3D-printed fake wasps help explain bad animal mimicry

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2:44Welcome back to the Nature podcast. This week, working out why imperfect animal mimics exist. And the archaeologists who took to the sea in their own handmade Stone Age canoe. I'm Sharmini Bundel. And I'm Benjamin Thompson.

3:06Mimicry is rife in the animal kingdom, with animals often pretending to be something they're not. One of these forms of mimicry is called Batesian mimicry, where a prey animal evolves to look like something dangerous or unpalatable. Take hoverflies. These harmless insects have evolved a variety of ways to look like wasps. The idea is that looking like something harmful will put off potential predators or give them pause for thought, allowing the hoverfly to escape. But many hoverflies don't look that much like wasps. On the face of it, they're not very good mimics at all. In fact, imperfect mimics are found all over the place.

3:52And this speaks to a wider question that's had researchers puzzled. Why do imperfect mimics exist? Because surely, given enough time and evolutionary pressure, mimics should become indistinguishable from the animal they're trying to copy. And yet, imperfect mimics do exist, so they must be doing something right. This week, a team is trying to shed some more light on this conundrum with the help of some 3D-printed hoverfly-like mimics, and they have a paper about it in Nature. One of the team is Christopher Taylor from the University of Nottingham here in the UK. I called him up to find out more about the experiment, but first he explained the ways that researchers have tried to figure out why imperfect mimics exist.

4:40There's a few different approaches which people have taken to try and tackle this question. One way that you can take is to use real insects to see how predators respond to them, and that gives really useful data, but it can't tell us about what's happened in the past, and it can't tell us about things which could potentially evolve in the future. The other angle that you can take is to create sort of novel stimuli, novel patterns, which might be as simple as a coloured square on a piece of paper or something. And that gives you loads of flexibility, but it's not necessarily very realistic to what the predators are used to encountering.

5:19And so in this work, you've tried to combine a lot of that work and you've gone about making mimics in a essentially rather unusual way. you've actually 3D printed them and you're looking at hoverflies in particular. That's right. We wanted to take the best of those two methods to have something which is both realistic, but also that we can manipulate and play around with. And it turned out that the level of technology was just about there for us to be able to create these realistic full-colour life-size insects. And we can also play around, we can change certain attributes to create new combinations that aren't encountered haven't been produced in nature and so yeah you've made a gamut of mimics then i think at one end you've printed what is obviously a hundred percent a fly and at the other end of the scale it's a hundred percent obviously a wasp but that area in between gives you some space to play with what things look like that's right it can sort of create this sliding scale of mimicry if you like where you start with an image of a fly on one end and we chose a fly which was not a mimic at all.

6:26But then by working out how you can go from that appearance to a wasp, we can then pick various points along that scale. So we can select a point which is halfway to the wasp or 90 % of the way to the wasp. We can then print those and we can use those in experiments to essentially ask predators what they think of them. And the predators that make up a lot of your work are songbirds then and you've tested these different mimics on these birds to see could they figure out what was real in inverted commas and what wasn't tell me about that how did you go about that we wanted to work initially with some wild predators so we went to some woodland near cambridge where there was a well-studied population of great tits and we initially trained them to come to certain locations where they knew to expect food and they learned that they could open these small dishes to obtain a reward a mealworm which is really tasty to a small bird and once we had that set up we then placed on top of those dishes our little 3d printed models as a kind of signal and the fly models would be associated with the mealworm reward but the wasp ones wouldn't so once you train the birds then that wasp equals no reward you threw your mimics into the mix as well to see what the birds would go for and what did you see then when the mimics were there?

7:51From day one basically the birds started to generalise so they would initially target the flies that they recognised that they had already learnt were rewarding but then they would start to go for the most similar ones the mimics that were quite like those flies that had a low level of mimicry to the wasps and then they would start to broaden out and experiment a bit more and try the other appearances that were less familiar to them. And then over the course of a couple of weeks, they learnt more and more effectively which of the models were rewarding and which weren't, to the point where they would very consistently, very accurately pick out all of the mimics and either ignore the wasps or leave them right until the end.

