Moth Survival Strategies And A Rodent Thumbnail Mystery

FLORA LICHTMAN: I’m Flora Lichtman, and you’re listening to Science Friday. Today in the podcast, from wing tints to toenails, how evolutionary success is in the details.

ILIANA MEDINA: We found that the answer is very complicated, more than we thought.

FLORA LICHTMAN: If you are a moth trying to stay uneaten, there are competing strategies around camouflage. Some moths try to blend in. They’re drab. They go for earth tones. Other moths take the opposite approach. They’re bold and bright with colors that send the message, don’t eat me, I’m poison, I swear. But which one of these winged wardrobes works better?

Writing in the journal Science, researchers report on a study that involved placing about 15,000 fake paper moths in forests around the world to see what they could deduce about coloration. Here to talk about it is study author Dr. Iliana Medina. She’s a senior lecturer in biosciences at the University of Melbourne. Iliana, welcome to Science Friday.

ILIANA MEDINA: Hi, Flora. Very nice to be here.

FLORA LICHTMAN: We got to start in the methods today. 15,000 origami moths. Please talk me through how you did it and why you did it.

ILIANA MEDINA: Yes. So 15,000 moths could only have been done by lots and lots of people. So it was a very, very big collaboration and tons of hours of work by many, many other researchers. And yeah, this couldn’t have happened without them, really. So yeah, we had teams essentially in all the 21 locations that were placing around 700 of those moths in each of those locations.

FLORA LICHTMAN: Still a lot of moths, I would say. 700, to be responsible for 700 moths, that’s a lot. Please walk me through the logistics and tell me what they look like.

ILIANA MEDINA: So they are very basic triangles made out of paper, really. We have three different types of targets. So we have a black and orange one that’s meant to represent the classic warning colors that we see in nature, the most common ones. Then we have a black and sort of gray blue, which is an uncommon type of warning color, but that it’s equally bright and equally contrasting as the orange one. And then we have a cryptic type of target that is essentially a brown that blends in really well with most of the bark of the trees around the world.

FLORA LICHTMAN: And by cryptic, you mean camouflaged?

ILIANA MEDINA: Yes, exactly. Yes, I mean camouflage. So a color that blends in really well with backgrounds. And then we baited these paper moths with a mealworm, essentially to test whether predators were interested in attacking this particular target or not.

FLORA LICHTMAN: Could the origami moths pass for real moths? Did that matter?

ILIANA MEDINA: Yes, that’s a very good question. So we went with this experiment because we knew that it worked before. So there’s been smaller, more localized trials of this sort of experimental design before. So we knew that the birds would be attracted to those moths. And the truth is that a lot of animals don’t have the great visual acuity that we have as humans. So they would need to be very, very close to something to actually tell that it’s not a real moth.

FLORA LICHTMAN: OK, what was the question you were trying to answer here?

ILIANA MEDINA: We were trying to answer under which conditions each of those two strategies would be favored in nature. So as you mentioned, those two strategies being hidden, camouflaged, or being warningly colored, they have evolved many times in snakes, in amphibians, in moths, even in nudibranchs. So we were interested in understanding, yeah, under which conditions which of those strategies would be favored.

FLORA LICHTMAN: What did you find?

ILIANA MEDINA: We found that the answer is very complicated, more than we thought. So we found–

FLORA LICHTMAN: As always.

ILIANA MEDINA: Yeah. So the first thing we found is that there was no overall best strategy. So the success of each of those strategies completely depends on the context and the type of environment in which the strategy is happening. The other thing we found is that predators play a very big role in determining which strategy works best.

And essentially, one of the cool things that we found was that the intensity of these interactions, and how much predation there is in a forest, really affects how likely predators are to go for those warningly colored moths. And essentially, just bypassing the defense and just go for it, and be pushed by competition to try those things that otherwise they wouldn’t normally try.

FLORA LICHTMAN: So if you’re a bird and you’re living in a place where there’s lots of other birds competing with you for dinner, you’re more likely to try the brightly colored warning sign moth.

ILIANA MEDINA: Exactly. Yeah, yeah, yeah. That’s what we think is happening is competition would push these predators to try anything, essentially.

FLORA LICHTMAN: I mean, are some birds bigger risk takers than others?

ILIANA MEDINA: Yeah, that’s hard to say. I think there’s a lot of variation, and there’s a lot of differences in boldness between birds. But there’s definitely some birds that have higher cognitive abilities like ravens and things like that, that would probably behave differently from something much smaller.

