Zebra stripes effective at deterring horseflies and other insect pests

The origin of zebra stripes has long been a subject of creation myths and scientific debate. From an evolutionary perspective, the first answer that probably comes to mind is camouflage.
The zebra's bold patterns are a mixed blessing: when stationary, the contrasting black and white stripes offer little protection from the prying eyes of lions. This explains why zebras seek safety in numbers - the occasional loss of an animal is offset by the benefits of a herd-wide predator detection system.
As conspicuous as they are in broad daylight, the zebra's pattern must confer some kind of benefit for them to be alive in this day and age. In the natural world, conspicuousness is selected for a variety of reasons.
Sexual selection has helped to shape the ornate tail of the peacock, helping males advertise themselves to choosy females. For the poison dart frog, its bright colors promise a fast and painful death to any victim unfortunate enough to lay eyes upon it. Finally, a Batesian mimic, such as the viceroy butterfly, is an animal that impersonates a noxious species as an insurance against predators.
The zebra is neither poisonous nor a Batesian mimic, but, perhaps, its body patterns come to use in a similar fashion. When a zebra is in motion, its body patterns may create a kind of dynamic optical illusion. The motion dazzle hampers the lion's ability to predict the zebra's speed and direction, improving its chances of escape. The rapidly-moving stripes can also interfere with the lion's perception of spatial location.
Add this to the fact that several dozen zebras will probably be bolting off in different directions during a lion attack, the lion will surely have a hard time deciding who to have for supper. At least, this is what one would think given the copious examples on camouflage as a defense mechanism.
As convincing as the camouflage theory sounds, the real story behind the zebra stripe has nothing to do with hungry lions. A research team from Hungary and Sweden recently found that zebra stripes are ideal for warding off an entirely different type of predator: blood-sucking parasites.
The idea is not new - a different group of researchers formulated the same hypothesis in 1981. This group, however, found solid proof for their claim by conducting experiments with light and dark stripes at a horsefly-infested farm.
The horsefly is the terror of grazing animals; it delivers vicious bites and can facilitate the transfer of diseases across distant populations. According to team leader G??bor Horv??th, horseflies are attracted to horizontally polarized light because horizontally polarized water reflections help them determine where they can mate and sow their eggs. In addition, blood-sucking female tabanids have the uncanny ability to pinpoint the location of their victims using polarized light reflected from their hides.
Horseflies are more attracted to dark horses than to white horses. Zebra embryos start out with dark skin, but develop their white stripes before birth. The team wanted to test whether the zebra's hide evolved to disrupt their attractive dark skins and to decrease their appeal to voracious bloodsuckers.
To find their answer, the researchers trekked to a horsefly-infested horse farm near Budapest, Hungary. To test the attractiveness of various black and white striped patterns to horseflies, the team varied the width, density and angle of the stripes, as well as the direction of polarization of the light that they reflected.
The insects then voted for the winning body pattern by trapping themselves in the oil and glue spread across the stripes. True to the hypothesis, the narrowest stripes attracted the fewest number of flies, indicating that zebras were selected for coat patterns that elicit the minimum possible response from dipteran parasites.
As follow-up, the team tested the attractiveness of horse models with varying degrees of skin tone and stripes, predicting that the striped horse would attract an intermediate number of flies. To their surprise, the striped model was the laggard among flies. The results are pretty representative of what happens in nature: when the researchers measured stripe widths and polarization patterns in actual zebras, they observed the same trend.
The consensus, then, is that zebra stripes are a wonderful adaptation: females are attracted to it, while horseflies think that it is the most horrendous body pattern to graze the African plains. Not to mention that lions are quite confounded by them.
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