February 3, 2023

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Scientists are re-investigating why zebra stripes mysteriously repel flies

For the current study, Tombak, then a graduate student at Princeton, and her team wanted to test stripe width to see if narrower stripes might be even more repellent to flies — a potential evolutionary advantage that would explain the difference between zebra species. They also limited their experiment to close-range encounters to rule out the theory that repulsion required an illusion that could only happen at a distance. Hence the Plexiglas box.

A lab student, Lily Reisinger, built the box and set up the experiment. For each trial, the team hung two skins with clothespins, released the flies, let them circle for a minute, and then counted how many landed on each skin. First, they tested an impala pelt versus one from a plains zebra, which has broad stripes. Then the impala vs. a Grevy’s zebra, which has narrower stripes. Finally, they played the skins of 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 either zebra skin. And over the 100 laps, the team found no apparent 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 combine input from thousands of photoreceptors, each pointing in slightly different directions from the rounded surface of their eye. Your color sense is limited. And although they can perceive movement and polarized light, and can process images ten times faster than our eyes, these images are very low-resolution.

But like you, flies are fooled by the “barber pole” illusion – that famous diagonal red stripe that seems to spiral endlessly upwards. “Outside of a hair salon, there’s this rotating pole that looks like it’s going up, but it’s just spinning,” says Tombak. It creates an incorrectly perceived direction of movement and also an incorrect speed. A zebra’s stripes, she thinks, create a similarly confusing sense of motion that should make it difficult for the flies to gauge the timing and speed for a smooth landing. “You can imagine that with a moving fly, it’s just tons of objects going by very quickly,” she says. And it makes sense that this illusion would work up close as the fly approaches land.

Narrower strips should produce an even stronger barberpole illusion – “an increased perceived speed effect,” as Tombak puts it – and thus greater repulsion. One has tested painted stripes up to 5 inches wide, which is beyond what a real zebra has. Instead, she says her team’s results show that “within the range of stripe widths naturally occurring in zebras, the width doesn’t matter that much of a difference.”

Of course, that begs the question of why zebras have stripes of different widths — but Ted Stankowich, an evolutionary ecologist from California State University Long Beach who wasn’t involved in the work, says all that really matters is that zebras cross them to have. Additional variations could arise from random genetic drift or separate adaptations designed to confuse predators. “Once you have stripes, you have this anti-fly effect,” he says. “Selection from many other sources can affect this trait.”