Zebra stripes have fascinated people for millennia, and there are a number of different theories to explain why these wild horses should be so brightly marked. A handful of laboratories around the world – including one lead by UC Davis wildlife biologist Tim Caro – have been putting these theories to the test. A new paper from Caro’s group, led by Ken Britten at the UC Davis Center for Neuroscience, puts a hole in one idea: that the stripes confuse biting flies by breaking up polarized light.

The paper is published in the May 25 issue of PLOS ONE.

Britten said Caro brought him in as a vision specialist on the project to understand why flies would or would not be attracted to zebra stripes. Horse flies and tsetse flies are common in the regions of Africa where zebras live. Not only do they suck their host’s blood, but they also carry diseases – for example tsetse notoriously spread trypanosome parasites that cause disease in wild animals, livestock and people (the human disease is known as sleeping sickness). Horse flies in particular can carry diseases fatal to equids.

It’s also known that many flies, mosquitoes and other insects are attracted to dark objects. Work by Hungarian researchers suggests that light reflected from black shapes is polarized. Unlike humans, many insects can distinguish polarized light, so this might be the signal that they use to track prey. The Hungarian team proposed that the white stripes break up the polarization pattern from black stripes, thereby confusing the flies.

Lab experiments with black and white spheres support the idea, but what do flies actually see under field conditions in Africa? Caro, Britten and UC Davis undergraduate Timothy Thatcher put this to the test by photographing wild zebras in Tanzania. Using a polarizing filter, they were able to measure the polarization of light glinting off the stripes of real zebras under field conditions.

Light reflected from black stripes was indeed polarized, they found. But, to their surprise, so was the light reflected from white stripes, although not quite to the same degree. Overall, they found that the black and white pattern made very little difference to the polarization signature of the zebra – not enough for an approaching fly to see, anyway.

“It’s a pretty strong blow to the polarization hypothesis,” Britten said. “It does not appear to be true in the field, in Africa.”

Nonetheless, there’s still strong evidence that the stripes are an anti-fly defense so some other mechanism must be involved. From a fly’s-eye point of view stripes might break up the image of a large, dark animal so it looks like lots of smaller objects and not an animal at all, Britten speculated. We still need to know more about fly vision and how they track their prey, he said. The key to zebra stripes may lie in the eyes and minds of flies.

The work was supported by the National Eye Institute (NIH), UC Davis and the National Geographic Society.