The zebra got its stripes to ward off flies

Zebras’ stripes have baffled biologists since Charles Darwin. Many hypotheses have been proposed regarding their purpose, but despite hundreds of years of study, there remains disagreement. In an attempt to end the debate, researchers have pitted various models against each other and systematically analyzed data from past studies. Their results reveal the one reason zebras have stripes: to ward off flies.

A handful of ideas regarding zebras’ stripes have found some support among biologists. One proposed that the dark and light bands change how air flows around a zebra’s body and help in heat management, which could go a long way in the hot tropical areas that zebras live in. Another proposed that the stripes are used by zebras as a way of social interaction. They may use them to identify other zebras and for bonding as a group in the wild. A third proposal suggested zebras use the stripes as camouflage. While stripes are clearly visible in the day, there was some thought that they helped hide the animals at dawn, dusk, and in the night.

All these ideas were shot down when tested rigorously. Two others, however, remained intriguing.

The first was that the stripes are used to dodge predators. It’s called the “motion dazzle hypothesis,” and it suggests that predators are confused by zebras’ stripes and cannot understand their movement. Research published in the journal Zoology in 2013 used a simulated visual system to show that zebra stripes do interfere with visual perception. But this is a difficult hypothesis to test in the field.

Martin Stevens at the University of Exeter has researched the motion dazzle hypothesis by getting human subjects to catch moving striped objects on a computer. “It’s an artificial experimental system,” he admitted.

The second proposal was that stripes helped keep flies at bay. Zebras are especially susceptible to biting flies due to the flies’ geographic spread. These flies, which include the tsetse fly, stomoxys stable flies, and tabanid biting flies, are particularly prevalent in areas with high temperature and humidity, exactly the areas where zebras are normally found.

Bites from these flies can be nasty—and quite literally draining. About thirty flies feeding for six hours on just one horse can draw as much as 100 mL of blood. The flies can usually number in the hundreds around one animal.

Zebras have shorter hair than other equids—the family that includes horses, donkeys and zebras—which may also increase their susceptibility to attack. Also, four diseases that are fatal to equids have been found in Africa, meaning that investing in anti-biting defenses such as stripes is especially important for zebras compared to non-African equids.

It’s possible that the dazzle effect acts on flies, deterring them from biting. “Stripes clearly have a number of functions,” Stevens said, “and these could be interacting in zebras.”

Revealing maps

In the new research, just published in Nature Communications, Tim Caro and his colleagues at the University of California in Davis didn’t perform experiments. Instead, they used ecological and observational data on zebras’ geographical locations and related factors. It’s the first time that a comparative approach has been applied to find the reasons for zebras’ characteristic coloration. Caro thinks his findings may have nailed the answer at last.