Comedy maestro Bill Bailey has a song about zebras, in which he casts their black and white stripes as a message of racial harmony. (“In a world of confusion/ We all need a sign/ If only we could live side by side/ Like the stripes down a zebra’s spine.”) Which, honestly, makes about as much sense as the most commonly cited hypothesis about zebra stripes—that they're a form of camouflage.
“The zebra is conspicuously striped,” wrote Darwin, on one of his less insightful days. The idea that its black-and-white coat might help it blend in rather than, say, stand out seems preposterous, but there are two ways in which this could work. First, the black stripes could match dark tree trunks while the white ones match shafts of light between the trunks. Alternatively, the stripes break up the zebra's outline, making it harder to identify as a juicy piece of horse-shaped steak. Both ideas have been around for a while, but neither has been tested well.
The problem is that we've always looked at zebras through the wrong eyes—ours. Human eyes are exceptionally good at resolving detail in daylight, so “we have a very odd appreciation of the coat of a zebra,” says Tim Caro from the University of California, Davis. By contrast, their main adversaries—lions and hyenas—have eyes with poorer resolution, but greater sensitivity at dawn, dusk, and darkness. So Caro, together with Amanda Melin from the University of Calgary worked out what zebras look like to these predators.