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EvolutionEvolution

When Keeping Flies at Bay, Zebras Can Thank Their Lucky Stripes

To solve the mystery of zebra stripes, these researchers looked to a horse of a different color.

ByKatherine J. WuNOVA WondersNOVA Wonders
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Zebra stripes are striking, but in the eyes of a hungry fly trying to land, they might spell disaster. Image Credit: John Storr, Wikimedia Commons

When you hear hoof beats, think horses. Unless, that is, you’re on safari with Tim Caro.

Caro, an evolutionary ecologist at the University of California, Davis has spent a hefty chunk of his decades-long career chasing after zebras and their distinctive stripes, trying to figure out why these eccentric equines look the way they do.

The mystery of zebra stripes has plagued evolutionary biologists (albeit, a pretty specific subset of them) for over a century. Several theories have been batted back and forth, but Caro is among those who stand staunchly by one in particular: Stripes safeguard against bloodthirsty bugs.

Today, in the journal PLOS ONE, a team of researchers led by Caro publishes the latest piece in the puzzle, showing that horseflies appear to have trouble touching down on zebra-patterned flanks. Beyond bolstering the case for the benefits of stripes, the results might also point to potential strategies for deterring nippy insects from setting their sights on vulnerable flesh—including, perhaps, that of humans.

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With their stark, vivid allure, zebra stripes are distinctive, to say the least. So perhaps it’s no surprise that this captivating coloration caught the eye of Charles Darwin, who famously debated the subject in the 1870s with his colleague Alfred Wallace. In the decades since, scientists have posited that stripes arose to protect against hungry predators, help individuals identify each other in big herds, or keep skin cool under the hot savannah sun.

There’s not much evidence for most of these ideas, however. The only theory that still holds water, Caro says, is that stripes deter biting flies. The stakes for these creatures, after all, are high: Zebra coats aren’t very thick, and one nibble from a fly can usher in the myriad deadly microbes that cause trypanosomiasis, equine infectious anemia, African horse sickness, and equine influenza.

But while previous studies have shown that flies can be thwarted by stripes (at least, compared to more monochromatic options), exactly how this aerodynamic aversion plays out has remained unclear.

Finding that answer, of course, meant looking at things from the perspective of the fly. But Caro’s modus operandi has typically been to take the zebra’s point of view (in some of his most famous experiments, Caro has even donned zebra stripes to mimic the vivacious visage of his subject of study). So Caro teamed up with Martin How, an ecologist and animal vision expert at the University of Bristol.

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Tim Caro often dons stripes in the name of science. While this particular onesie was never used for fieldwork, Caro has previously adopted the guise of a zebra to test some of his theories. Image Credit: Maurice Weiss

Together, the researchers and their colleagues monitored the flight behaviors of flies buzzing around a group of captive zebras and horses. From a distance, horses and zebras of all shades lured in thirsty flies to about the same extent. But things changed upon close approach. Though the flies sailed in to sate themselves on the hides of horses, they had lower rates of success landing on zebras, often simply ricocheting off their festive flanks, or missing them entirely.

Next, the researchers conducted a stripy switcheroo, draping both species sequentially with black, white, and black-and-white striped coats. Once again, the insects seemed flummoxed in close proximity of striped surfaces, and had difficulty maneuvering during touchdown. In the end, flies had a much easier time landing on horses costumed in plain black or white than on those draped with striped cloth.

“It’s remarkable how effective stripes are at reducing landing success,” How says. “It’s a really dramatic effect, and we see it consistently—that’s the most startling thing.”

Much like aircraft, airborne insects probably need to decelerate as they prepare for final descent, Caro explains. “We think it’s about that last half second,” he says. “It’s a dangerous situation. The fly has to slow down and make sure it doesn’t break a leg or hit its eyes or head against the object.” But stripes might just befuddle the flies into crash landings, distracting them from their original mission: nabbing mouthfuls of blood.

It’s not yet clear what it is about stripes that discombobulate the flies, but Caro, How, and their team are currently testing a few theories. One possibility is that stripes create an optical illusion that disrupts the insects’ ability to estimate distance. After all, moving stripes can look pretty wonky: Think of the common barber pole illusion, in which the rotating poles found outside of traditional barber shops seem to show stripes traveling upwards.

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As a part of their experiment, researchers placed coats of different colors and patterns on horses to test if this changed the behavior of flies trying to land on their skin. Image Credit: Tim Caro, UC Davis

When pestered, the horses and zebras behaved a little differently as well: While zebras were constantly lashing their tails and galloping away to fend off flies, horses were far less vigilant, responding with just the occasional body twitch or swish of the tail. Tails can only reach so far, though, so differences in biting can’t just be chalked up to horsey nonchalance. But it seemed zebras were going to greater lengths to avoid the mouthparts of their pint-sized nemeses.

This may all fall in line with the ultimate goal of disease avoidance, but that theory still needs rigorous testing. Additionally, Carol Hall, an animal behaviorist and equine expert at Nottingham Trent University in the UK who was not involved in the study, points out that stripes could still have other, non-insect-related perks. “This is a useful step along the way,” she says. “But I don’t know if there’s enough evidence to say that’s it, that’s all that’s going on.”

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In the meantime, it probably wouldn’t hurt to dust off striped apparel for equines and humans alike—especially if your next vacation destination overlaps with horsefly territory.

“A couple of years ago, I would’ve said I’m sure [wearing stripes] wouldn’t have any effect on avoiding flies,” Caro says. “But now I’m willing to entertain the idea that it might be useful for humans.” Football referees, rejoice.

And these effects might not be restricted to just horseflies, Caro says. Many insect eyes are built in similar ways—which means there’s a chance stripes could addle bug brains galore.

There’s not yet evidence of that, but such concerns may be increasingly relevant as the climate continues to change, Hall says. With global temperatures rising, tropical locales are expanding, allowing many insects that carry human diseases to flourish—and it’s a constant struggle to keep hungry mouths at bay.

“The more we can find out about stopping animals getting bitten, the better we will be at stopping the spread of disease,” Hall says.