The colourful patterns on butterfly wings create optical illusions during flight, making it harder for birds to judge the speed and direction of a flying butterfly.
When butterflies fly, their wings create optical illusions – so observed biologists from the University of Exeter and the University of Essex (Nature, 30 September). These deceptive illusions make it difficult for birds to catch a flying butterfly. A beak will often snap shut beside, behind, or even in front of the insect.
The researchers compare the visual effect to the barber’s pole that often hangs outside a barbershop. Red, white and blue lines spiral upwards, and when the pole rotates, the lines appear to be genuinely moving upward.
Zigzag movement
It was already known that many butterflies are difficult to catch because of the zigzag movement they make in flight, which renders their trajectory rather unpredictable. The fact that the patterns on their wings also blur the gaze of birds makes it even harder to judge how fast the animals are flying and in which direction.
Using a high-speed camera, the researchers filmed a butterfly in flight. A computer model then helped them view the footage as a bird would see it. When played back in slow motion, a downward movement was visible, even though the insect was in reality flying upwards.
This gave the biologists reason to investigate further. They caught five species of butterfly in a garden and filmed the animals taking off from the ground at just over a thousand frames per second. The footage of these butterflies showed a stronger optical illusion than would have been the case had the wings carried no pattern.
The biologists then scanned images of 397 European butterflies from the Collins Guide to Butterflies. These were used to digitally recreate and analyse the butterflies’ flight, allowing the researchers to identify patterns that cause the greatest confusion. Stripes in particular proved misleading – just as the stripes on a barber’s pole fool the brain.
Computer game
Using five species of butterfly, the researchers also created a computer game in which participants had to catch butterflies by tapping them on a touchscreen. This allowed them to test how misleading the wing markings are to human observers. One hundred volunteers took part. Patterns that were predicted in theory to cause more confusion did indeed result in more misses.
The researchers then used a computer model to simulate the evolution of butterflies, modelling the development of 57,600 wing patterns. When the algorithm was set to select for markings capable of causing confusion, the resulting wings closely resembled those of butterflies that actually exist. The biologists take this as an indication that deception may be a driving force behind the evolution of wing patterns in butterflies.
Camouflage
Wouter Halfwerk of Vrije Universiteit Amsterdam finds it a thoroughly interesting study. He himself researches, among other things, the camouflage of moths. ‘This is also a form of camouflage.’ Whereas animals that are sitting still typically break their camouflage the moment they move, for butterflies the camouflage only begins once they take flight. ‘It is moving camouflage’, says Halfwerk. This places butterflies in the same category as stick insects and chameleons, which mimic the movement of plants through their stilted motions.
The novelty of this study lies in its approach, says Halfwerk. ‘They look at stripes in a different way, and at how these mislead the visual systems of predators.’ One caveat, however, is that the research is very digital in nature. The biologists have not analysed footage of birds attempting – and failing – to catch a butterfly. ‘In the past I have trained great tits myself for a study’, Halfwerk recounts. It is possible, but it is a great deal of work.
Evolutionary puzzle
Can we now say that butterflies have striking colour patterns because they use them to confuse birds? It is an evolutionary puzzle, Halfwerk explains. ‘It may also be that these colours are so important in mate choice that the disadvantage of standing out and being eaten is outweighed by the advantage of being selected.’ It is known that, in a number of butterfly species, appearance plays a role in finding a mate.
Another possibility is that the animals use their colours to signal that they are toxic. ‘Monarch butterflies, for example.’ Other butterflies are not toxic but mimic toxicity through their colour patterns. There is also the idea that wing patterns help regulate body temperature. All of these factors may play a role in butterfly evolution, says Halfwerk. In some species, creating an optical illusion will matter more than in others. ‘It really is a fascinating mechanism.’
It would be interesting to look at other animals through the lens of optical illusions as well – zebras, for instance, and writhing snakes. And the carnival of fish on a coral reef. ‘Personally, I think of raptors’, Halfwerk adds. ‘Why do they have stripes on their underside? Perhaps so that their prey doesn’t notice them.’