Researchers in central Japan painted plain black cows with white stripes, stood them in a pasture, and photographed the flies that landed on them. The striped cows drew about half as many biting flies as the same animals left unpainted. Published in PLOS ONE in October 2019 by Tomoki Kojima and colleagues at the Aichi Agricultural Research Center and Kyoto University, the result does not settle the zebra question on its own. What it does is add a clean real-world data point to a theory assembled mostly from flies, models and cloth coats: that the stripes work as a visual insect deterrent.
That theory has been gaining ground for a decade.
Older explanations for zebra colouring, camouflage and heat management among them, have held up less well under testing.
What the study actually did
Six pregnant Japanese Black cows, a breed that is naturally all black, were assigned to three treatments under a Latin-square design: black-and-white stripes, black stripes only, and no stripes at all. Each cow rotated through all three over the trial, so every animal served as its own comparison. Each set of stripes was waterborne lacquer, drawn freehand at roughly four to five centimetres wide, and took about five minutes a cow to apply. It faded within a few days, which is partly why the design worked at all. The fieldwork ran across the fly seasons of 2017 and 2018, and the paper came out the following year.
Counting was done from photographs. On the black-and-white cows, the total number of biting flies on legs and body was close to half that on the unpainted and black-striped animals. Traps nearby caught mostly stable flies, with some horn flies and the occasional horse fly. These cows also spent noticeably less time on the twitching, stamping and tail-flicking that cattle use to shake insects off, about twenty per cent less.
One detail carries more weight than the halving itself. Cows painted with black stripes only, near-invisible on an already-black animal, attracted no fewer flies than the bare controls. Had the paint’s smell been doing the work, those cows would have benefited too. They did not, which points to the visual pattern rather than the chemistry.
What the cows add to the zebra theory
The idea that stripes deter biting flies did not start with these cows. It was built earlier and elsewhere, largely by Tim Caro and collaborators, whose 2014 paper in Nature Communications found that striping across equid species tracks the distribution of biting flies, and whose 2019 work put striped and plain coats on live horses and watched what the insects did. Meanwhile the rival explanations have weakened. A 2018 paper in Scientific Reports by Gabor Horvath and colleagues reported that stripes do not cool zebras, denting the thermoregulation case.
What the cows contribute is an independent test on an animal that never evolved the pattern.
If painting stripes onto a plain black cow halves the flies that land on it, the effect is coming from the stripes themselves, not from anything a zebra happens to carry in its biology. That is the sense in which the study adds weight. It shows the mechanism the theory depends on, working in the field, on a fresh subject, rather than the reason zebras themselves carry the pattern. Corroboration is exactly what that is supposed to look like.
What the stripes seem to do to the flies
The mechanism is visual rather than chemical, and the working reading is that stripes spoil a fly’s final approach rather than warding it off from a distance. Caro’s horse study described tabanid flies homing in normally from far off, then failing to decelerate properly in the last moment before landing, so they bumped the surface or sheered away. Current explanations turn on how narrow stripes disrupt the brightness and polarised-light cues the insects rely on to judge a surface. That account is not settled, and the cow paper’s authors say so plainly. Calling the stripes a repellent is a shade generous. They do not keep flies away so much as ruin the touchdown.
Whether it is any use
Biting flies are a real cost to cattle farming, through lost grazing time, stress and reduced weight gain, and the usual defence is pesticide, to which flies keep evolving resistance. A paint job that halves fly landings without chemicals is an appealing idea, and the authors present it as one. The obstacle is durability. Their lacquer faded within days, while a working fly season runs three to four months, so any practical version would need stripes that last without repainting the herd each morning. The trial also ran on six animals. This is one small study, not an established method, and it would need repeating at scale and across breeds before it meant much on a working farm.
The work has had a second life regardless. In September 2025, Kojima and his co-authors were given an Ig Nobel Prize, the award for research that makes people laugh and then think, in the biology category.
What to watch is mundane next to the finding: whether anyone can make the stripes last a full fly season, and whether the halving holds when the trial is repeated on more than six cows and across more than one breed.
