Scientists have long known that the stripes of a zebra herd create an optical illusion that 'dazzles' predators, confusing them as to where one animal ends and another begins. However, until now, they didn't know exactly how it worked.
In a new study, researchers from the University of Queensland, Australia, and the Royal Holloway University of London used computer models to track the moving pattern of zebra stripes, and have shown that the complex patterns produced by them flood a predator's vision with 'erroneous motion signals', similar to two separate optical illusions.
The first happens due to a problem called aliasing, and is commonly called the 'wagon wheel effect'. Our eyes capture the world around us at a certain 'frame rate', similar to how a movie is filmed. When we watch a wheel or a propeller spin, eventually the spinning becomes so fast that the 'frame rate' exceeds what our eyes can handle. Depending on how fast they're moving, the wheel spokes or propeller blades can appear to suddenly stand still or they can look like they reverse direction and spin backwards. This happens with any kind of repetitive movement, though, so as the zebras move around, the motion of their stripes causes a similar effect, not just for us but for any predators or insects that might be after them as well. Since they typically move about in herds, the effect is amplified and can cause confusion as to which direction the zebras are moving.
The second illusion is called the 'barber pole illusion', and it's due to how our eyes are structured. Each individual rod or cone in our eye collects light from a specific point on an object, and it's only by processing all the information together that we get a picture of what the object is. However, in the case of a moving object that has diagonal lines on it, like a barber pole or a zebra, each bit of information our eyes receive shows the movement of the stripes, and the brain makes certain assumptions based on that apparent movement. So, as the barber pole spins, it looks like the stripes are climbing upwards, and as the zebra moves around, it looks like its stripes are moving around on its body.
This short video gives a little demonstration of the effects, just with two zebras running around:
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Zebra stripes stand out pretty well against the grasslands they live in, but they would have to give an evolutionary advantage for the species. If they didn't, the zebra wouldn't have survived until now or it would have been a completely different colour (or pattern). This new study shows exactly what that advantage is, and could lead us to understand other uses of this kind of pattern, both in nature and otherwise.
"We suggest that these illusions cause pests and predators to mistake the zebra's movement direction, causing biting insects to abort their landing maneuvers and chasing predators to mistime their attacks," said Dr. Martin How, the lead author of the study, according to BBC News.
"The results have implications for the study of patterning in animals — there are many other species such as humbug damselfish or banded snakes that use apparently conspicuous black and white stripe body patterns. The results also might help us understand how similar camouflage might function in man-made situations, such as the large-scale 'dazzle' camouflage patterns used on battleships."
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