8:35And so why that is, is something you looked at as well. But in this case, she went from songbirds to chickens, chicks specifically, to see what it is about mimicry that birds pick up on. We moved to a lab setting so that we could make use of a slightly more controlled environment and a larger number of different types of models where we could take, for example, the colour of a wasp but project that onto the shape of a fly. Or we could take a model which was the size of a wasp but still had the shape and the colour of a fly. And this meant that we were able to start to figure out which aspects of the appearance were having the strongest effect on the predator's behavior.

9:18And what was the wasp characteristic then that stood out? The chicks seemed to be really strongly affected by the color of the models that they were looking at. Even if the color was slightly different to a wasp, they were able to recognize that if the color was like the wasp, the next thing they looked at was whether the size was also similar to a wasp we're talking about a difference of a couple of millimeters the chicks were picking up on this the other characteristics the the shape and the pattern seem to have less effect on what the chicks did so this is a clue that potentially if you're a mimic you don't have to be as good at mimicking the shape and the pattern as long as you've got the color and the size similar to the wasp that you're mimicking.

10:03But that's for birds, two very different sorts of birds. The animal kingdom is a big place. And so you looked at the responses of other predators as well and saw some differing results. That's right. The things which might fool a bird aren't necessarily the things which would fool, for example, a spider, which would also be in a similar situation of probably wanting to avoid attacking a wasp but would happily attack a fly. so we did some similar experiments where we took praying mantises jumping spiders and crab spiders they went through a similar learning process and then we tried them out on our intermediate 3d printed models and the praying mantis and the crab spider are sit and wait predators they tend to stay in one spot and wait for the prey to come to them and their eyesight is okay but not brilliant and they were fooled by fairly intermediate mimics that were kind of halfway between a fly and a wasp whereas the jumping spiders were more discerning they weren't as good as the birds had been but they weren't fooled by the 50 50 mimic they were only fooled by a mimic that was kind of three quarters of the way to the wasp birds and invertebrates are quite different how easy is it to compare the two there are limitations with comparing the data that we had from the birds and from the invertebrates because it's very different training a bird to training an invertebrate and there were a number of differences between the experiments you know the experiments with the great tits were done in the wild the experiments with the spiders were done in the lab so the comparison between the birds and the invertebrates needs to be treated with a little bit of caution but even within the invertebrates as I said we saw variation in how they responded to the prey.

11:59So it seems like then you can fool some of the predators some of the time and your evidence then suggests that really it's what the predator is and maybe where it lives in its mode of action that is important for how good a mimic needs to be. That's right yeah if you kind of extrapolate that to what might be happening in the wild if you've got a hoverfly which is vulnerable to birds that will be under a strong evolutionary pressure to be a very accurate mimic because we know that the birds are really good at making that distinction but if you've got another species which is perhaps smaller lives in more enclosed environments where it's less likely to be visible to a bird then those ones might still benefit from a level of mimicry that might fool a spider, for example, but they don't have the same impetus, the same incentive to reach that really high level of mercury.

12:53And this is the evidence you're putting forward. But of course, there are limitations to this work. We've covered on the podcast many times about the kind of non-visual communication that goes on or signals that come from prey and predators. I guess you've used your 3D models. They look accurate, but they don't move in many of these experiments. And presumably they smell like plastic or whatever they smell like. It's just not necessarily what a fly would smell like in the wild. Our work was focused specifically on the visual aspect of the mimicry and how good does their appearance have to be. But indeed, there's a whole other world out there of predators with different senses and mimics which might need to target those senses in order to fool a predator.

13:32It's much harder to study those areas because they're not necessarily as obvious to us as humans. And it struck me that there are a lot of theories as to why non-perfect mimics exist and your paper's out now showing what may be the case what do you think people who maybe subscribe to some of those other theories might say about this work it's important to appreciate that mimicry is a very broad phenomenon it crops up in loads of different species and our study has been focused on hoverflies and how they interact with their predators it may well be that other factors come into play in some of those other systems and of course this ultimately is evolutionary theory this is evolution at work does this act of mimicry developing over time does this speak to wider thoughts on evolution do you think i think so it's an example of a broader question about how long does natural selection need to act before a species has kind of reached the peak of what it needs in adaptation.

