Why Do Zebras Have Stripes?

A zebra munches on grass in a field in Africa. Nearby, a horsefly is buzzing around looking for food too. The insect bites a nearby animal and drinks its blood. The fly is tiny, but it can be a big problem. It can pass on deadly diseases as it feeds. Luckily for the zebra, the hungry fly doesn’t land on it. Why?

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

For at least 150 years, scientists have pondered why zebras have stripes. Some guessed the patterns distract predators, allowing zebras to escape attacks. Others thought the stripes might keep zebras cool. Caro followed these steps of the scientific process to try to solve the mystery.

Zebras live in parts of the world that have the most horseflies that carry disease. But most zebras don’t catch these diseases. Caro wanted to know: Do zebra stripes stop flies from biting? He designed a plan to find out.

Caro visited a farm in the United Kingdom where zebras, horses, and horseflies all lived. There he could compare how horseflies landed on horses and on zebras. He also dressed horses in different coats: one that was all black, one that was all white, and one that had a black-and-white zebra pattern. These outfits allowed Caro to observe how the horseflies reacted to the different colors and patterns.

His team used video cameras to record flies near the horses and zebras. The footage showed each fly’s flight path.

From a distance, the flies seemed equally interested in landing on zebras and horses. But when Caro and his team looked at the videos zoomed-in and in slow motion, they saw two very different behaviors: When a horsefly was about to land on a horse, it slowed down and extended its legs right before landing. But when a fly neared a zebra, it didn’t slow down at all. “It flies past the zebra or bounces off of it,” says Caro.

His team also noticed that when a horse wore a zebra pattern, flies didn’t land on the stripes. Instead, they landed on the uncovered head of the horse which didn’t have any stripes.

The scientists found that far fewer flies landed on zebras than on horses. And horses dressed up as zebras attracted fewer flies than horses wearing no coat or all white and all black coats.

As Caro suspected, evidence showed that the stripes discourage flies from landing. Scientists don’t know why the stripes help. But Caro’s findings show that the stripes are an adaptation that helps keep the flies away.