Zebra Stripes Protect Against Biting Flies, Scientists Say

Analysis by Dr. Karen Shaw Becker

Story at-a-glance -

  • Researchers report that unlike other equine animals like most horses and donkeys, zebras, which have stripes, are better able to avoid fly bites
  • Thermographic measurements of wild impala, buffalo and zebra show that zebras’ body temperatures are no cooler than these other species living in the same climates
  • Researchers started paying more attention to the fly-biting protection concept after studies published in the 1980s noted that horseflies and tsetse flies seemed to avoid landing on striped surfaces
  • Among all the species of equids, when there are stripes, the intensity of the color patterns seemed to correlate most with areas of the world where biting flies are the most prevalent, namely Africa and Asia
  • Camera footage of fly activity showed that flies decelerated when approaching brown or black horses, but they didn’t decelerate when they approached zebras

For at least the last century and a half, scientists, including Victorian-era biologist Charles Darwin, have speculated about why zebras have stripes. There are plenty of other animals without stripes, so it was a perplexing challenge.

In recent years, the dialogue has intensified, but a consensus has been reached that the stripes are somehow helping these uniquely beautiful animals (categorized as Equus with horses, donkeys and other equine species, about 60 of which are already extinct1) to avoid biting flies.

There were other hypotheses, however. Possibilities included a camouflage to escape predators, a cooling system of sorts or a social "signature" among their peers, but these have been largely abandoned due to recent research. That zebra stripes are there to keep biting flies at bay better than a solid color "sounds preposterous at first blush," Discover Magazine remarks,2 but research supports it, especially if you look at the science behind the other three possibilities.

Why Other Zebra Stripe Theories Weren't Working

Sometimes proving something works best if researchers approach a puzzle from the opposite direction. In the case of why zebras have stripes, scientists explored each possibility by demonstrating reasons why the other suppositions were unlikely.

For the temperature regulating theory, some researchers theorized that black stripes must absorb solar heat and white stripes reflect it. Others suggested stripes might cause convection currents to flow along a zebra's back to render a cooling system. Scientists said both possibilities were "improbable," but they needed science to rule them out. According to Discover Magazine:

"Careful experiments in which large water barrels were draped in striped or uniform colored pelts, or were painted striped or unstriped, showed no differences in internal water temperatures. Moreover, thermographic measurements of zebra, impala, buffalo and giraffe in the wild show that zebras are no cooler than these other species with whom they live."3

Some scientists maintained that while zebras stand out prominently to human eyes in trees or grasslands (known as savannas in Africa), their stripes may be confusing to lions, spotted hyenas and other predators because their poorer vision makes it difficult to tell how many there are.

Those animals can only see zebras at close range — about the same distance they could hear and smell them. Zebras are the preferred prey of lions,4 which kill more than one may expect. A PLOS One study5 is just one of several to this effect, causing scientists to conclude that stripes must not be that effective as an anti-predator defense.

As for helping zebras interact with members of their own species, each individual does have unique striping patterns, rather like the fingerprints of humans. However, researchers note that horses of similar colors recognize other horses by sight and sound.6 Further, horses with stripe patterns don't perform the ritual of social bonding known as grooming on each other any more than unstriped equid species do. The rare unstriped individual zebras aren't shunned by their peers, and they breed just as successfully.7

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Experiments to Verify the Fly Biting Theory

Researchers started paying more attention to the fly-biting protection concept as the main reason for stripes after studies published in the 1980s emerged noting that horseflies and tsetse flies seemed to avoid landing on striped surfaces. Among all the species of equids, when there are stripes, the intensity of the color patterns seemed to correlate most with areas of the world where flies are the most prevalent, namely Africa and Asia. In other words:

"Wild equids indigenous to areas where annoyance from horseflies is prolonged over the year are those most likely to have marked striping patterns. We think that the reason equids need to be striped in Africa is that African biting flies carry diseases such as trypanosomiasis, African horse sickness and equine influenza which can be fatal to equids.

And zebras are particularly susceptible to probing by biting fly mouthparts because of their short cropped coats. Having a fur pattern that helped evade flies and the deadly diseases they carried would be a strong advantage, meaning stripes would be passed on to future generations."8

How do stripes deter flies? Scientist determined this with a series of experiments conducted with horses and zebras at a livery in Somerset, U.K., due to the abundance of horseflies in the summer. Scientists stress that flies don't see as well as humans, and reported that while both animals had a similar number of horsefly "approaches" (probably because of the equines' smell) zebras had far fewer landings. Meanwhile:

"Around horses, flies hover, spiral and turn before touching down again and again. In contrast, around zebras flies either flew right past them or made a single quick landing and flew off again."9

Monitoring Fly Landings on Zebras Versus Horses

Looking closely at camera footage of fly activity, researchers could see that before landing, flies decelerated when approaching brown or black horses, but they didn't decelerate when they approached zebras. Instead, they either flew straight past or literally bounced off.

Next, they placed black, white or zebra-striped coats on the same horses, covering all but their heads and lower legs, as a control for possible behavioral or aromatic attraction. Again, the flies failed to land on the stripes. The horses' heads, however, attracted as many landings (and bites) as ever. One of the best proofs of this theory is that two different companies market striped horse coats, which reportedly "really do work."10

In essence, the optical illusion once ascribed to lions and hyenas is actually true about flies.11 Zebras, it seems, have their own protection, and that understanding "has real implications for the horsewear industry, with the potential to make riding and horse maintenance less painful for horse and rider alike."

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