Fairy tales about the origin of zebra stripes are abundant. blame sunlight filtered through tree leaves for tanning the zebras hide, claim the dark lines are scorch marks, acquired after stumbling into fire during a fight with a baboon. Scientists’ ideas of the stripes’ origins are less fanciful, but no less varied. From thermoregulation to signalling to other zebras, a lot of theories have been floated. Fewer theories have been tested, and only recently did support arise for one hypothesis: avoiding fly bites.
Despite their name, horseflies do not limit themselves to horses. There are well over of horsefly that target a wide variety of animals, including humans. However, horseflies earned their moniker due both to horses, donkeys and zebras’ extreme prevalence, and the risks these bugs pose equids. Amongst other diseases, horseflies are infection vectors for , a retrovirus from the same genus as human immunodeficiency virus (HIV).
Within the Equus family, all 7 extant members have long tails to help flick away pesky insects. But the 3 living species of zebras have an additional tool in their anti-fly arsenal in their fur patterns.
Japanese Black cows painted with white to mimic the zebra coat received deerfly bites compared to those painted with black stripes, or no stripes at all.
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Don’t go painting yourself or your animals to avoid deerfly bites just yet though. There is one important specific you need to know first: the critical width. Black and white stripes present a sort of optical illusion to deerflies. believe that stripes narrower than a (approximately 5 cm) trigger what we call the . This illusion makes a wheel, propeller or other regularly rotating object appear as if it is spinning backwards. You can see an example of it .
When faced with black and white stripes, horseflies approached their target faster and failed to decelerate in the final stages of their flights, before contacting zebra surfaces. It’s not just painted mammals that seem to work though. that horseflies will avoid landing on horses wearing a striped blanket, or other surfaces with or small (<10 cm in diameter) .
