Why Do Zebras Have Stripes? Solving a 150-Year-Old Puzzle

The black-and-white stripes of the zebra, one of the best-known animals of the African savannah, are among nature’s most striking patterns. They have puzzled biologists since Charles Darwin and Alfred Russel Wallace. Over more than 150 years many ideas have been proposed — and recent experiments have shed light on the most likely answer.

Black on white, or white on black?

First, a common question. Shave a zebra and you will find black skin underneath. In the hairs, the pigment-producing cells — melanocytes — are active in some areas and switched off in others. Active areas form the black stripes; areas without pigment form the white ones. That is why it is more accurate to call a zebra “a black animal with white stripes”.

There are three species of zebra:

Hypothesis 1: confusing predators

For many years the most popular idea was that stripes fool predators. In a running herd, so the theory goes, the stripes blur together and make it hard for a lion to pick out a single animal — a form of motion dazzle. Or the stripes break up a zebra’s outline in tall grass and at dusk.

But this hypothesis has weakened in recent years:

• Zebras are among the animals lions hunt most often. Stripes do not seem to deter them much.
• Calculations based on animal visual acuity suggest predators such as lions and hyenas can only clearly see zebra stripes from a few tens of metres away. Further off, a zebra looks like a grey-toned horse.
• Rather than hiding zebras in grass, the stripes make them more conspicuous on the open savannah.

Hypothesis 2: keeping cool

Another idea is that black stripes heat up more in the sun than white ones, and the temperature difference creates small air currents above the skin that help the animal cool down.

Studies testing this have produced conflicting results. Experiments comparing the temperature of striped and plain water barrels found no meaningful difference, while some field studies found that zebras in hotter areas have more pronounced stripes. The cooling hypothesis remains disputed.

Hypothesis 3: fending off flies

The explanation with the strongest support today is that stripes protect zebras from blood-sucking flies. In Africa, biting insects such as horseflies and tsetse flies are not just a nuisance — they carry deadly diseases such as trypanosomiasis and African horse sickness. Zebras have shorter hair than horses, making them more vulnerable.

In 2014, Tim Caro of the University of California and colleagues compared the distribution of zebra species and subspecies with environmental factors. The factor that best explained how strongly striped zebras were was where biting flies are most active. Predator density and temperature did not show nearly as strong a link.

The farm experiment: why can’t flies land on stripes?

In 2019, Caro’s team filmed horseflies on a farm in England that kept both zebras and horses. The results were revealing:

• Flies approached zebras and horses about equally often — the stripes did not deter them from a distance.
• When approaching horses, flies slowed down to land and touched down easily.
• Flies approaching zebras did not slow down. They bumped into the zebra, bounced off or flew past without landing.
• As a result, far fewer flies landed on zebras than on horses.

The researchers went one step further: they dressed horses in black, white and striped coats. Horses wearing striped coats had markedly fewer flies landing on their bodies — but there was no difference on their uncovered heads, showing that the effect came directly from the stripes.

How do stripes trick a fly’s eyes?

A fly’s compound eyes are excellent at detecting motion but poor at seeing detail. Scientists think stripes may confuse flies in two ways:

• A motion illusion: as the fly approaches, the stripes may send its motion-detection system misleading signals — as if things were moving the wrong way, or not at all. The fly cannot judge how fast it is closing in and misses the moment to brake.
• A vanishing target: from afar, the fly sees the zebra as a single mass. Up close, that mass breaks into black and white fragments and the fly “loses” its landing spot.

Some research also shows that horseflies use reflected polarised light to find water and hosts. Black and white stripes polarise light differently and may disrupt this navigation system too.

Is this knowledge useful?

Yes. The findings are already being applied to livestock:

• Some horse owners use striped fly rugs to protect their horses.
• In a study in Japan, painting black-and-white zebra stripes on cows significantly reduced the number of biting flies and fly-avoidance behaviours — a possible natural alternative to insecticides.

Every zebra is unique

Whatever their purpose, stripes have one certain side effect: every zebra’s pattern is unique. Like human fingerprints, no two zebras share the same stripes. Researchers can track individual zebras for years by comparing patterns in field photographs with computer software.

Whether zebras recognise each other by their stripes is less certain. Foals are thought to learn their mother’s pattern after birth, but scent and sound probably play important roles too.

In short

Zebra stripes have been a biological puzzle for 150 years. Popular explanations such as confusing predators or keeping cool now rest on weak or disputed evidence. The strongest evidence suggests stripes protect zebras from disease-carrying biting flies: the flies approach, but fail to slow down before reaching the striped coat and so often cannot land. The zebra’s stylish pattern turns out to be a highly practical fly repellent.