The mystery of zebra stripes and horseflies has been revealed. A study published Thursday in the Journal of Experimental Biology provides evidence that zebra stripes may discourage horseflies from biting. Prior to the study, researchers thought the zebra stripes were designed to confuse predators. While this may still be the case, the deterring impact of zebra stripes appears to be aimed primarily at horseflies.
“We demonstrate that a zebra-striped horse model attracts far fewer horseflies (tabanids) than either homogeneous black, brown, grey or white equivalents,” the researchers write in the study’s abstract.
Horseflies, which can carry diseases and distract their victim from feeding or drinking, are unwelcome visitors to zebras and other animals that graze. According to Gábor Horváth, one of the study’s authors, horseflies are attracted to horizontally polarized light because sunlight that reflects off of water is horizontally polarized. When horseflies and other aquatic insects discover water they can mate and lay eggs. The female variety of horseflies, however, are also attracted to linearly polarized light that reflects from the hides of their victims.
Zebra embryos, which start out with dark skin and develop white stripes before being born, raised an important question with the researchers. Did the zebra develop stripes over time in order to deter disease-carrying horseflies and other insects?
In order to determine whether a zebra’s stripes really deter horseflies, the team traveled to Budapest and set up camp at a horse farm with plenty of horseflies. The team attempted to attract horseflies using black and white striped patterns that varied in width. Using oil and glue to capture the horseflies, the narrowest black and white striped strips attracted the fewest insects.
The team of researchers also measured the attractiveness of white, dark and striped horses to horseflies. Although the team expected the results to reveal that white horses were the least attractive, they found that the striped version captured the least number of horse flies.
“We created an experimental set-up where we painted the different patterns onto boards,” researcher Susanne Akesson of Lund University posited to BBC Nature. “We put insect glue on the boards and counted the number of flies that each one attracted,” Ms. Akesson added.
While researchers have not eliminated the possibility that zebra stripes might have benefits besides the deterrence of horseflies, they believe that a zebra’s striped coat pattern is a key factor in repelling disease-carrying insects.
“We believe that escaping biting flies, which are annoying to their hosts and transmit lethal diseases, would be a very important selection factor, which may have a much stronger effect than the benefits of striped coat patterns suggested previously,” Ms. Akesson told LiveScience.
Until recently, zebra stripes were thought to be primarily designed for camouflage in the wild. Scientists believe that zebra stripes act as camouflage in two ways. First, the wavy stripes help the zebras blend into the grasses around them. A lion, one of the animal’s natural predators, is colorblind which means that it is unable to tell the difference between the grass and the zebra. Second, scientists believe that a zebra is able to use its stripes to blend in with other zebras in a herd. When a predator spots a herd of zebras, it sees a mass of black and white stripes instead of individual zebras. Although these uses for zebra stripes are still relevant, the study’s authors believe that they are no longer the primary reasons that a zebra has stripes.
“We conclude that zebras have evolved a coat pattern in which the stripes are narrow enough to ensure minimum attractiveness to tabanid flies,” the team of researchers posited in the Journal of Experimental Biology, adding that “the selection pressure for striped coat patterns as a response to blood-sucking dipteran parasites is probably high in this region [Africa].”
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