How butterflies use optical illusions to elude and escape predators

By Stephen Beech

Butterflies use optical illusions to escape predators, reveals new research.

Their flapping patterned wings create motion illusion effects — just like a traditional barber's pole — that "scramble" predators' aim, say scientists.

The new study, by researchers from the University of Exeter and the University of Essex, provides the first evidence that a butterfly's flight and wing pattern work together to create "powerful" visual illusions that confuse predators' sense of speed and direction, making them misdirect attacks.

The findings, published in the journal Nature, help solve the longstanding puzzle why butterflies have striking patterns that make them stand out, yet remarkably few predators specialize in catching them midflight.

In contrast, moths are hunted by a range of visually guided aerial predators.

George Hancock, from the University of Exeter, said: "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.

"The illusions interfere with the predator's most basic visual targeting system and disrupt the final 'ballistic attack' in the tens of milliseconds when it commits to grabbing its prey, with no time to change course.

"As a result, birds and other predators will often simply miss."

He explained that the underlying mechanism works much like the "barber-pole illusion" — the red-and-white striped pole outside barbershops, where the stripes appear to travel upward even though the pole is only spinning in place.

Jolyon Troscianko, also from the University of Exeter, said: "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.

"Combined with their unpredictable flight paths, this patterning creates 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.

More from this section

Antarctica’s ice loss is locked in, and extra snowfall won’t save it – that’s bad news for low-lying coastal areas elsewhere

+3

Pills or no pills? Study weighs antidepressant risk in pregnancy

+2

James Webb telescope reveals ‘cosmic house of mirrors’ galaxy cluster

"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.

"First I checked this wasn't a coding error, that I'd swapped up and down somehow, but then it dawned on me that this would be a perfect way to confuse attacking predators."

The team used a high-speed camera to film real butterflies taking off at over 1,000 frames per second to capture how their patterns behave in flight, as seen through the eyes of a bird.

The researchers then 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.

The team then used a computer simulation to "evolve" more than 50,000 wing patterns from random starting points, selecting for patterns that created the most motion confusion.

Patterns converged on designs strikingly similar to those found in nature, suggesting that this illusion is a major force shaping butterfly wing evolution.

Hancock said: "It might seem strange that a butterfly would evolve to be so visible, especially as very few European species are toxic or unpalatable.

"But our study reveals the wide variety of different patterns butterflies can use to create illusions that confuse predators."

He said butterflies and moths are also well known for using camouflage while resting, but movement breaks camouflage.

The new study shows how, once airborne, highly visible butterflies evolved their colors for a completely different line of defense to solve that problem.

Troscianko said: "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."

He added: "Our study gives us a genuinely new way to analyze motion vision, and 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.

"It opens up an exciting new research avenue."