Why do zebras have stripes? The answer is smaller than you think.

Ask ten safari guides why zebras have stripes and you will get at least four answers. The stripes break up the animal's outline in long grass. They dazzle a charging lion so it cannot pick one zebra from the herd. They act as a cooling system, with black and white bands heating unevenly to set up tiny air currents. They let zebras recognise one another. Each sounds plausible. Each has been tested. Only one survives.

Testing the theories against the map

In 2014 a team led by biologist Tim Caro at the University of California, Davis, took an unusual approach. Instead of watching zebras, they looked at where striping occurs across all seven living species of wild horse, zebra and ass, and how strong it is on different parts of the body. Then they laid those patterns over maps of everything the theories predicted should matter: woodland cover, the ranges of lions and hyenas, temperature, and the distribution of biting flies.

Camouflage failed. Striping did not line up with wooded habitat. Predator confusion failed too; stripes were no stronger where lions were more common. Temperature showed no clear relationship. The one variable that tracked striping almost perfectly was the presence of tabanid horseflies and tsetse flies. Where the flies' breeding conditions were best, the animals were most heavily striped, and the striping was strongest on the legs and face, where flies prefer to bite.

Then they watched the flies

A map is suggestive, not proof. So in 2019 Caro's group ran the experiment. On a farm in England they filmed horseflies approaching captive zebras and domestic horses kept in the same paddocks, and then dressed horses in striped, black and white coats. The flies circled zebras and horses at similar rates, but their landings told a different story. On zebras, and on horses wearing striped coats, the flies failed to slow down on their final approach. They bumped into the animal or veered off at the last moment, landing a quarter as often as on plain coats. Somewhere in the last metre, the stripes seem to break the fly's ability to judge its landing.

Why would that matter enough to drive the evolution of such a striking pattern? Because in Africa, horseflies and tsetse flies carry diseases that kill equids: trypanosomiasis, equine infectious anaemia, African horse sickness. And zebras have unusually short hair, which leaves their skin more exposed to a fly's mouthparts than a buffalo's or an antelope's. A coat that quietly cuts fly bites by three-quarters is a survival advantage, generation after generation.

What to look for on safari

Next time you are at a sighting, look at where the stripes are boldest. On a plains zebra the legs and head carry the finest, most contrasting pattern, while the stripes fade on the rump and belly. That is not an accident of design. It is a map of where a fly would most like to land.

And if a guide tells you it is camouflage, you can be polite about it. The idea has been around for a century. It just happens to be wrong.