Why Do Zebras Have Stripes? Four Theories and the Experiments That Tested Them

The oldest idea is that stripes hide the zebra: they break up its outline in long grass, or blend into the dappled shade of woodland, or make it hard to see at dusk when lions hunt. The trouble is that zebras live mostly in open grassland, stand in herds, and are noisy. In 2016 researchers measured how far away a lion or hyena could resolve zebra stripes, given what is known about their eyesight, and found that at the distances predators begin a hunt the stripes merge into grey and the zebra is as visible as any other horse-sized animal. By the time a lion can see the stripes it can also smell and hear the zebra. Camouflage is out.

The second idea is that a herd of striped animals running together is a dazzling mass in which a predator cannot pick a target or judge speed and direction, the logic behind the dazzle camouflage painted on ships in the First World War. It is a plausible idea and hard to test on lions. Experiments with people chasing striped targets on a screen found that stripes sometimes helped and sometimes hindered, and the field data are unhelpful in another way: lions kill zebras at about the rate their abundance predicts, so whatever the stripes do, they do not seem to make zebras harder to catch.

The third idea is that black stripes heat up more than white ones, setting up small convection currents over the skin that cool the animal, which would explain why the most heavily striped zebras live in the hottest regions. Measurements do not support it. Thermal cameras show that a zebra's surface is no cooler than that of a uniformly coloured animal standing next to it, and water-filled barrels painted with stripes, grey, black or white heated up according to their average darkness, with no stripe bonus. Zebras do sweat more efficiently than most animals, but that is a separate adaptation.

The fourth idea, first suggested in the 1930s and largely ignored, is that stripes deter biting flies. Horseflies and tsetse flies in Africa carry diseases, including the trypanosomes that cause sleeping sickness, and they bite through hair. The evidence for this one has kept coming:

• •Sticky traps painted with stripes catch far fewer horseflies than plain black or white ones
• •Across the zebra's range, the intensity of striping tracks the abundance of biting flies better than it tracks predators, temperature or habitat
• •Horses wearing striped coats on a farm in Somerset were landed on by flies a quarter as often as the same horses wearing plain coats
• •High-speed video shows that flies approach a striped surface normally but fail to slow down in the last instant and bounce off or overshoot, as if they cannot judge the landing
• •Painting white stripes on Japanese cattle cut fly bites by half

The mechanism seems to be that stripes disrupt the way a fly uses the pattern of light flowing across its eyes to control its landing, so it misjudges the surface. The question that remains is why zebras and not the antelope grazing beside them. The best guess is hair: a zebra's coat is unusually short, so a fly's mouthparts reach the skin, and the diseases flies carry are more dangerous to horses than to antelope. Zebras also happen to be the only wild horses left in fly country, so there is no stripeless control to compare.

The lesson of the zebra is partly about method. The camouflage and dazzle theories were attractive because they were easy to imagine; the fly theory was ignored for decades because it was not. The experiments that settled it were cheap and slightly ridiculous, involving painted barrels and horses in coats, and they are the reason a question argued over since Darwin now has an answer most biologists accept.

Under the coat, a zebra's skin is dark all over. The stripes are made by the hair: pigment cells lay down melanin in the hairs of the dark bands and hold back in the pale ones. The pattern is set down while the foal is still an embryo, and biologists think it arises from the kind of self-organising chemical process first described mathematically by Alan Turing, in which interacting signals produce regular bands. Because the pattern forms early and then stretches as the body grows, stripes near the rump tend to be broader than those on the neck.

The three living species are striped differently. Grevy's zebra has many narrow stripes, while plains zebras in the south of their range often show pale brownish shadow stripes between the black ones and fading stripes on the legs. The quagga of southern Africa had stripes only on its head, neck and front half. It was hunted to extinction in the nineteenth century, and in 1984 it became the first extinct animal to have its DNA studied, which showed it was a form of plains zebra rather than a separate species. That fits the fly idea: the southern populations, living where biting flies are fewer, were the least striped.

No two zebras have the same stripes, and even the two sides of one animal differ, so the pattern works like a fingerprint. Field biologists use this to count and follow zebras without catching or tagging them. They photograph the flanks of the animals they meet, and computer programs compare each picture against a catalogue, recognising individuals from the spacing and branching of their stripes. Repeated surveys show which animals survive, move, breed or disappear, so researchers can estimate population sizes from sightings alone, a method especially useful for the endangered Grevy's zebra of Kenya and Ethiopia.

The quagga's close relationship with plains zebras opened an unusual possibility. In 1987 a project in South Africa began selectively breeding plains zebras that showed reduced striping on the hindquarters and legs, choosing the least striped foals in each generation. After several generations some animals look strikingly like the old museum skins and paintings, with stripes fading out along the back half of the body. Critics point out that this recreates the quagga's appearance rather than the animal itself, since its other genes and behaviour cannot be bred back, and the project is a test of how much of a lost form survives in its relatives.

Zebra stripes gave their name to one of the best known road markings in the world. Britain began experimenting with pedestrian crossings in the 1930s, and the familiar black and white striped design was trialled in 1949 and introduced nationally in 1951, chosen because high-contrast stripes are easy for drivers to see.

The crossings became zebra crossings, and in 1969 one in north London became world famous when the Beatles walked across it for the cover of their album Abbey Road. That crossing has since been given listed status for its cultural importance. The name shows how a vivid animal pattern can become a shorthand for any bold arrangement of black and white bars.

Of the four serious theories for zebra stripes, camouflage fails because predators cannot resolve the stripes at hunting distance, confusion has no field support, and cooling has been measured and found absent. Biting flies, the least intuitive, is backed by traps, geography, coated horses and high-speed video: flies cannot land properly on stripes, and zebras, with short coats and fly-borne diseases, are the animal that most needed the protection.