“We can make hypotheses about the adaptive adaptations all day long,” he says. And Stankowich, who calls this experiment “the most natural test I’ve seen so far,” points out that sometimes it takes standing in a field with some pelts, being surrounded by flies, to make progress.
But when you’re an animal and you’re outside all the time and you don’t have the protections in the shelter that we do, it really does affect you.” While the biologists agree that the mystery of how stripes work isn’t quite solved, the answers are a little bit closer. “As a field ecologist, you get reminded of every time you go into the field-it really sucks to be bitten, they really drive you crazy. “This work has given me a much greater appreciation for biting flies as an evolutionary force,” Tombak says. “It would be interesting if something similar is happening here,” she says.įrom an evolutionary standpoint, an optical illusion is beneficial because the zebras don’t need to waste energy shooing flies by twitching their tails or stomping. And up close, their bright colors tell predators not to mess with them. She says it’s common for camouflage to work both near and far, in different ways: “This idea of a two-stage protection is quite well-established.” For example, from afar, poison dart frogs blend into their environment. Tombak’s evidence doesn’t rule out the possibility that the stripes might play interesting tricks from afar, according to Anna Hughes, a psychologist at the University of Essex who has studied how predators perceive zebra stripes. “Selection from many other sources can impact that trait.” “Once you’ve got stripes, you’ve got this anti-fly effect,” he says. Additional variation could come from random genetic drift, or separate adaptations meant to confuse predators. That, of course, begs the question of why zebras have stripes of different widths-but Ted Stankowich, an evolutionary ecologist from California State University Long Beach who was not involved in the work, says all that really matters is that zebras have them. Instead, she says, her team’s results show that “within the range of stripe widths that occurs naturally in zebras, width doesn’t make that much of a difference.” But, she says, only a couple of previous studies examined stripe width, and they rarely involved real pelts one tested painted stripes up to 5 inches wide, which is beyond what any real zebra has. Narrower stripes should create an even stronger barber pole illusion-“an enhanced perceived speed effect" as Tombak puts it-and thus stronger repulsion. And it makes sense that this illusion works close-up, as the fly is on approach to land. “You can imagine for a moving fly, just tons of objects are passing by at a very fast rate,” she says. A zebra’s stripes, she thinks, create a similarly disorienting sense of movement, which should make it harder for flies to gauge the timing and speed for a smooth landing. It creates a false perceived direction of motion, and false speed as well. “Outside of a barber shop, there’s that rotating pole that looks like it’s going up, but it’s just rotating,” says Tombak. And while they can sense motion and polarized light and process images 10 times faster than our eyes, those images are very low-res.īut like you, flies get fooled by the “barber pole” illusion-that famous diagonal red stripe that seems to spiral infinitely upwards. Flies have “compound eyes” that combine input from thousands of photoreceptors, each pointing in slightly different directions from their eye’s rounded surface. Why does it work? First, it’s helpful to know that flies don’t see the world as you do. And over the 100 rounds, the team found no obvious difference between stripes of different widths.
Zebra stripe popcorn skin#
The flies chose the impala skin about four times as often as they chose either zebra skin. It could be thermoregulation,” says Tombak. It could be some kind of social adaptation to help zebras recognize each other.
Ecologists have long scratched their heads about what evolutionary advantage could support such a conspicuous change. In African landscapes that are green, brown, blue, and yellow, painting your butt with sharp streaks of black and white seems like a death wish. In fact, the fact that zebras have stripes at all is still sort of a surprise. “That was a surprise because previous studies had indicated that there might be a difference,” says Tombak, who is currently a postdoctoral researcher at Hunter College. They also found that narrow zebra stripes don’t repel flies any better than wider ones. Tombak’s team agrees there is an illusion-but since they restricted the flies to a 4-foot-wide box, they argue that the mechanism happens up close, not from afar. Writing in Scientific Reports this month, they describe how their experiment in Kenya led to two discoveries that buck some previous theories.
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