Why zebras are so fabulously striped has been a topic of debate among scientists for over a century. From temperature control to predator evasion or even just being fashion-forward, many ideas have been thrown around but a new study published in the journal Proceedings of the Royal Society B investigated the idea that parasitic flies could well have been the driving force for this coloration.
Led by Professor Tim Caro, researchers at the University of Bristol's School of Biological Sciences have been examining theories to explain the zebra’s stripes for a decade. Some of their previous research found that blood-sucking horseflies, a common pest for zebras, would approach a horse dress in stripes as often as a horse with a plain covering, but the flies wouldn’t land on the striped horse. (When scientists don’t have a zebra to hand, they improvise)
The researchers weren’t sure why this was happening but suspected a particular kind of optical illusion could be to blame in dazzling the flies and forcing them to either bump into the “skin” or fly away altogether.
"The aperture effect is a well-known optical illusion that, in human vision, is also known as the barber-pole effect,” said lead author Dr Martin How in a statement. “Moving stripes, such as those on the rotating barber-pole signs outside barbershops, appear to move at right angles to the stripe, rather than in their true direction, so the pole appears to move upwards, rather than around its axle.
"We set out to see if this illusion also takes place in the eyes of biting flies as they come to land on striped hosts.”
When flies approach a surface, they alter their speed to match how quickly it’s expanding across their vision, helping them to avoid unfortunate crash landings. The researchers suspected this “optic flow” could be disrupted by the aperture effect as the stripes make the surface appear further away than it really is. Without being able to carry out their landing calculations the flies would either slow down and never land or crash into their target.
They put their theory to the test using a range of horse coverings with patterns, but their results revealed that the aperture effect couldn’t explain the phenomenon entirely. The horses sporting the zebra’s pattern did deter flies, but the flies were also deterred by horses wearing checked coats. A checked pattern wouldn’t disrupt the optic flow in a way consistent with the aperture effect, so you would expect the flies to land on this surface without trouble. However, the study revealed that flies had a really hard time with checks and hardly landed on rugs with this pattern at all. Thus, the zebra’s “aperture effect” is not unique in deterring tabanid horseflies, other patterns can be effective too.
You might think this result would leave the researchers who have long been investigating the purpose of zebra stripes feeling somewhat disillusioned, but Professor Caro remains optimistic.
"Not only do these exciting studies bring us closer to understanding one of the world's most iconic and photogenic species, they will be of great interest to farmers attempting to reduce the damage caused by fly bites and even general horse-wear companies."
And so, the search for answers continues. After all...
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