The reason zebras have stripes have stumped biologists since Darwin’s day. Early guesses included camouflage, cooling, and social signaling. But recent studies point to one function above all others — a defense system against biting flies.

Not For Camouflage

It’s obvious that zebras are highly visible on the savanna. Their bold black-and-white stripes contrast with the dry grass and acacia trees. They spend little time in wooded areas where disruptive patterns might help.

There is a long standing theory that zebra stripes make it harder for predators to single out a target in a fleeing herd. Tim Caro, a zoologist at UC Davis and author of Zebra Stripes, gathered extensive evidence against the camouflage theory. Predators like lions can detect and kill zebras with ease. Zebras don’t rely on stealth. They rely on speed. Caro’s data shows no reduction in zebra predation linked to stripes.

Daniel Rubenstein at Princeton believes more work is needed. He’s testing lion responses to striped vs. solid decoys. But for now, there’s no concrete evidence that stripes confuse big cats.

Stripes Repel Flies

Flies, particularly biting ones like horseflies and tsetse flies, avoid striped surfaces.

In a 2019 study published in PLOS ONE, Caro and colleagues ran a clever experiment. They placed striped coats on domestic horses. The result: far fewer flies landed on striped horses compared to those in solid-colored coats. The flies hovered just as often but failed to land.

“Something is stopping the fly from realizing that it’s close to making a landing,” said Caro. “Stripes are exerting an effect to the very last second.”

A 2014 study by Caro’s team also found that striping is most intense in regions with high fly density. This geographic pattern supports the biting fly deterrent hypothesis over all others.

Some scientists thought zebra stripes worked by creating a “barber pole illusion.” This illusion happens when diagonal stripes seem to move in strange ways as an observer gets closer, like on the spinning poles outside old barbershops. The theory was that the flies’ visual system became disorientated during approach, disrupting their ability to land.

That theory was tested in another study led by Martin How of the University of Bristol. He and his team dressed horses in two types of patterned coats: traditional zebra-style stripes and a black-and-white checkered pattern. Both patterns repelled flies equally well, disproving that stripes alone produced a “barber pole” illusion. It showed that the disruption probably isn’t tied to motion perception, but rather something more general like high-contrast pattern recognition. Note that the horses’ bare heads, left uncovered, still attracted bites.

Rubenstein is using virtual reality and fly-eye simulations to further understand how patterns interfere with insect perception. The full mechanism isn’t clear yet, but one thing is: stripes stop landings.

Erica McAlister, fly expert at London’s Natural History Museum, put it bluntly: female biting flies are relentless as they need blood to nourish their eggs. In the process, they transmit diseases like trypanosomes, equine influenza, and African horse sickness. Their bites can cause serious distress. Repelling them is a serious evolutionary advantage.

Stripes May Be Cooler

The thermoregulation hypothesis remains debated. Black stripes absorb heat, white stripes reflect it. In theory, this temperature difference could create tiny convection currents across the skin, helping sweat evaporate faster.

Zebras, like horses and humans, rely on sweating to cool down. They produce a surfactant called latherin that spreads sweat along the hair, improving evaporation.

A 2020 study by Alison and Stephen Cobb found that black stripes heat up 12–15°C (22–27°F) more than white ones during the hottest hours of the day. They also noted that black hairs stand up at midday, which could help vent heat.

Rubenstein’s team used mixed-species herds to compare body temperatures. Preliminary results show that zebras may indeed stay a few degrees cooler than their unstriped neighbors. But the correlation between striping and environmental heat is weak. Striping intensity maps better to fly density than to temperature zones.

Conclusion

It’s possible zebra stripes serve more than one purpose. Evolution rarely favors a single benefit. A trait that repels flies and cools the body slightly may gain a strong foothold over generations. But as of now, the clearest, most testable advantage is fly deterrence.

Stripes mean fewer bites. And fewer bites mean a better chance to survive.

That’s why zebras have them.