About 30 miles north of the equator, in central Kenya, Kaia Tombak and her colleagues stood beside a plexiglass box. Tombak, who studies the evolution of animals’ social behavior, was dressed for the power of the Savannah sun in a light, long-sleeved shirt and pants. A gang of flies buzzed nearby, and Tombak wondered whether she’d be better off wearing stripes.
That was why her team was here: to study the fly-repelling power of stripes. Inside the box hung two foot-wide pelts from carcasses found nearby. One was from a tan impala. The other was from a zebra. In between the two was a petri dish trapping 20 or so flies. A teammate tugged on a fishing line, yanking the dish open. The flies scattered and found new landing spots within seconds. To nobody’s surprise, they avoided the zebra pelt. “It really does work,” says Tombak.
Biting flies slurp their meals from the blood of Savannah animals. At best, the flies are annoying. At worst, they transmit disease. Scientists have known since the 1980s that zebra stripes repel flies, and many believe that zebras evolved their distinctive stripes because of this advantage. But researchers still don’t actually know why the stripes work. Most theories suggest some visual illusion. Perhaps, up close, the stripes affect how biting flies perceive a zebra’s motion. Or from afar, stripes may scramble the outline of the animal’s body. For Tombak’s team, this raised an irresistible question about how a parasite, rather than food or mating strategies, could drive evolution.
Writing in Scientific Reports this month, they describe how their experiment in Kenya led to two discoveries that buck some previous theories. Tombak’s team agrees there is an illusion—but since they restricted the flies to a 4-foot-wide box, they argue that the mechanism happens up close, not from afar. They also found that narrow zebra stripes don’t repel flies any better than wider ones. “That was a surprise because previous studies had indicated that there might be a difference,” says Tombak, who is currently a postdoctoral researcher at Hunter College.
In fact, the fact that zebras have stripes at all is still sort of a surprise. In African landscapes that are green, brown, blue, and yellow, painting your butt with sharp streaks of black and white seems like a death wish. Ecologists have long scratched their heads about what evolutionary advantage could support such a conspicuous change. “It could be to confuse predators. It could be some kind of social adaptation to help zebras recognize each other. It could be thermoregulation,” says Tombak.
More than one answer could be right, but the “shooing flies” theory began its ascendance after the first mention of this zebra superpower in a 1981 study. Subsequent experiments have shown that stripes repulse flies outdoors, in labs, on rugs, on plastic models, on painted cows—“we’re at a point where the phenomenon is well established,” says Tombak. But “there have been comparatively fewer studies using actual real animal pelts.”
For the current study, Tombak, then a PhD candidate at Princeton, and her team wanted to test stripe width to see if narrower ones might be even more repulsive to flies—a potential evolutionary advantage that would explain the difference between zebra species. They also restricted their experiment to close-range encounters to rule out the theory that the repulsion required an illusion that could only happen at a distance. Hence the plexiglass box.
An undergraduate from the lab, Lily Reisinger, built the box and set up the experiment. For each trial, the team hung two pelts with clothespins, unleashed the flies, let them circle for a minute, and then counted how many landed on each pelt. First, they tested an impala pelt vs. one from a plains zebra, which has wide stripes. Then the impala vs. a Grevy’s zebra, which has narrower stripes. Finally, they pitted the skins from the two zebra species against each other. They tested 100 rounds for each pair.
The flies chose the impala skin about four times as often as they chose either zebra skin. And over the 100 rounds, the team found no obvious difference between stripes of different widths.
Why does it work? First, it’s helpful to know that flies don’t see the world as you do. Flies have “compound eyes” that combine input from thousands of photoreceptors, each pointing in slightly different directions from their eye’s rounded surface. Their sense of color is limited. And while they can sense motion and polarized light and process images 10 times faster than our eyes, those images are very low-res.
But like you, flies get fooled by the “barber pole” illusion—that famous diagonal red stripe that seems to spiral infinitely upwards. “Outside of a barber shop, there’s that rotating pole that looks like it’s going up, but it’s just rotating,” says Tombak. It creates a false perceived direction of motion, and false speed as well. A zebra’s stripes, she thinks, create a similarly disorienting sense of movement, which should make it harder for flies to gauge the timing and speed for a smooth landing. “You can imagine for a moving fly, just tons of objects are passing by at a very fast rate,” she says. And it makes sense that this illusion works close-up, as the fly is on approach to land.
Narrower stripes should create an even stronger barber pole illusion—“an enhanced perceived speed effect" as Tombak puts it—and thus stronger repulsion. But, she says, only a couple of previous studies examined stripe width, and they rarely involved real pelts; one tested painted stripes up to 5 inches wide, which is beyond what any real zebra has. Instead, she says, her team’s results show that “within the range of stripe widths that occurs naturally in zebras, width doesn’t make that much of a difference.”
That, of course, begs the question of why zebras have stripes of different widths—but Ted Stankowich, an evolutionary ecologist from California State University Long Beach who was not involved in the work, says all that really matters is that zebras have them. Additional variation could come from random genetic drift, or separate adaptations meant to confuse predators. “Once you’ve got stripes, you’ve got this anti-fly effect,” he says. “Selection from many other sources can impact that trait.”
Tombak’s evidence doesn’t rule out the possibility that the stripes might play interesting tricks from afar, according to Anna Hughes, a psychologist at the University of Essex who has studied how predators perceive zebra stripes. She says it’s common for camouflage to work both near and far, in different ways: “This idea of a two-stage protection is quite well-established.” For example, from afar, poison dart frogs blend into their environment. And up close, their bright colors tell predators not to mess with them. “It would be interesting if something similar is happening here,” she says.
From an evolutionary standpoint, an optical illusion is beneficial because the zebras don’t need to waste energy shooing flies by twitching their tails or stomping. “This work has given me a much greater appreciation for biting flies as an evolutionary force,” Tombak says. “As a field ecologist, you get reminded of every time you go into the field—it really sucks to be bitten, they really drive you crazy. But when you’re an animal and you’re outside all the time and you don’t have the protections in the shelter that we do, it really does affect you.”
While the biologists agree that the mystery of how stripes work isn’t quite solved, the answers are a little bit closer. And Stankowich, who calls this experiment “the most natural test I’ve seen so far,” points out that sometimes it takes standing in a field with some pelts, being surrounded by flies, to make progress. “We can make hypotheses about the adaptive adaptations all day long,” he says. “But until you actually test them in the field, like people are now doing with zebra stripes, it’s never going to be certain what the function actually is. It’s great to see people doing this stuff.”