FLORA LICHTMAN: How did the camouflaged moths fare? Did you learn anything about that strategy?

ILIANA MEDINA: Yeah. So we found that the camouflage strategy, the success was highly dependent on the context and things like the illumination. So forests that have very high light levels that were very bright in those forests, that camouflage strategy would do worse.

And it would also be affected by how common that type of camouflage prey is in a forest. So essentially, if it was forest where the predators were very used to seeing lots of other camouflage prey, then they would do worse. And we think that happens because predators can form search images and essentially get really good at finding a specific type of prey, even if they are camouflaged.

FLORA LICHTMAN: Some of the moths in your study were this vibrant kind of unnatural blue color. How did they fare? Were the birds freaked out by them?

ILIANA MEDINA: Yeah, that was an interesting finding as well. So we use that blue black color mainly because it is rare, as you say. So in nature, not many species have that particular combination to advertise that they are toxic or just distasteful. That’s quite rare. And then what we found is that that familiarity with the colors matters.

So those type of prey did actually poorly in places where the predators were used to the common red and black and orange and black warning signals. If there were lots of those signals or butterflies with those types of colors in a community, then the moths with this uncommon pattern, the blue and black, would do worse.

So familiarity with the signal is important. And that helps us understand really why so many species use exactly the same colors across the world and across communities. Those colors work well, and if they already exist, then the prey is going to do better, because predators are already trained to avoid.

FLORA LICHTMAN: That’s interesting. Do you feel like this is the definitive moth coloration experiment, like we’re done?

ILIANA MEDINA: No, I don’t think that really ever happens in science. But I think it teaches us lots of things. Just that trying to compare different factors in the same study and trying to understand how all those factors interact, that brings us closer to understanding how that happens, which is impossible to do with smaller scale studies. And just the power of collaboration too, just knowing that we can do things like this, because there’s lots of researchers around the world that are interested and keen to participate.

FLORA LICHTMAN: Yeah, how many countries were you in?

ILIANA MEDINA: I think this was in 16 different countries and 21 locations.

FLORA LICHTMAN: Amazing. Do you think the results are generalizable to other creatures beyond moths? Snakes or frogs.

ILIANA MEDINA: Yeah, I think so. I mean, some aspects of it. So things like the light, the effect of the light on the camouflage, that is something that had been already been found in smaller scale studies. And then I think that definitely would apply to a lot of other animals that use camouflage as a strategy. And yeah, certain things I think that are related to the learning of predators would apply, again, to other types of animals.

Yeah, so I think that there’s generalizations, but there’s also differences and things to consider when we think about expanding our conclusions to other groups. So other taxa might use their colors for different types of things as well, not only as an anti-predator defense. So that’s important to consider.

FLORA LICHTMAN: Iliana, thank you so much for joining me today.

ILIANA MEDINA: No worries. Thank you so much for the questions. Very happy to chat.

FLORA LICHTMAN: Dr. Iliana Medina is a senior lecturer in biosciences at the University of Melbourne. Don’t go away, because after the break, we take a look at your nails. Well, actually, the thumbnails of the rodent nearest to you.

RAFAELA MISSAGIA: If you show a hand of a rodent, most people, they would not guess it was a rodent hand.

FLORA LICHTMAN: Stick around. You hear a lot about a superpower of primates, the opposable thumb. It helps us grip and grasp and hold. But it turns out that among rodents, the special appendage upgrade is slightly different. I give you the thumbnail.

Researchers dug into the science of the thumbnail and argue that, in part, it’s what helped make rodents so successful. Joining me now to talk about it are two authors of that paper. Anderson Feijo is a mammal curator at the Field Museum in Chicago, and Rafaela Missagia is an assistant professor at the University of Sao Paulo in Brazil. Welcome to you both to Science Friday.

ANDERSON FEIJO: Thank you for having us.

RAFAELA MISSAGIA: Yeah, thank you.

FLORA LICHTMAN: OK, why would you want a thumbnail instead of a claw?

ANDERSON FEIJO: Yeah, sure. Basically, a nail will allow you to have more dexterity in the fingers that have the nails. So the claw would grow over your fingertips and will kind of get in the way. So by having a nail, you have more space to have a very high dexterity on those fingers. So that’s why, for example, primates are so good in using tools. Among other things, we believe their nails were important factor on allow them to have a very good dexterity and handling.

RAFAELA MISSAGIA: I just wanted to add the tip of our fingers and rodents’ fingers are very vascularized, so they are very useful for tactile sense. They are not opposable, but they’re used in a way to manipulate food. So they are not opposable as primates, but they do have this combination of two thumbs to be able to do the manipulation.

