Moth Survival Strategies And A Rodent Thumbnail Mystery

Moths’ evolutionary split between bright warning colors and subtle camouflage depends on the context. Plus, mysteries of the rodent thumbnail.

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Ira Flatow?

  Science Friday is supported by MathWorks, creator of MATLAB and Simulink software for technical computing and model-based design. MathWorks, accelerating the pace of discovery in engineering and science. Learn more at mathworks.com.

• 0:17Flora Lichtman?

  I'm Flora Lichtman, and you're listening to Science Friday.

• 0:23Flora Lichtman?

  Today in the podcast, From Wing Tints to Toenails: How Evolutionary Success Is in the Details.

• 0:31Speaker 02

  We found that the answer was very complicated, more than we thought.

• 0:39Flora 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.

• 1:24Flora Lichtman?

  Iliana, welcome to Science Friday.

• 1:26Speaker 02

  Hi, Flora. Very nice to be here.

• 1:28Flora 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.

• 1:38Speaker 02

  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.

• 2:03Flora 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.

• 2:14Speaker 02

  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 is 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.

• 2:51Flora Lichtman?

  And by cryptic, you mean camouflage?

• 2:54Speaker 02

  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.

• 3:13Flora Lichtman?

  Could the origami moth pass for real moths? Did that matter?

• 3:17Speaker 02

  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, you know, to be very, very close to something to actually tell that it's not a real moth.

• 3:51Flora Lichtman?

  Okay, what was the question you were trying to answer here?

• 3:54Speaker 02

  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, you know, 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.

• 4:22Flora Lichtman?

  What did you find?

• 4:23Speaker 02

  We found that the answer was very complicated, more than we thought.

• 4:28Flora Lichtman?

  As always.

• 4:29Speaker 02

  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 bypass any defense and

• 5:12Speaker 02

  just go for it and be pushed by competition to try those things that otherwise they wouldn't normally try.

• 5:20Flora 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?

• 5:33Speaker 02

  Exactly. Yeah, that's what we think is happening. Competition would push these predators to try anything essentially.

• 5:40Flora Lichtman?

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

• 5:44Speaker 02

  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 sort of higher cognitive abilities, like ravens and things like that, you know, that would probably behave differently from something much smaller.

• 6:08Flora Lichtman?

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

• 6:13Speaker 02

  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.

• 6:58Flora Lichtman?

  Hmm. 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?

• 7:08Speaker 02

  Yeah, that was an interesting finding as well. So we, we use that blue and black color mainly because it is, it is rare, as you say. So in nature, not many species have that particular combination to advertise that they are, you know, 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, you know, red and black and orange and black warning signals.

• 7:46Speaker 02

  If there were lots of those signals or, you know, 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, that helps us understand really why so many species use exactly the same colors across the world and across communities. There's, those, those colors work well. And if they already exist, then, you know, the prey is going to do better because predators are already trained to avoid it.

• 8:24Flora Lichtman?

  That's interesting. Do you feel like this is the definitive moth coloration experiment? Like, we're done?

• 8:32Speaker 02

  No, I don't think that really ever happens in science. But I think it was, you know, it teaches us lots, lots of things. Just that, you know, 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, you know, with smaller scale studies. And just the power of collaboration too, just knowing that we can, you know, do things like this because there's lots of researchers around the world that are interested and keen to participate.

• 9:13Flora Lichtman?

  Yeah, how many countries were you in?

• 9:15Speaker 02

  I think we were in, this was in 16 different countries and 21 locations.

• 9:21Flora Lichtman?

  Amazing. Do you think the results generalizable to other creatures beyond moths? You know, snakes or frogs?

• 9:32Speaker 02

  Yeah, I think so. I mean, some aspects of it. So, you know, things like the light, the effect of the light on the camouflage. That is something that had been already been sort of found in different, smaller scale studies. And then I think that definitely would apply to a lot of other animals that just camouflage is a strategy. And yeah, like certain, 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, other taxidermy, you know, might use their colors

• 10:17Speaker 02

  for different types of, of things as well, not only as an antipredator defense. So that's, that's important to consider.

• 10:25Flora Lichtman?

  Iliana, thank you so much for joining me today.

• 10:28Speaker 02

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

• 10:32Flora Lichtman?

  Dr. Iliana Medina is a senior lecturer in biosciences at the University of Melbourne.

• 10:42Flora Lichtman?

  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.

• 10:50Speaker 03

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

• 10:55Flora Lichtman?

  Stick around.

