The mystery of why zebras have their characteristic stripes has puzzled researchers for more than a century.
In recent decades, Professor Tim Caro has been at the forefront University of BristolThe School of Biological Sciences has investigated and discredited many popular theories, such as their use as camouflage of predators, a cooling mechanism through the formation of convection currents, and a role in social interactions.
Stripes that are used to confuse predators is another general explanation, but it is also flawed when you look at the scientific data. Instead, mounting evidence suggests that they are parasitic flies that are confused by the distinctive pattern of the zebra.
In a new paper published today in the magazine Procedures of the Royal Society B, Bristol scientists have now provided significant depth to this hypothesis by limiting the possible mechanism.
Earlier, the same researchers had shown that blood-sucking horse-flying horses would come in striped carpets smoother than regular carpets, but then failed to land or slow down as they approached.
The mystery of why zebras have their characteristic stripes has puzzled researchers for more than a century. Credit: Amelia Gillard, University of Bristol
In essence, stripes blind the flies, forcing them to collide with the skin or fly away altogether. In their new study, they investigated a potential mechanism that explained how the streaks lead to this outcome: the aperture effect.
Lead author Dr. Martin How, also from Bristol’s School of Biological Sciences, said: “The aperture effect is a well-known optical illusion which, in human vision, is also called the barber-pole effect. Moving stripes, such as those on the rotating barber pole signs outside barber shops, seem to point at the strip, instead of in the real direction, so that the pole looks upwards as near its axis.
“We set out to see if this illusion also occurs in the eyes of biting flies, when they land on strip hosts.
‘As each fly approaches a landing surface, it will adjust its speed according to how fast the surface expands over its vision, enabling a slow and controlled landing.
However, stripes could disrupt this ‘optical stream’ due to the aperture effect, leading the fly to believe that the landing plane is farther away from reality. Thus, the fly does not succeed or land successfully. ”
Three zebras drinking. Credit: Amelia Gillard, University of Bristol
Despite relying on visual ecologists, this study found that the aperture effect is not the mechanism behind flight banding by comparing fly landings on horses with stripe versus controlled carpets.
Since controlled carpets provide visual input free of the aperture effect, one would expect flies to land on them without any problems. However, flights had real difficulties with this pattern – barely landing on controlled or striped carpets. Thus, stripes themselves are not unique to restricting tabanide horseflies, other patterns may also be effective.
Professor Caro, the senior co-author of this paper, added: “Not only will these exciting studies bring us closer to understanding one of the most iconic and photogenic species in the world, they will be of great importance to farmers who ‘ t try the damage caused by fly bites and even general horse companies. ”
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Reference: August 19, 2020, Procedures of the Royal Society B.
This study was funded by the Institute for Advanced Studies at the University of Bristol and the Royal Society.
