Science2 publishers2 min readPublished
Butterfly wing patterns throw off predator strikes with a barber-pole motion illusion
Exeter and Essex researchers report in Nature that butterfly wing patterns in flight create motion illusions, widespread across about 400 European species. The finding is presented as helping explain why so few predators hunt butterflies on the wing, though the tests used bird-vision models and human volunteers.
The Scientist · Science desk
What happened
- Jolyon Troscianko said the wings clap together on the upstroke and peel apart on the downstroke, so the stripes shift angle like those on a spinning barber's pole.
- The team filmed real butterflies taking off at over 1,000 frames per second and analysed the footage through a computer model of bird vision.
- In a touchscreen experiment, 100 volunteers acted as predators and tried to catch virtual butterflies.
- A simulation evolved more than 50,000 wing patterns from random starts, selecting for motion confusion, and they converged on designs strikingly similar to real butterflies.
- Troscianko said the illusion can make a predator miss entirely or strike a less vulnerable part of the wing, such as the hindwing or tail.
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Why it matters
- constraint Troscianko ties the illusion to butterflies' unpredictable flight paths, so any bird trial has to separate the pattern's effect from erratic flight before the stripes get credit for the misses.
- capability Troscianko calls the pairing of high-speed footage with a bird-vision model a genuinely new way to analyse motion vision, applicable to flapping bird wings and flicking lizard and fish tails.
- precedent Zebra and snake stripes, long suspected of confusing predators' motion perception without firm evidence, become testable with the same approach.
The illusion acts at the last moment of an attack. "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," said George Hancock of the University of Exeter [4]. At more than 1,000 frames per second, each frame of the team's footage lasts a millisecond or less. A strike window of tens of milliseconds therefore spans tens of frames [1].
The effect first looked like a bug. "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," said Jolyon Troscianko, also of Exeter [8]. "First I checked this wasn't a coding error, that I'd swapped up and down somehow," he said [18]. Hancock described the result in terms of the predator's estimate. The stripes and spots interfere with how visual systems guess direction and speed, he said, "boosting false motion cues while hiding the butterfly's true heading" [13].
Each strand of the study tests a different link. Filmed flights place the illusion inside a computer model of bird vision [9], and a model is still not a bird. The species survey says such patterns are common [2]. In the touchscreen trial the patterns had to fool human eyes and fingers [10]. In the simulation, selecting for motion confusion alone produced designs strikingly similar to real wings [11]. The authors take that convergence as a sign the illusion is a major force shaping butterfly wing evolution [17]. That makes the simulation a test of sufficiency. It cannot exclude other selective pressures arriving at similar designs.
The ecological claim is looser. Few predators specialise in catching butterflies in mid-air, while moths are hunted by a wide range of visually guided aerial predators [3]. The illusion fits that difference, but a contrast between two insect groups is a correlation. The work does offer a reason butterflies can afford to be so visible. "It might seem strange that a butterfly would evolve to be so visible, especially as very few European species are toxic or unpalatable," Hancock said [12]. Butterflies and moths both use camouflage at rest, and movement breaks it [16].
I think the illusion itself is well supported by this design. The explanation for the scarce aerial specialists is plausible, pending tests with birds. The phys.org account describes no trials with live birds, and it does not report how often, or by how far, the volunteers missed [10].
What to watch
- Trials with live birds striking at flapping, patterned targets, with miss rates compared across patterns and flight paths.
- The Nature paper's own touchscreen figures: how much illusion patterns raised volunteers' miss rates or shifted strikes toward hindwings and tails.
- Whether butterfly groups with strong illusion patterns face fewer aerial specialist predators than groups without them.