For the current study, Tombak, then a PhD student at Princeton, and her team wanted to test the width of the stripes to see if the narrower stripes might be even more loathing flies—a potential evolutionary advantage that could explain the differences between zebra species. They also limited their experiment to close-range collisions to disprove the hypothesis that repulsion requires an illusion that can only occur at long distances. Hence the plexiglass box.
An undergraduate student from the lab, Lily Reisinger, built the box and set up the experiment. For each experiment, the team hung two pelts with clothespins, released the flies, let them hover for a minute, then counted how many landed on each pelt. First, they tested an antelope skin against one from plain, wide-striped zebra. Then antelope compared to Grevy’s zebra, which has narrower stripes. In the end, they pitted the skins of two species of zebra against each other. They tested 100 rounds for each pair.
The flies chose antelope skin four times more often than they chose zebra skin. And in more than 100 rounds, the team found no discernible difference between stripes of different widths.
Why does it work? First, it’s helpful to know that flies don’t see the world the way you do. Flies have “compound eyes” that incorporate input from thousands of photoreceptors, each of which points in slightly different directions than the circular surface of their eyes. Their sense of color is limited. And although they can sense motion, polarized light, and process images 10 times faster than our eyes, those images have very low resolution.
But like you, flies are fooled by Illusion “barber pole”—the famous red diagonal line that seems to spiral upwards endlessly. Tombak said: “Outside a barbershop, there was a spinning pole that looked like it was going up, but it was just spinning. It creates a falsely perceived motion direction and also a false speed. She thinks the zebra stripes also create a similar sense of disorienting motion, which would make it difficult for flies to determine the time and speed for a smooth landing. “You can imagine for a moving fly, there are tons of objects passing by at very fast speeds,” she said. And it makes sense for this illusion to work up close, when the fly is approaching the ground.
Narrower stripes give the illusion of an even stronger barber pole—the “enhanced perceived speed effect” as Tombak puts it—and thus a stronger repulsion. several previous studies examined stripe widths and they were rarely associated with genuine leather; one trial stripe paint up to 5 inches wide, far beyond what any real zebra has. Instead, she says, her team’s results show that “within the width of a stripe that occurs naturally in zebras, the width doesn’t make much of a difference.”
Of course, that raises 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 that all that really matters is that zebras have them. Additional variation may come from random genetic drift or distinct adaptations intended to confuse predators. “Once you have stripes, you get this fly repellent,” he says. “Selection from many other sources can influence that trait.”
