Why Zebras Have Stripes

A herd of zebras stand in a field in Africa munching on grass. Nearby, a horsefly buzzes around looking for something to eat. These insects bite animals and drink their blood. But for some reason, the fly doesn’t bother the zebras. Could it have something to do with their stripes?

“We grow up learning pandas are black and white, leopards are spotted, and zebras are striped,” says Tim Caro, a biologist at the University of California, Davis. “But not enough people ask ‘Why?’”

Some researchers have suggested that the zebra’s black-and-white pattern distracts predators, allowing zebras to escape attacks. Alternatively, some people thought the adaptation might somehow keep zebras cool. Still others, like Caro, wondered whether a zebra’s stripes warded off biting flies. After studying zebras for more than a decade, Caro believed this was the most plausible hypothesis—he just needed to design an experiment to investigate it.

A herd of zebras munches on grass in a field in Africa. Nearby, a horsefly buzzes around. It’s looking for something to eat. These insects bite animals and drink their blood. But the fly doesn’t bother the zebras. Why? Could it have something to do with their stripes?

“We grow up learning pandas are black and white, leopards are spotted, and zebras are striped,” says Tim Caro. He’s a biologist at the University of California, Davis. “But not enough people ask ‘Why?’”

Some researchers thought that the zebra’s black-and-white pattern might distract predators. That could allow zebras to escape attacks. Other people thought the adaptation might somehow keep zebras cool. But Caro and others wondered if a zebra’s stripes keep biting flies away. Caro had studied zebras for more than a decade, and he believed this was the most likely hypothesis. He just needed to design an experiment to look into it.

Horseflies can be found all over the world. Some of the insects carry dangerous diseases, which they can pass on as they feed. But zebras rarely catch these illnesses. To find out why, Caro and Martin How, a biologist at the University of Bristol in the United Kingdom, visited a farm that raises both zebras and horses. They wanted to see if the flies interacted differently with the two types of animals.

The team videotaped the flightpaths of horseflies near the animals. From a distance, the flies seemed equally attracted to zebras and horses. But when the scientists watched the videos—zoomed in and in slow motion—they noticed two different behaviors: When a horsefly was about to land on a horse, it slowed down. But when a fly neared a zebra, it didn’t slow its flight (see Buzz Off). “It flies past the zebra or bounces off it,” says Caro.

Horseflies live all over the world. Some of them carry dangerous diseases. The insects can pass these diseases on as they feed. But zebras rarely catch these illnesses. To learn why, Caro teamed up with Martin How, a biologist at the University of Bristol in the United Kingdom. They visited a farm that raises both zebras and horses. Would the flies act differently with the two types of animals?

The team videotaped horseflies flying near the animals. From far away, the flies seemed just as attracted to zebras as to horses. But then the scientists watched the videos, both zoomed in and in slow motion. They noticed something different. When a horsefly was about to land on a horse, it slowed down. But when a fly neared a zebra, it didn’t slow down (see Buzz Off). “It flies past the zebra or bounces off of it,” says Caro.

Next, Caro and How covered horses at the farm with three differently colored blankets: black, white, or black-and-white striped like a zebra’s coat. These outfits allowed the biologists to observe whether different coat colors and patterns caused horseflies to behave the way they did.

Caro found that when a horse wore a zebra pattern, it attracted fewer flies than horses wearing an all-white or all-black blanket. And while flies mostly didn’t land on the bodies of horses dressed as zebras, they did land on their uncovered heads, which lacked stripes.

The evidence Caro collected showed that stripes discouraged flies from landing. Scientists don’t yet know exactly why stripes help keep flies away. That’s just one more mystery waiting to be solved.

Next, Caro and How covered horses at the farm with blankets. The blankets were either black, white, or black-and-white striped like a zebra’s coat. The biologists wanted to see how these outfits affected the horseflies. Did different coat colors and patterns cause them to behave differently?

Caro found that a horse wearing a zebra pattern attracted fewer flies. Horses wearing an all-white or all-black blanket attracted more. And flies mostly didn’t land on the bodies of horses dressed as zebras. But they did land on their bare heads, which lacked stripes.

Caro’s evidence showed that stripes discouraged flies from landing. But why do stripes help keep flies away? Scientists don’t know yet. That’s just one more mystery waiting to be solved.