14:38And this is a question that can be applied in all sorts of study systems. But the advantage that we have looking at mimicry is that these traits, these characteristics are quite conspicuous ones. And they're ones which, as you've heard, we've been able to manipulate in ways that might be more difficult for other biological examples. Christopher Taylor there. To read his paper, look out for a link in the show notes. Coming up, the solar-powered sea slug that stole its chloroplasts. Right now, though, it is time for the Research Highlights with Dan Fox.

15:17After the death of the ancient Egyptian pharaoh Hatshepsut in around 1458 BC, her successor and nephew, Thutmose III, ordered the destruction of her name and image from temples. But did the new king hate his aunt, as some researchers have proposed? Or was there some other motive? To investigate, a researcher studied field notes and artefacts from 1920s excavations of Hatshepsut's mortuary temple. Evidence suggests that there were many statue fragments with nearly intact faces, suggesting that animosity towards Hatshepsut was limited. The researcher proposes that the statues were simply deconstructed by breaking them down, a ritualistic and routine process of retiring old statues that neutralised their power, essentially returning them to being merely stone.

16:09After being deconstructed, the block-like bodies of the statues were probably used as building materials and the less reusable heads were discarded, meaning that some of the damage might simply be utilitarian. Find the breakdown of that research in antiquity. Killer whales turn kelp stalks into tools that they use to groom each other while cleaning their own skin too, observations suggest. During aerial observations of an endangered group of killer whales, researchers noticed animals breaking off the ends of bull kelp stalks with their teeth. A whale would then press the piece against the body of a partner and the two would roll the kelp between them for up to 12 minutes.

16:53This behaviour was seen across all social groups, age classes and sexes, however it was more common between close relatives or similarly aged partners. The researchers think that the kelp scrub might remove dead skin from both partners' bodies. If more studies confirm this, it would be the first case of tool manufacture by any marine mammal. Scrub up on that research over at Current Biology.

17:23Finally on the show, it's time for the briefing chat, so-called, because we have chosen some stories from the Nature Briefing, Nature's weekday email newsletter with a roundup of all the latest science news. So you've been having a look, Ben. What have you picked for us this week? Yeah, the article I picked is in Nature, and it's based on a paper in the journal Cell. And it's a curious one, to be honest with you. It's about a sea slug, a solar-powered sea slug. And the story is about how they appear to have stolen a backup food source from another organism, which is kind of unusual. It's got all my favourite things, this story.

18:00It's got theft, animals, and quite a lot of alliteration of stealing sea slugs. Tell me about sea slugs. Slugs that live in the sea? Pretty much, actually. They do look, in some cases, a lot like slugs. But some of them have got some added fancy, Charmini. I mean, these amazing froms and colours and all sorts of different things, right? They really are wonderful looking things. And in this work, researchers have made a discovery that they describe in the article as, quote, the wildest thing that we had seen, end quote. And decades ago, researchers discovered that certain sea slug species store the chloroplasts from the algae they eat.

18:40Now, chloroplasts, of course, being organelles, tiny factories within some cells that allow photosynthesis to occur. Now, the sea slugs eating this algae turns them bright green. And in this case, we're talking about sea slugs from the genus Elysia. I'll just send you a picture. Have a look, see what you think. Oh, yes. So this is why you said solar-powered sea slugs, because they've actually got these stolen chloroplasts in them. Right. And oh, yes, I've got a picture here. Podcast listeners, I'm going to describe this sea slug to you. It is indeed very frilly, quite a yellowy green. Yeah, it's like a blob with frills all along both sides of it.

19:16Wonderful. Yeah, I think it looks a little bit like a lettuce leaf, doesn't it? Yes, now that you say that, yeah. And so we know then that these slugs eat algae. They consume chloroplasts. The chloroplasts are functioning given this green colour. But what wasn't understood is how these chloroplasts stick around, how they're stored, because they don't have support from the algal cell they came from. So they shouldn't be kept, right? Because it's the organelle of another creature. Yeah. How do you then keep that alive in your own body? Exactly right. And so the team wanted to know. And what they did was they used chemical tags and attached those to new proteins made by the sea slug.

19:56And they wanted to see where these proteins went. And it turned out that most of the proteins in the chloroplasts were made by the slug. So not the algae they were stolen from. Oh, right. Okay. So it's not even just like, oh, we'll just like borrow these chloroplasts. They're actively like replacing and adding proteins into it to, I guess, keep it going. Yeah. To maintain it. It seems like that's the case. And it gets a bit weirder. So looking under the microscope, they saw that the chloroplasts were being kept in special compartments in the slug's guts. And these compartments were membrane bound.

