Why do zebras have stripes? Perhaps to dazzle away flies

Pablo Tucker
February 23, 2019

Now, researchers have added evidence to a theory that the primary objective of the stripes is for avoiding blood-sucking parasites - horse flies.

Stripes did not deter flies from a distance as they circled horses and zebras at similar rates. It seems the stripes do something that messes with the flies own movement, making it hard to land. The flies tended to fly over or glance off the zebras; when it came to horses, many more flies were able to stick the landing.

"We showed that horse flies approach zebras and uniformly colored horses at similar rates but that they fail to land on zebras - or striped horse coats - because they fail to decelerate properly and so fly past them or literally bump into them and bounce off", said behavioral ecologist Tim Caro of the University of California-Davis, lead author of the research published in the journal PLOS ONE. But what about a zebra's stripes?

The experiment, which took place at a horse farm in the United Kingdom, involved both horses and zebras, which were dressed sequentially in black, white, and the black-and-white striped coats. Now, a new study suggests that the stripes may have a different objective entirely.

They studied photos and video footage of zebras to test the theory, which suggested the markings make optical illusions so it's hard for predators to focus on individual animals.

Scientists conducted an unusual experiment involving zebras and horses dressed in black and white striped costumes. Consequently, any horse flies that did successfully land on zebras spent less time there compared to those landing on horses, with few staying long enough to probe for a blood meal. He added that the most likely mechanisms for the deterrent effect are either that the "sudden reveal" of the stripes on close approach either surprised the insects and made them veer off, or interfered with their perception of how fast objects were moving past them, affecting their ability to land.

African horse flies carry diseases such as trypanosomiasis (which cause fever, headaches, joint pains and itching and can later on include behavioural changes, confusion and poor coordination) and African horse sickness that causes wasting.

Horses, however, primarily twitch and occasionally swish to ward off flies. Zebras swish their tails nearly continuously during the day to keep flies off, stop eating when flies bother them, and run away if the flies are particularly persistent.

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