Brightly-coloured caterpillars are commonplace in the garden. The colour acts as a warning to birds that they taste unpleasant and shouldn't be eaten. But other caterpillars - which you probably haven't noticed - prefer to go undercover, camouflaging themselves cryptically to blend-in with the garden greenery. But why would different species use such different tactics to avoid becoming a blackbird's lunch? That's a mystery that Hanna Kokko has been trying to solve and she told Kat Arney about how she used some Plasticine and a bike to do so...
Hanna - One thing that a caterpillar needs to do is to grow but the other very important thing it needs to do is to avoid being eaten, and some caterpillars seem to do this by trying to hide as well as they can. So, they have cryptic coloration, it's really hard to find them in vegetation, but others seem to do exactly the opposite. They look extremely brightly coloured, they have stripes, they have hairs, all kinds of things, and these are basically signals that if you're going to eat me, something nasty will happen, so they are often toxic. But the question is, why do we have both strategies in use in nature and what determines who does what?
Kat - How do you figure out what's going on?
Hanna - The method was actually really clever here. So, we had this PhD student with a bicycle and she was going around the entire area and placing tiny little artificial larvae that everybody had created out of Plasticine, and these could be either cryptically colored, so, hard to find, or they could have this sort of very bright orange spots. And she placed them in vegetation, and the nice thing about Plasticine is that if you're a bird that tries to attack this larvae, you can see that later, because of course the bird can't eat it but there's these beak marks that are left there later. So we can come back to the same places again, five days later and then see which larvae have, so to speak, died.
Kat - I don't think I've ever heard of a scientific experiment involving a bicycle and Plasticine before, that's a great idea.
Hanna - It was me who came up with that, but it's great.
Kat - So, what did you find? I mean, which were the kind of the little brown and green larvae, did they get eaten, or the brightly colored ones?
Hanna - There was a really clear change. Actually, two changes over the season. So, very early in the season when there's only adult birds around, they are still incubating, there's no fledglings - the bright coloration was actually a protection. In the middle of the season, it was very different patterns, it was actually better to be cryptic and then late in the season, it switched again. So, the interpretation is that in the middle of the season, we have these young birds, they have no idea about life, they just go for whatever's easy to find, so that's when the brightly colored caterpillars get attacked, they get eaten. The birds are learning but in the process, they are killing their prey. Later in the season, they have actually learned that, "Ooh, this brightly colored thing might actually be very distasteful, so therefore, I better avoid it." So, late in the season, it's again better to have these warning colors from the caterpillar perspective. We looked in addition to this artificial experiment, we looked at almost 700 species, all the relevant species of caterpillars that actually occur in Finland where this study was conducted and it looked like they did have different strategies depending on when in the season they are growing. If it was a very early occurring species, so that the larvae are eating their things in early June and so on, then they were bright. In the middle of the season, they were trying to hide as well as they could and the late occurring species, they were, again, really bright which is kind of cool because it totally matches the survival pattern that we get in the Plasticine experiment.
Kat - This is a lovely example, I guess, of the insect population almost working in tandem with the bird population. Do you think this is going to happen for other systems if you look at that?
Hanna - Of course they have totally different goals. I mean, the birds want to find food and the caterpillars want exactly the opposite. They don't want the birds to find food. But I could imagine that this sort of species, our species communities in general, there can be interactions between species that you wouldn't initially think about because essentially, what's happening here is that the late occurring caterpillars, they are kind of exploiting the, you could call it the education effort of the earlier occurring species that are teaching their predators year after year again, that these are the colors you should be avoiding.
Kat - Hanna Kokko from the University of Zurich.