• 11:00Speaker 04

  Science Friday is supported by PBS, presenting Going Wild. If you love learning about the natural world and meeting the scientists who help us understand it, be sure to check out the new season of Going Wild with Dr. Ray Wynn Grant. It's the chart-topping and Webby Award-winning podcast from the folks at Nature on PBS, now with a video companion series. In this season of Going Wild, host acclaimed wildlife ecologist Dr. Ray Wynn Grant heads out into the field to explore the animals, people, and stories that make nature worth saving. Stops include a hibernating black bear den in Arkansas and an underwater shipwreck site off the Florida coast. If you're looking for a place to start, check out the season's debut episode, where Ray joins beloved biologist and influencer Jordan King in the Florida Everglades

• 11:45Speaker 04

  to learn about his unlikely journey into science and how to study a wetland when it's not very, well, wet. So don't miss out. Follow Going Wild with Dr. Ray Wynn Grant on Apple Podcasts, Spotify, YouTube, or wherever you get your podcasts. Support for Science Friday comes from Science Sandbox, an initiative of the Simons Foundation.

• 12:08Flora Lichtman?

  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 Faizeho is a mammal curator at the Field Museum in Chicago, and Rafaela Misagia is an assistant professor at the University of Sao Paulo in Brazil. Welcome to both to Science Friday.

• 12:47Speaker 05

  Thank you for having us.

• 12:49Flora Lichtman?

  Yeah, thank you. OK, why would you want a thumbnail instead of a claw?

• 12:53Speaker 05

  Yeah, sure. Basically, a nail allow you to have a more dexterity in the finger 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 an important factor on allow them to have a very good dexterity and handling.

• 13:30Speaker 06

  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 use 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.

• 13:52Flora Lichtman?

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

• 13:58Speaker 05

  Yeah, so another big find of your 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 fifty-five millions ago, already had nails on their thumb. So for us, that shows that thumb nails 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.

• 14:43Speaker 05

  If you think about like an acorn or another kind of nuts, it's a very hard shell outside that basically prevent other animals to consume. 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 have a special teeth, specialized muscles,

• 15:12Speaker 05

  the ability to hold their food with their thumbs. And in that case, the thumb nails plays a key role and allow them to have this dexterity.

• 15:22Speaker 05

  Is another key piece of the puzzle to explain how they are able to access those resource and because of that evolve in different groups, in different environments.

• 15:33Flora Lichtman?

  I'm thinking of Darwin's beaks, you know, it'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.

• 15:42Speaker 05

  Yes, exactly. It's pretty much a similar scenario.

• 15:45Flora Lichtman?

  Do all rodents have thumbnails?

• 15:48Speaker 06

  So we found that approximately eighty-five, eighty-six percent 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. That's 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.

• 16:29Flora Lichtman?

  Right, they graze.

• 16:30Speaker 06

  Yeah, yeah, exactly. So that's a good evidence of a functional relation between the thumb nails and being able to handle food.

• 16:39Flora Lichtman?

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

• 16:42Speaker 05

  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 like to be clear, claw is not just a long nail. They are different. They have a different composition.

• 17:09Flora Lichtman?

  Raphael, I'm going to 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?

• 17:20Speaker 06

  The thing is, for mammals in general and as evolutionary biologist, we tend to overlook some structures. We have some descriptions, like species descriptions, that mention the thumb, the presence of a thumb nail in rodents, but they always describe them as vestigial or rudimentary, making the impression that they are like not functional.

• 17:47Speaker 03

  And that's one of the beautiful things about this study. I can speak for myself that I was studying in schools when we started this project, and I never even thought about looking into nails.

• 18:01Flora Lichtman?

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

• 18:06Speaker 03

  Yeah, with respect and with more love, maybe. I was so surprised when I saw the first pictures of rodent's hands and thumbnails, because they are short as our nails. And they really look like a nail. Like, if you show a hand of a rodent, most people, they would not guess it was a rodent hand.

• 18:32Flora Lichtman?

  Oh, my gosh, I know. The pictures, which we'll put on our website at sciencefriday.com/thumnail, are pretty shocking. I feel it's going to be like my iPhone background, you know?

• 18:43Speaker 05

  Yeah. And for me, before this project, I never thought on rodent 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.

• 18:55Flora Lichtman?

  Thank you both so much for taking time to talk to me today.

• 18:58Speaker 03

  Yeah, thank you. Thank you for the interest in our study.

• 19:01Speaker 05

  Yeah, it was a pleasure to talk to you.

• 19:04Flora Lichtman?

  Anderson Fajoe is a mammal curator at the Field Museum in Chicago, and Raffaela Misagia is an assistant professor at the University of Sao Paulo in Brazil.

• 19:18Flora Lichtman?

  Today's episode was produced by Charles Burquist. I'm Flora Lichtman. Thanks for listening.