Somewhere in central Japan, someone stood in a paddock with a roll of masking tape and a can of white lacquer, painting a plain black cow to look like a zebra, for reasons that had nothing to do with fashion. The cow had no opinion on the matter.
The flies did.
The painting was part of a real experiment, and it worked. Tomoki Kojima and colleagues at the Aichi Agricultural Research Center and Kyoto University published the results in PLOS ONE in October 2019. Black cows wearing painted white stripes attracted about half as many biting flies as the same animals left unpainted.
Why the black stripes mattered as much as the white ones
Six Japanese Black cows were used, a breed that comes in exactly one colour. Each animal rotated through three conditions in a Latin-square design: painted with black-and-white stripes, painted with black-only stripes, and left unpainted. Every cow served as its own control, which sidesteps the awkward possibility that some cows are simply more delicious than others.
Including a black-only condition is what makes the result stick. It rules out the simplest alternative, that flies were reacting to the smell of fresh lacquer rather than the pattern itself. If that were true, the black-striped cows would have drawn just as few flies as the zebra-striped ones.
They did not.
The numbers, as amended in a 2020 correction issued by the same team, are blunt. Counting flies on the legs, unpainted cows carried an average of 86.7, cows with black-only stripes 73.1, and zebra-striped cows 40.2, roughly half. On the body, where fly numbers ran lower overall, the proportional drop was even bigger: 42.1, 38.6 and 15.3, meaning zebra-striped cows carried a little over a third as many as the unpainted ones. The striped cows also spent less time throwing their heads, beating their ears, stamping and flicking their tails: the signs of a cow losing an argument with an insect.
The flies in question
What exactly was landing on them? Sticky traps set around the pasture gave the answer: stable flies made up around 78 per cent of the catch, horn flies most of the rest, with a scattering of horse flies. All of them feed on blood, and none of them is a minor nuisance. Livestock under sustained fly pressure graze less, bunch together for protection, and lose condition, which is why most farms just reach for insecticide, and why resistance to it is only a matter of time.
What a stripe does to a fly
Picture a horse fly zeroing in on a warm flank at full speed. A separate 2019 study led by Tim Caro and Martin How, also in PLOS ONE, filmed exactly that moment on a farm in North Somerset, comparing captive zebras with ordinary horses. The flies circled both animals at much the same rate. Landing was a different story. Video showed them approaching the zebra fast and failing to slow down, often bouncing off the striped flank rather than settling on it.
The same horses were then dressed in black, white and striped cloth coats. Only the striped coat repelled the flies, while the horses’ bare heads were bitten at similar rates regardless of coat colour. As the University of Bristol put it, the pattern seems to overwhelm a fly’s vision in the instant before touchdown, once the stripes are close enough for its coarse eyesight to resolve.
A follow-up published in the Journal of Experimental Biology in 2023 narrowed the field further. High contrast is what puts flies off, not polarised light and not the disruption of optic flow. The current best guess is that thin, crisply outlined stripes break up the large dark patches a horse fly uses to recognise a target.
The theories that fell over
Zebra striping has attracted explanations for more than 150 years, most of them plausible until somebody checked. Camouflage struggles because predators cannot resolve the stripes at any useful distance. The cooling hypothesis fares no better. It holds that black and white surfaces generate small convection currents that cool the body. Gábor Horváth and colleagues tested this in 2018, covering water-filled barrels in real and artificial hides on a Hungarian horse farm for four months. Writing in Scientific Reports, they reported no meaningful temperature difference between the striped barrels and the grey ones.
Six cows is still six cows
How far can six cows really take a theory? The painted-cow result comes from a single study, a sample size that would fit in one paddock, tested on one breed in one prefecture. A neat design and a big effect are worth something, but a halving observed in six animals falls well short of a proven farming technique.
There are loose threads too. Skin twitching bucked the trend, rising slightly in the striped cows instead of falling, an anomaly the published data doesn’t explain. White lacquer is also not a permanent fix: rain, mud and hair growth all work against it.
Laugh first, think second
None of that stopped the work finding an audience. In September 2025 the Kojima team was awarded an Ig Nobel Prize in biology, the annual honour for research that makes people laugh and then think. Kojima told reporters in Boston that winning one had been his dream since running the experiment.
The idea has already reached real farms in Japan. As reported by Understanding Animal Research, a three-year survey of cattle farmers turned up at least one convert. A Wagyu farmer said he was “really surprised that no insects came,” after years of keeping cattle off pasture entirely to dodge the flies.
A question that occupied Darwin and Wallace, and generations of biologists since, might end up being settled less by a laboratory than by a few sceptical farmers watching their cows for flies that stop landing.
