parks and we found that a zebra's coat is much thinner. The mouth-parts of these flies can easily penetrate their skin. If they didn’t have black and white stripes they might really suffer from biting fly annoyance.