Scientists examine zebra stripes through the eyes of a fly.
The alternating black and white stripes that traverse a zebra’s body are both mesmerizing and vexing.
Striking visuals in nature almost always serve a crucial function, from the flashy yellow and red warning signals of poisonous frogs to the brilliant plumage that some male birds use to entice females.
Zebra stripes have several purported functions. For example: black and white stripes merge into a grayish tint in the twilight, which could conceal zebras from hungry lions and hyenas; the stripes may degrade a predator’s ability to target a single zebra moving in a group by creating an effect called motion dazzle; or convection currents created by air moving at different speeds over the black and white parts of the pelage could help to keep zebras cool.
Though these are attractive ideas, none have actually been backed up by experimental evidence.
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New research published in the journal PLOS ONE explores a different explanation for the zebra’s stripes — one that invokes the visual system of blood-sucking flies.
Dreaded horse flies transfer serious blood-borne diseases to their equine targets through bites. The researchers suspected that stripes might offer zebras protection from horse flies by countering the flies’ natural attraction to polarized light.
“A lot of insects can detect polarization because of the details of the anatomy of their photoreceptors,” lead author Kenneth Britten from the University of California, Davis told The Science Explorer.
“In our eyes, the molecules that pick up the light are all randomly oriented, so they can't pick up the polarization. But in fly eyes, the membranes hold those molecules in a particular orientation, allowing them to pick up the plane of polarization.”
Shiny surfaces, like glass or glossy black hair, reflect light waves oriented in the same direction, Britten says. This polarized light is similar to that reflected by pools of water, where horse flies lay their eggs.
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However, in white hair, some of the light is scattered rather than reflected, which might serve to cancel out the polarized light reflected by the black stripes. In agreement with this idea, previous studies have shown that biting flies manifestly avoid striped surfaces.
To examine what the flies really see as they approach a zebra, the researchers took photographs of zebras gathered around a watering hole in Tanzania.
Through analysis of the photos, the researchers found that the white stripes reflected more polarized light than expected. Furthermore, they found that although the horse fly’s eye would blur the black and white stripes at a distance of around 10 meters away (the point at which the flies are thought to start zeroing in on a zebra using vision, rather than scent), the zebra was still distinct enough from the surrounding dry grass at close range to attract the flies.
So if white stripes do not in fact dilute the polarized light reflected by a zebra’s body, why have prior studies found that biting flies avoid landing on black and white striped surfaces?
The researchers suggest one possibility is that the stripes might confuse flies that are trying to land on zebras in motion by creating visual illusions like the wagon-wheel effect and the barber pole illusion.
While this study offers new insights into why bold stripes evolved in zebras, as is often the case in scientific research, there is still a question mark at the end.
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