For the current study, Tomak, who was a PhD student at Princeton, and his team wanted to test the width of the lines to see if a smaller number might exist. more aversion to flies—a possible evolutionary advantage that may explain differences in zebra species. He also restricted their attempts to close encounters to dispel the theory that resistance required deception that could only be done at a distance. Hence the plexiglass box.
Lily Reisinger, an undergraduate in the lab, designed the box and set up the experiment. In each trial, the team hangs two shells with clothespins, releases the flies, lets them spin for a minute, and then counts how many land on each point. First, they tested the impala’s bill against the zebra’s, which has broad stripes. That’s the impala versus Grevy’s zebra, which has narrow stripes. Finally, he dropped the skins of two kinds of zebras. They tested 100 times for each group.
The flies chose the fly skin about four times as often as they chose the zebra skin. And over 100 times, the team found no clear difference between the different stripes.
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 combine input from many cells called photoreceptors, each pointing to a slightly different part of the eye’s orbit. Their sense of color is limited. And even though they can detect motion and bright light and process images 10 times faster than our eyes, the images are very low quality.
But just like you, flies are fooled by the “barber pole” illusion—that familiar red diagonal line that seems to spin endlessly. “Outside the barbershop, there’s a rotating antenna that looks like it’s going up, but it’s just rotating,” says Tomak. It creates a false sense of identity, and a false sense of urgency. He thinks that the zebra’s stripes also cause a disturbance in its movement, which should make it easier for the flies to know when and how fast to land. He said: “You can imagine a flying fly, tons of things are passing by very quickly. And it makes sense that this trick works close up, because the fly is getting close to landing.
Fewer strokes are supposed to make barbers look more powerful—”more interest” as Tombak puts it—and to do so is frowned upon.” 5 inches long, which is longer than a real zebra has.” On the contrary, he says, the results of his group show that “within the long lines of the lines that are found naturally in the zebra, the width does not make that much. difference.”
Of course, that raises the question of why zebras have stripes of different widths—but Ted Stankowich, a biologist at California State University Long Beach who was not involved in the work, says the most important thing is that zebras have them. More variation can come from random moves, or different changes meant to confuse enemies. “When you have bruises, you have an allergy to flies,” he says. “Choosing from many other factors can affect this behavior.”
