Why do zebras have stripes?

The prime suspect: biting flies

Scientists have puzzled over the zebra's coat for over 150 years⁴. In 2014, a team led by biologist Tim Caro compared the stripes of zebras, horses and asses with where those animals live^{1, 2}. Stripes went with places whose climate suits biting flies^{1, 2}. The number of belly stripes also matched where tsetse flies occur¹. Camouflage, avoiding predators, managing heat and social life found no consistent support¹.

Why would flies matter so much? In Africa, horseflies can spread diseases that can be fatal to zebras, such as trypanosomiasis³. Zebra hair is also shorter than a biting fly's mouthparts, so flies can reach the skin easily^{2, 3}.

In a study published in 2019, researchers filmed horseflies around captive zebras and horses on a farm in England^{3, 4}. Flies circled and briefly touched both animals with no clear difference, but far fewer of them landed on zebras^{3, 4}. They came in fast and did not slow down, and a fly has to slow down to land^{3, 4}. In only five recorded landings on zebras, flies stayed 1.20 seconds on average, versus 10.06 seconds on horses. The authors put this down to the zebras' behavior, not their stripes³.

How stripes trip up a fly

In a second test, the same horses took turns wearing black, white and striped coats^{3, 4}. In the striped coat they got far fewer touches and landings, yet flies landed on their bare heads just as often³. So the pattern itself makes the difference, not the animal's smell or behavior^{3, 4}. Earlier, a striped model horse attracted far fewer horseflies than plain-colored ones, and in tests with striped patterns, narrower stripes attracted fewer flies⁵.

Exactly why the flies fail is still not settled^{3, 7}. One idea was the 'aperture effect', where straight edges create false signals of movement⁶. But flies avoided checked rugs about as much as striped ones, and that ruled the idea out⁶. Another idea was polarized light, because light and dark stripes reflect light polarized in very different ways⁵. Yet a 2023 study found no evidence that horseflies are drawn to coats that reflect more polarized light⁷. The authors' working hypothesis: thin, sharp stripes leave no large dark patches, and horseflies are drawn to such patches at close range⁷.

What about lions and heat?

Camouflage looks unlikely. Estimates of how lions and hyenas see suggest that zebra stripes are hard for them to make out beyond about 50 meters in daylight, or 30 meters at twilight⁸. From farther away, a zebra simply looks gray⁸. Zebras see stripes better than their predators do, but the authors doubt zebras need them to recognize each other, since horses manage without⁸.

Heat is hotly debated. Across 16 sites where plains zebras live, temperature predicted a lot of the variation in their stripes, and that study found no evidence for the biting-fly idea⁹. But when scientists wrapped water-filled barrels in hides, the inside of the striped barrels was no cooler than the inside of the gray ones¹¹. A study of two zebras in Kenya measured black stripes 12–15 °C hotter than white ones and proposed that small air currents above the stripes help cool the animal¹². A 2021 lab test found that such air currents get blown away by the weakest wind¹³.