These optical illusions are why butterflies are so hard to catch

Nick Howe (0:24)

Welcome back to The Nature Podcast. This time, how butterflies can trick you into thinking they're flying one way, when they're really going another. I'm Nick Petruchow.

Butterflies are hard to catch, perhaps uniquely so. A new research in Nature proposes that this is down to their patterns, and how they confuse predators. Now, you may think that butterflies are pretty and colourful to ward off predators. Don't eat me, their stripes say, I'm poisonous. Or that they're useful to attract mates. A dull butterfly may not stand a chance against a charming, colourful competitor. And you'd be right. But that doesn't explain everything. For instance, butterflies in Europe aren't poisonous, but still have the bright patterns that suggest they are. It seems these patterns may also be doing something more. And the new study reveals something new. These patterns are sending predators the wrong way with a confusing motion cue.

Curious about these confusing colourations, I made a trip to Cornwall to meet the authors of the new Nature paper.

That's Jolion Trashanko, one of the authors of the new study, as he welcomed me into his office. He is a visual ecologist, who studies how animals interact with the world they see. And while there are a lot of ways that animals use visual cues to avoid predators, camouflage being the obvious example, he told me that butterflies may be uniquely good at this.

Jolyon Trashanko (2:29)

The really, really telling interesting thing is that there are no birds that have evolved to really specialize on catching butterflies in flight. There are lots of things we'll try and eat, moths and other insects, but butterflies, despite being the most obvious thing, they're so visually salient, you know, there's nothing much that really specializes on eating them on the wing. When you see a bird try and catch a butterfly, they sort of just try and chase around it, they can never actually grab it. So this is an interesting tell that there's something weird going on with butterflies.

Nick Howe (2:56)

It's rare, but some birds do actually catch butterflies, but they do it when they've landed.

When a butterfly is in flight or on the wing, there's no known predators that specialize in catching them. In fact, I even got to see quite how hard it is to catch butterflies in person.

George Hancock (3:15)

Oh, no. Oh, sod.

Nick Howe (3:21)

This is George Hancock, one of Gillian's colleagues and another of the authors on the new paper, trying and failing to catch a butterfly. Now, in fairness to him, he did quite expertly catch another two butterflies later on, but those had landed. Butterflies in flight seem particularly hard to catch, even with a net. So Gillian and George were curious why this was, and they thought that they could look to advance camera technology to figure it out.

These days, high-speed footage that you can slow down to a crawl is fairly commonplace. So Gillian tried to find some of this footage and run it through a vision model, which replicates how birds' eyes see to understand what was the cause of the butterfly's unique weirdness.

Jolyon Trashanko (4:07)

I just went on YouTube and took a high-speed video of a butterfly taking off. It was a BBC and Natural History video, and I just thought, right, I'll take this, put it through the model, see what happens.

And I thought I'd made a big mistake. It was colour coded, so green was upwards motion, and purple was coded as downwards motion. And this butterfly was taking off, and it was just loads of this purple, this downwards motion was really powerful. And this is before I'd really conceptually thought about how this visual illusion might work. And I thought initially, I've made an error, my code's gone wrong, so I did lots of double checking, and then, no, it was true.

Nick Howe (4:42)

Despite the butterfly clearly moving upwards as it took off, Gillian's model was identifying downwards motion.

Jolyon Trashanko (4:49)

This is really this eureka moment of realizing, wow, the way the stripes are angling in the different points in the wing beat cycle, they are turning into what we call a barber pole effect. They are creating sideways and downwards motion from a wing that's actually moving up.

Nick Howe (5:04)

The barber pole effect gets its name from, you guessed it, barber's poles. These are the often red and white rotating poles that you see outside of barbers. They're made of a cylinder painted with a diagonal spiral stripe. This spins sideways, but looks as though it's moving up or down. This is because the information about the movement is ambiguous to our brains.

We don't know what's happening to the stripes behind the pole. But because the stripes are diagonal, our brains interpret the movement as going up or down, as if they're twisting around the pole and then disappearing off the end.