Zebra’s Stripes Would Help Repel Flies Due to Optical Illusion [Study]

In recent years, scientists discovered the purpose of zebras' black and white stripes: to protect them from biting insects. What, though, is it about stripes that prevent a biting fly from landing on a zebra and sucking its blood?

Zebras' characteristic stripes are said to have developed as a result of the fact that scientists have known they repel insects since the 1980s. However, scientists are still uncertain as to why the stripes are effective. Most hypotheses propose some optical illusion. Perhaps the stripes impact how biting flies perceive a zebra's movements up close. Or stripes may muddle the animal's body form when viewed from a distance.

Zebra Stripes Repel Flies

A study published in PLOS ONE last 2019 mentioned that stripes thwart fly landings. But in a separate report published last month in Scientific Reports, researchers explained how their experiment in Kenya produced two findings that defy several accepted assumptions.

According to Kaia Tombak and her coworkers, flies cannot escape zebras because of an optical illusion. They also discovered that broader zebra stripes do not repel flies more effectively than narrower ones.

"That was a surprise because previous studies had indicated that there might be a difference," Tombak; currently a postdoctoral researcher at Hunter College, said in a Wired report.

To determine if thinner stripes would be even more repellent to flies-a potential evolutionary benefit that might explain the variation between zebra species-Tombak, who was then a Ph.D. candidate at Princeton, and her colleagues tested stripe width for the present study.

Additionally, they limited their study to near contacts to disprove the idea that the repulsion needed an illusion that could only occur at a distance. Therefore the plexiglass container.

Stripes Making Optical Illusion

Lab undergrad Lily Reisinger built the box and continued the experiment, Wired further reported. For each test, the researchers hung two pelts with clothespins, let the flies fly, and counted how many landed on each after circling for a minute. They compared a broad-striped plains zebra pelt to an impala fleece. Then the impala will confront a Grevy's zebra with narrower stripes. Researchers then compared the two zebra skins. Each couple underwent 100 rounds.

Flies chose impala skin roughly four times more than zebra hides. During the 100 rounds, stripes of different widths showed no difference. Why?

First, remember that flies see the world differently. Flies' "compound eyes" use thousands of photoreceptors that point in diverse directions from the eye's spherical surface. They're colorblind. They have low resolution but can process pictures ten times faster than human eyes. They detect motion and polarized light.

The "barber pole" illusion, that diagonal red line that spirals higher, fools flies too. There's that revolving pole outside a barbershop that appears like it's moving up. But Tombak said it's just turning. It distorts speed and direction. She thinks the zebra's stripes confuse flies' sense of motion, making it harder to land. Since the fly is landing, this illusion should work up close.

Narrower stripes should increase the barber pole illusion and aversion. She says that few studies often utilize painted stripes-examined stripe width. One study examined painted stripes up to 5 inches wide, broader than zebra stripes. Her team found that "width doesn't make that much of a difference within the range of stripe widths that occurs naturally in zebras."

This naturally begs the question of why zebras have stripes of varying lengths, but Ted Stankowich, an evolutionary ecologist at California State University Long Beach who was not involved in the study, says the only thing that matters is that they do. Variation may occur via predator deception or genetic drift.

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