FLORA LICHTMAN: What’s the evidence for thumbnails being the secret sauce for rodents? I mean, is this an evolutionary history story?

ANDERSON FEIJO: Yeah. So another big find of our study was that nails on rodents’ thumbs are actually very ancestral state. And we found with a very high confidence that the ancestral going back 55 million years ago already had nails on their thumb. So for us, that shows that thumbnails were present across most of the rodent evolutionary history.

One of the reasons to explain how rodents become so successful, and one of the most common mammals in the world, was because they were able to utilize and consume seeds and nuts. And this is a very hard resource to access. If you think about an acorn or another kind of nut, it’s a very hard shell outside that basically prevent all the animals to consume it. But rodents were able to access this resource.

And before, basically their specialized teeth were the main factors associated with that ability to access those foods. But we’re now saying that in addition to having special teeth, specialized muscles, the ability to hold their food with their thumbs, and in that case, the thumbnails, plays a key role and allow them to have this dexterity. It’s another key piece of the puzzle to explain how they are able to access those resources and because of that, evolve in different groups, in different environments.

FLORA LICHTMAN: I’m thinking of Darwin’s beaks. There’s just a variation. This is another tool that they use that they’re sort of an adaptation to get at the food they need.

ANDERSON FEIJO: Yeah, exactly. It’s pretty much a similar scenario.

FLORA LICHTMAN: Do all rodents have thumbnails?

RAFAELA MISSAGIA: So we found that approximately 85%, 86% of species that we analyzed have nails, which is a big portion of rodent diversity. But the interesting thing is that most of them have nails and do food manipulation. As Anderson said, it’s an ancestral condition. Most rodents are like that.

But we have some exceptions that appear many times. So one of the exceptions is that we have rodents that don’t have anything at all. They don’t have nail or claw. So they don’t handle food. They eat orally, just as cows. One example is the capybara.

FLORA LICHTMAN: Right, they graze.

RAFAELA MISSAGIA: Yeah, yeah, exactly. So that’s a good evidence of a functional relation between the thumbnails and being able to handle food.

FLORA LICHTMAN: Are claws just longer nails or are they their own thing?

ANDERSON FEIJO: They’re not the same thing. So claws and nails are basically made of keratin, but they develop differently, at least as far as we know. And for example, even the bone underneath have a different structure to support either the claws or the nail. So to be clear, a claw is not just a long nail. They are different. They have a different composition.

FLORA LICHTMAN: Rafaela, let me ask you this. Do you feel like thumbnails are kind of overlooked? I mean, they’re this tiny appendage. I don’t think we give them much thought. What’s the bigger story about this?

RAFAELA MISSAGIA: The thing is, for mammals in general, and as evolutionary biologists, we tend to overlook some structures. We have some descriptions, like species descriptions, that mention the thumb, the presence of a thumbnail in rodents, but they always describe them as vestigial or rudimentary, making the impression that they are not functional. And that’s one of the beautiful things about this study. I can speak for myself that I was studying skulls when we started this project, and I never even thought about looking into nails.

FLORA LICHTMAN: Do you look at your own thumbnails with more respect now?

RAFAELA MISSAGIA: [LAUGHS] Yeah, with respect, with more love, maybe. I was so surprised when I saw the first pictures of rodents’ hands and thumbnails, because they are short as our nails, and they really look like a nail. If you show a hand of a rodent, most people they would not guess it was a rodent hand.

FLORA LICHTMAN: Oh my gosh, I know. The pictures, which we’ll put on our website at sciencefriday.com/thumbnail, are pretty shocking. It’s going to be my iPhone background.

ANDERSON FEIJO: Yeah, and for me, before this project, I never thought on rodents’ thumbnails. But now every time I see a squirrel, it’s the first thing that comes to my mind. Like, oh my God, that’s amazing. I can only picture the thumbnail in my mind.

FLORA LICHTMAN: Thank you both so much for taking the time to talk to me today.

RAFAELA MISSAGIA: Yeah, thank you. Thank you for the interest in your study.

ANDERSON FEIJO: Yeah, it was a pleasure to talk with you.

FLORA LICHTMAN: Anderson Feijo is a mammal curator at the Field Museum in Chicago, and Rafaela Missagia is an assistant professor at the University of Sao Paulo in Brazil. Today’s episode was produced by Charles Bergquist. I’m Flora Lichtman. Thanks for listening.

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