zebra stripes
When it comes to zebra stripes and flies, it’s all about optical illusions. Researchers know that zebra stripes deter flies and probably affect their vision, too. But what flies are—or aren’t—seeing when they spot a zebra still remains a mystery.

It appears, however, that researchers have eliminated one hypothesis from the list of possibilities: They recently determined that it’s unlikely that polarization is what confuses flies, said Kenneth Britten, PhD, of the Department of Neurobiology, Physiology, and Behavior at the University of California, Davis (UC Davis).

Polarization is a physics concept that—very simply put—describes the way the electric vectors in light waves move in certain distinct patterns. While humans can’t detect that polarization, horseflies (“tabanids”) can. Considered “polarotactic,” horseflies are very attracted to certain kinds of polarization. The horizontal polarization of water-reflected light, for example, leads them to water sources where they can mate, reproduce, and rehydrate. Linear polarization leads blood-sucking female horseflies to food sources such as equids and cattle.

For years, many researchers believed zebra stripes affected light polarization in a way that repelled flies. But scientists are now shedding a new angle of light on that theory.

Britten and his fellow researchers studied high-resolution digital images of 21 plains zebras in Tanzania. They found that while the stripes do affect polarization, they’re still more polarized—and hence more appealing to the flies—than surrounding dry grasses, for example. They also determined the flies have such poor eyesight that the polarization modifications wouldn’t make much of a difference to them from a distance of more than 10 meters (30 feet).

Instead, the stripes might create optical illusions to these flies’ “rudimentary” vision, Britten said. “We don’t know that for certain, of course,” he said. “It’s just another hypothesis that could explain the presence of stripes to deter flies. But there are some observations that suggest it. In particular, a model of how flies see motion predicts that stripes will bias their perception of motion. Since a fly landing on a surface uses the ‘looming’ of the surface to help it land, it might make mistakes on landing.”

But before getting to the point of landing, flies might avoid striped targets altogether from afar. “At greater distances, the stripes might cause the fly to not perceive the overall outline of the animal and maybe not approach it as it would a darker animal,” he said.

Whether this discovery could help develop fly-deterring methods for unstriped horses remains to be determined, he added.

Britten directed individuals interested in learning more about zebra striping to the recently released book Zebra Stripes, by Tim Caro, BSc, PhD, a professor of wildlife, fish, and conservation biology at UC Davis.

The study, “Zebras and Biting Flies: Quantitative Analysis of Reflected Light from Zebra Coats in Their Natural Habitat,” was published in PLoS One.