Butterflies’ wing stripes can make predators miss
A computer simulation of over 50,000 wing patterns converged on designs strikingly similar to those found in nature, the University of Exeter said.
Stripes and spots on many butterflies’ wings create visual illusions that scramble a predator’s sense of speed and direction, so attacks often miss, researchers at the universities of Exeter and Essex have said.
A bird attacking a butterfly will often find its beak snapping at thin air. According to the research, that split-second miss is not just bad luck but is caused by a visual illusion from the butterfly’s wings.
The study, published in the journal Nature, provides the first evidence that a butterfly’s flight and wing pattern work together to create such illusions, according to a news release from the University of Exeter.
It helps solve a long-standing puzzle, the release says: butterflies have striking patterns that make them stand out, yet remarkably few predators specialise in catching them mid-flight.
“The stripes and spots on many butterflies’ wings interfere with the way visual systems try to guess the direction and speed of moving things, boosting false motion cues while hiding the butterfly’s true heading,” said Dr George Hancock, from the University of Exeter’s Centre for Ecology and Conservation.
The illusions disrupt the final “ballistic attack”, Hancock said, in the tens of milliseconds when a predator commits to grabbing its prey, with no time to change course. “As a result, birds and other predators will often simply miss.”
The underlying mechanism works much like the barber-pole illusion, the red-and-white striped pole outside barber shops, where the stripes appear to travel upward even though the pole is only spinning in place.
“Butterfly wings deform as they fly – clapping together on the upstroke and peeling apart on the downstroke – so their stripes shift angle and point in different directions as they flap,” said Dr Jolyon Troscianko, from the University of Exeter.
“The first slow-motion video of a butterfly I put through our bird-vision computer model was glowing with downwards motion even though the butterfly was moving up,” Troscianko added.
How the illusion was tested
The team filmed real butterflies taking off at over 1,000 frames per second to see how the patterns behave in flight, as seen through a bird’s eyes. They looked at the patterns of around 400 European butterflies and found that motion illusion patterns are widespread.
To test how patterns affected real attacks, 100 volunteers acted as “predators”, trying to catch virtual butterflies on a touchscreen.
Dr Anna Hughes, from the Department of Psychology at the University of Essex, said: “Butterflies’ beautiful wings are not just for show, they are vital for survival.”
Troscianko said the team thinks motion confusion is “likely to be far more widespread in nature than previously realised, from the flapping wings of birds to the flicking tails of lizards and fish”.