Why Do Zebras Have Stripes? What the Science Finally Says
It is one of the oldest questions in biology: why does the zebra wear such bold black-and-white stripes? Charles Darwin argued about it, generations of naturalists guessed at it, and for more than a century it stayed an open mystery. Now, after a run of clever field experiments, scientists are converging on an answer — and it is not the one most people expect.
The Old Theories That Didn't Hold Up
For decades, the leading explanations sounded intuitive. Maybe stripes are camouflage that breaks up the body in tall grass. Maybe they dazzle and confuse charging lions. Maybe they help zebras recognise one another, or keep them cool. One by one, careful testing weakened each idea: lions catch zebras about as readily as other prey, the cooling effect proved negligible, and stripes do little to hide an animal on the open plains.
The Hypothesis That Won: Biting Flies
The strongest evidence points somewhere unexpected — small, blood-sucking flies. Biologist Tim Caro and colleagues compared horses and zebras and found that biting flies (horseflies and their relatives) approached both at similar rates, but landed on the striped zebras far less often (UC Davis). A separate behavioural study of tabanid flies around zebras and horses reached the same conclusion: stripes sharply reduce successful landings (PLOS One).
How the Stripes Actually Work
The flies aren't repelled from a distance — they're foiled up close. As a fly closes in, the high-contrast stripes seem to disrupt its ability to decelerate and make a controlled landing, so it overshoots, bumps into the animal, or veers away at the last moment. Stripes don't hide the zebra; they jam the fly's final approach. It's a tiny advantage with big stakes, because biting flies drain blood and carry diseases that can sicken or kill.
Key Takeaways
- Classic theories (camouflage, predator confusion, cooling) don't hold up well to testing.
- Experiments show biting flies land far less often on striped surfaces.
- Stripes appear to disrupt a fly's controlled landing rather than deter its approach.
- Avoiding disease-carrying flies is a strong evolutionary driver.
Frequently Asked Questions
So why do zebras have stripes? The best-supported answer is that stripes deter biting flies by disrupting their ability to land.
Don't stripes work as camouflage? Evidence for camouflage or predator confusion is weak; the fly hypothesis is far better supported.
Who studied this? Much of the key work comes from Tim Caro, Martin How, and colleagues using field experiments with real flies.
The zebra's coat turns out to be less about lions and more about insects — a reminder that evolution's biggest pressures are sometimes the smallest creatures. Explore more striking animals in the Creature Atlas encyclopedia.