Zebras May 'Dazzle' Predators and Insects With Two Optical Illusions At Once
When considering camouflage, typically a creature blending into its environment comes to mind. But two researchers argue persuasively in a study out in December's issue of Zoology that zebras employ a different kind of illusion: motion camouflage. Though they are not the first researchers to propose motion camouflage as the reason that zebras possess their stunning stripes, Martin J. How, a psychologist from at the Queensland Brain Institute at the University of Queensland, Australia, and Johannes M. Zanker, a physiologist at Royal Holloway University of London, may be the first to demonstrate the mechanism by which zebras "bedazzle" not only their would-be predators, but pesky insects as well.
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"Our simulations demonstrate that the motion signals that these coat patterns generate could be a highly misleading source of information," How and Zanker wrote in Zoology. "The observer's visual system is flooded with erroneous motion signals." Those signals correspond to two visual illusions associated with motion that humans are familiar with: the wagon-wheel effect, whereby a wheel appears to be going in the opposite direction it actually is; and the barber-pole illusion, whereby a diagonally striped rotating pole appears as though the stripes are moving down rather than around.
How and Zanker compared "motion output" from 10 horse and 10 zebra images and generated histograms — graph-like representations of frequency --- of the distribution of motion for flank, rump, and background regions of the horses and zebras. The zebras' stripes elicited much stronger motion signals than did the horses' coats. Motion signals from the zebras' coats also varied greatly depending on where the stripes were located on their bodies.
"These two illusory effects act together to confuse biting insects approaching from the air, or possibly mammalian predators during the hunt, particularly when two or more zebras are observed moving together as a herd," How and Zanker wrote, pointing out that Charles Darwin was wrong when he argued that a zebra's stripes couldn't have been about camoflouge, since the animals run in open savannahs. The two researchers think on that point, anywhow, Darwin was wrong. "The functional significance of the zebra coat stripe pattern is one of the oldest questions in evolutionary biology," they wrote, and they think they now have helped answer that question.
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