Zebra stripes confuse tiny predators, not the big ones

It turns out a zebra's flashy black and white coat has nothing to do with camouflaging it from big predators and everything to do with disorienting the little pesky ones. New research has determined that zebra stripes keep biting flies from from landing on them.

Naturalists have long speculated about the function of the zebra's stripes, and the camouflage theory has been the most popular notion. But evolutionary ecologist Tim Caro from the University of California, Davis, found the camouflage theory just didn't hold up to scrutiny.

"Zebras are the preferred prey of lions" said Caro in an interview with Bob McDonald on CBC's Quirks & Quarks. "So if this was an anti-predator or defense mechanism perhaps confusing the lions or camouflage zebras in some way [...] It's not working very well."

But, there was one theory that did hold up in recent experiments by Caro and his colleagues. They found that Zebras stripes were very useful in helping the Zebra avoid biting flies, like the horse fly, which in Africa often carry fatal diseases.

Stripes work, whether you're growing them or wearing them

To investigate this theory Caro observed captive zebras and horses on a farm in Britain. The Zebras there are tame, and are kept for conservation and breeding programs.

Caro closely observed flies attempting to land on horses and zebras, and one of his colleagues did video analysis as well. To ensure that it was only visual cues that the flies were making a difference, they also observed flies trying to land on horses wearing zebra-striped coats.  

They both found that horse flies were equally attracted to both the zebras and the horses.  

"Zebras received just as many investigations of horse flies coming in to land on them as did the horses themselves so there's no difference in these animals being attracted to horseflies," said Caro.  

However, when it came to landing on the zebras and the horses with striped coats he found the horse flies had trouble. "At the very last minute, they veer away."  The flies either broke off their approach, or in some cases bounced off the zebras. They seemed to consistently approach too quickly and so couldn't make a controlled landing.

As a result, flies were only able to land on zebras about a fifth as often as they were able to land on horses.

A not so smooth landing

This controlled landing, said Caro, is critical for flies. "Landing for a fly is very important part of its flight because you musn't bump into something otherwise you might damage a leg or damage your eye, and so you want to land in a controlled way."

Caro says that it isn't immediately clear what it is about the zebra's stripes that are disorienting the flies and causing their difficulty in landing.

One possibility is that the stripes disrupt what's called "optic flow" which is the perception of the apparent motion of objects in the visual field.

Another hypothesis is that the the stripes may look something like a forest to the flies, with the dark stripes resembling trees and the white spaces as gaps between them.

Whatever the explanation, it clearly works for the zebra. And that's particularly important, said Caro, because of the threat biting insects like horseflies pose in Africa, where they can carry a range of debilitating and potentially fatal diseases and parasites.

Trying out stripes of a different colour

The fact that flies not only had difficulty landing on with real zebras, but also on horses in striped coats opens up the possibilities for understanding better just what about the stripes is so disruptive to the flies.

In future work he hopes to test coats with different arrangements of stripes — thicker and thinner, horizontal as well as vertical, and different shadings and colours as well.  This should give more insight into what aspects of the stripes are most important, and how the flies visual systems are being disrupted.

Though zebra stripes seem effective at warding flies from horses during fly season, Dr. Caro wouldn't recommend it as horsewear trend just yet, though he speculates that perhaps such a coat might work on a human as well. "It raises another issue... should it be the horse or should it be the rider that's being protected?"

Zebra stripes disorient biting flies - From UC Davis