Zebra Stripes Confused Scientists for 150 Years. The Answer Was Flies.

Charles Darwin was puzzled by zebra stripes. Alfred Russel Wallace was puzzled by them. For 150 years, the question of why zebras are striped generated more competing hypotheses than almost any other question in animal coloration research. Camouflage. Predator confusion. Heat regulation. Social signaling. Individual recognition. Each hypothesis had proponents, each had evidence, and none was conclusive.

The answer, when it finally came from a series of controlled experiments published between 2012 and 2020, was not what anyone had predicted. Zebra stripes deter biting flies.

The Hypotheses and Why They Failed

The camouflage hypothesis was always the weakest. Zebras live in open grassland, are highly visible to lions and other predators, and their black and white coloration is conspicuous rather than concealing in virtually every habitat they occupy. Lions hunt zebras successfully and frequently. If the stripes were camouflage, they were not working.

The predator confusion hypothesis, which proposed that stripes create a visual illusion that makes it difficult for predators to single out an individual from a moving herd, had more support. There is some experimental evidence that striped patterns create motion dazzle effects that impair the ability of observers to accurately judge the speed and direction of moving objects. But field studies of lion predation on zebras versus other prey species have not found that zebras are harder to catch than similarly sized unstriped animals.

The heat regulation hypothesis proposed that the boundary between black and white stripes creates small-scale air circulation that cools the animal. Thermal imaging studies have found temperature differences between black and white stripes, but the magnitude of the effect is too small to provide meaningful thermoregulatory benefit.

The Fly Experiments

Tim Caro at the University of California Davis spent years systematically testing each hypothesis and published a comprehensive analysis in 2014 concluding that the evidence best supported the fly deterrence hypothesis. His subsequent experimental work, conducted with colleagues including Martin How at the University of Bristol, provided the most direct evidence yet.

In one experiment, horses were dressed in striped coats and observed alongside uncoated horses. Biting flies, primarily horseflies and tsetse flies, landed on the striped coats at dramatically lower rates than on the uncoated animals. The flies approached the striped horses at normal rates but failed to slow down and land correctly, instead flying past or bouncing off. The stripes were not repelling the flies from a distance. They were disrupting the flies’ ability to execute a controlled landing.

How’s analysis of fly visual systems provided the mechanism. Biting flies use polarized light reflected from dark surfaces to locate suitable landing sites. The alternating black and white stripes of a zebra coat create a pattern that disrupts this polarization signal, making it difficult for the fly’s visual system to identify a stable landing target. The fly sees the zebra but cannot land on it effectively.

Why does this matter enough to drive the evolution of such a striking coloration pattern? Biting flies in sub-Saharan Africa transmit diseases including trypanosomiasis, equine influenza, and African horse sickness. They also cause significant blood loss and stress through their biting alone. An animal that can reduce fly landing rates substantially has a meaningful fitness advantage in environments where fly pressure is high.

Caro’s analysis found that the geographic distribution of zebra stripe intensity correlates with the distribution of biting fly species across Africa. Zebra populations in areas with higher fly pressure have more complete and more clearly defined striping. The correlation is not perfect, but it is consistent enough to support the hypothesis.

Three Species, One Pattern

There are three living zebra species. Plains zebras, Equus quagga, are the most numerous, with a population of several hundred thousand across eastern and southern Africa. Mountain zebras, Equus zebra, are found in southern Africa and are classified as Vulnerable, with a total population of around 35,000. Grevy’s zebras, Equus grevyi, are the largest wild equid and are classified as Endangered, with fewer than 3,000 individuals remaining in Kenya and Ethiopia.

Each species has a distinct stripe pattern. Plains zebras have broad stripes that extend onto the belly. Mountain zebras have narrower stripes and a distinctive grid pattern on the rump. Grevy’s zebras have the narrowest and most numerous stripes of any species, extending all the way to the hooves.

Whether these differences in stripe pattern reflect differences in fly pressure, differences in the social signaling function of stripes, or simply evolutionary drift in isolated populations is not fully resolved. Caro’s fly hypothesis explains the existence of stripes but does not fully account for the variation between species.

The quagga, a subspecies of plains zebra that was striped only on the front half of its body and unstriped on the hindquarters, was hunted to extinction in the wild by 1878. The Quagga Project in South Africa has been selectively breeding plains zebras with reduced striping since 1987, attempting to recreate the quagga’s appearance through selective breeding. Whether the resulting animals are meaningfully equivalent to the original quagga is a question that touches on deep issues in conservation biology about what a species actually is.

Darwin never got his answer. It took controlled experiments with horses in striped coats, high-speed cameras tracking fly behavior, and a detailed understanding of insect visual systems to find it. The stripes were always about the flies. The predators were a distraction.