Why do zebras have stripes?

Zebras most likely have stripes to keep biting flies off. Of all the explanations biologists have tried, that is the one the evidence supports best. Horse flies and tsetse flies are more than an itch for zebras: they spread diseases that can kill horses and their relatives. And flies, it turns out, hate landing on stripes.

The classic answer used to be camouflage. The naturalist Alfred Russel Wallace thought stripes made zebras harder to see at night. But zebras graze in open grassland, they are noisy and social, and they don't freeze when a predator is near. A 2016 study found that lions and hyenas can't even make out the stripes from a distance, so the stripes are useless for blending in. And lions have no trouble catching zebras when they get close.

The flies, meanwhile, were telling a clear story. As early as 1930 a biologist noticed that flies prefer to land on plain surfaces rather than striped ones. In 2019 Tim Caro and his colleagues watched flies around zebras and horses. The flies approached both, but they failed to land on the zebras, and they also failed on horses dressed in zebra-print coats. Exactly how stripes confuse flies is still a mystery, but the effect is real.