Zebras' Stripes Ward Off Flies

Feb 24, 2019

The question of why zebras have stripes goes back to the time of Charles Darwin.

One theory goes that they work as camouflage. Another theory is that they are an identification system that only zebras can understand.

Now we know that the stripes serve as an important protection against biting flies.

Tim Caro is professor of wildlife, fish, and conservation ecology at U.C. Davis. In a recent study, he looked at the intensity of striping across different species in the horse family.

“We found that the intensity of striping on those different species or subspecies was very closely related to the amount of biting fly activity over the course of the year,” Caro told Living Lab Radio.

Caro observed how flies interact with zebras at a conservation farm. He also watched horses wearing zebra-striped blankets. Then he watched how flies interact with solid-colored horses.

“When [the flies] are coming in to the horse, they slow down. They’re starting to decelerate from about half a second away before they make a controlled landing,” Caro said. “Whereas when they’re coming in to a striped pattern, be it a zebra or a striped horse coat, they continue to fly fast. They don't decelerate.”

The flies either fly right past the zebra or bounce off as if they haven’t seen it.

“And so, the zebras are doing something to affect the visual system of the flies just before they’re coming in to make a landing,” he said.

Should we all wear zebra stripes to avoid flies? Caro says he’s not yet ready to make that statement.

But he did clear up a question that we’ve had for years. Are zebras black with white stripes, or white with black stripes?

“In utero, they start off black…and then during development, while they’re in the mother’s womb, the white stripes start to appear.”

Here at Living Lab Radio, we’re going to call that black with white stripes.

Web post produced by Elsa Partan.

A horse wearing a zebra blanket.
Credit Tim Caro