University of Exeter researchers investigating the mysterious ability of European butterflies to evade avian predators despite their highly visible markings found that they use a ‘barber pole’ optical illusion that scrambles a predator’s sense of speed and direction, causing missed aerial attacks.
The research team behind the dazzling solution said a similar evolutionary survival benefit may explain striped patterns seen on other prey animals, such as zebras and snakes.
How Butterflies Employ the ‘Barber-Pole’ Optical Illusion
Many North American butterfly species are toxic or unpalatable to birds and other animals that would normally prey on them, helping them survive. However, many European species don’t enjoy the same advantage. Still, despite the vibrant colors that make them stand out from great distances, European butterflies continue to flourish.
Moths and butterflies can enjoy some level of camouflage when resting. However, when they take flight, birds can target them for a mid-air interception. While this ballistic attack often results in a captured moth, the Exeter team said a bird trying the same move on a butterfly will often find its beak “snapping at thin air.”
Exeter researcher Dr Jolyon Troscianko said that the likeliest explanation involves something called the ‘barber pole illusion,’ where the stripes on a spinning red and white pole appear to travel upward even though the pole isn’t moving up and down.
“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,” Dr. Troscianko explained.
Dr George Hancock, from the University of Exeter’s Centre for Ecology and Conservation on the Penryn Campus in Cornwall, said the team suspected that the stripes and spots found on many European butterflies could “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.”
Dr. Jolyon Troscianko said those markings, combined with a butterfly’s unpredictable flight paths, “create misleading motion illusions just like the barber-pole, causing a predator to miss entirely, or to strike a less vulnerable part of the wing, such as the hindwing or tail.”
Striped Patterns Confound Bird Vision Computer Models
To test their theory, the Exeter team captured butterflies taking off using high-speed cameras capable of 1,000 frames per second. Then, the researchers uploaded the videos into a specially designed program, and something unexpected happened.
“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 explained.
The briefly confounded researcher checked to make sure the contradictory readings weren’t a coding error in the model and that they hadn’t accidentally “swapped” up and down.
“But then it dawned on me that this would be a perfect way to confuse attacking predators,” Troscianko explained.
When the team compared the pattern of the test butterflies to “around” 400 European species, they said they found that the motion illusion pattern that was confounding avian predators “is widespread.”
“A Major Force” in Shaping Wing Evolution
After confirming the models were working correctly, the researchers enlisted 100 volunteers and asked them to behave like avian “predators” by trying to catch virtual butterflies on a computer touchscreen.
They also used the computer simulations to “evolve’ over 50,000 possible butterfly wing patterns from random starting points. Critically, the team said they selected for patterns “that created the most motion confusion.”
As hoped, the team found that the patterns ‘evolved’ by the computers “converged on designs strikingly similar to those found in nature.” The team said this result suggests that the barber pole illusion “is a major force shaping butterfly wing evolution.”
“It might seem strange that a butterfly would evolve to be so visible, especially as very few European species are toxic or unpalatable,” Dr Hancock said. “But our study reveals the wide variety of different patterns butterflies can use to create illusions that confuse predators.”
Zebras and Snakes May Employ Similar Evolutionary Visual Trick
Although the study focused on the optical illusion butterflies create, Dr. Troscianko said evolution may have found a similar survival advantage for other prey animals.
“The dazzling stripes on zebras and snakes have long been suspected of confusing predators’ motion perception, but firm evidence has been hard to come by,” the researcher explained.
Troscianko suggested that their findings could offer researchers studying other animals “a genuinely new way to analyze motion vision.”
“We think motion confusion is likely to be far more widespread in nature than previously realized, from the flapping wings of birds to the flicking tails of lizards and fish,” the researcher explained, adding that “it opens up an exciting new research avenue.”
Christopher Plain has spent the last six years as Associate News Editor and Head Science Reporter at The Debrief. Follow and connect with him on X, learn about his novels at plainfiction.com, or email him at christopher@thedebrief.org.