20:32And they named this structure the kleptosome from the Greek to steel. Now, this kleptosome has structures in it that actively keep the chloroplasts functioning. So more evidence that they are trying to maintain these chloroplasts and keep them going. So they've got these special stolen organelle storers. Presumably, this is so that they can use them for photosynthesis. Well, that was something that had been proposed, but it seems like that might not be exactly what's going on, at least according to this research. And so what the team did, they did an experiment where they compared these slugs with kleptosomes to a different species that didn't have them.

21:12So they don't have the chloroplasts. Now, these sea slugs without the chloroplasts, they didn't last very long in a starvation situation. They died after three to four weeks without any food. The sea slugs with the chloroplasts, they kept on going. They could survive for four months without food. But this is the kicker. After just a month, these green sea slugs turned orange like leaves in autumn. And so the photosynthesis has stopped. And it seems to be that the color change is caused by the chloroplasts breaking down. And the researchers in this work suggest that the chloroplasts could be an emergency food store.

21:54One of them describes the kleptosome where the chloroplasts are kept as like a moving refrigerator. And so when times are tough, they switch from storage to consumption. And they have this kind of snack, I suppose, kept to one side that can keep them going until they can find some more algae. So that sort of turns upside down what people have thought up until now. Yeah, I mean, I guess there's a lot to learn about these sea slugs, right? This is quite an unusual thing to find. But potentially it does speak more broadly about the evolution of organelles. Of course, eukaryotic cells, as we know them, the mitochondria that they contain, they were, as far as we know, another organism that was engulfed by a cell.

22:32The same with chloroplasts as well. so whilst on the face of it this is quite an unusual kind of animal oddity it could give researchers a bit more of an understanding about the long-term acquisition and storage i suppose of organelles and i just want to say also in this article there is a picture of an orange sea slug where it's a sad little orange lettuce color so if you want to have a look at the cute little sea slugs and their various shades of green and orange head over to the show notes and we'll put a little link to that for you there. Absolutely. Right. Well, let's move on to our second story this week.

23:06Sharmini, what have you got? So I've been reading a really nice story. There's a nature article about it in a science advances paper. And it's about some experimental archaeology, which is some archaeologists who weren't content with merely digging things up and trying to figure out what ancient people might have been doing that way. They set out to try it themselves. and they've proved that a speculative sea crossing could in fact have been achieved. So it sounds like then they were living their work. Yeah. Tell me a bit more about it. Yeah, this is intense archaeology. So imagine the globe, imagine the Earth, and we're going to zoom in on Taiwan and Japan.

23:48So what you've got is you've got the sort of long, thin Japanese islands right at the bottom of them. There's all these little islands, sort of island chain that goes down until nearly reaching Taiwan. Now, when people have looked at when these islands were settled, these are the Ryukyu Islands. They've got Paleolithic sites from around 30 ,000 years ago with stone tools. And the idea is that although people did come to Japan from the north as well, some Stone Age Paleolithic people may have travelled from Taiwan and settled from the south. But is this plausible? Is there a question? Well, my first question is that seems like probably quite a long way.

24:32What sort of distance are we talking? OK, we are talking about 140 miles. That's 225 kilometres. And from the top of some of the mountains in Taiwan, you can actually see one of the islands, right? So it's sort of tantalisingly there on the horizon. You know, it's very plausible that Paleolithic people looked out and thought, you know, there is something there. But in order to get to it, not only is it a sort of major distance, these are very low-lying islands, right? So as soon as you come back down again, you can't see what you're looking for. You can't see the island at all. So you'd have had to sit out with no visual guide in that sense.

25:12And then the other thing is the Curitio Current, one of the world's strongest ocean currents just so happens to be making its way past these islands and in order to get from Taiwan to the Ryukyu Islands you'd have to cross it and it is streaming along at over a meter per second it is really really strong and that would have been a really big challenge. So I'm guessing the question is how did the people do it and of course we are a very resourceful species. And I guess there's options. Swimming is off the table, but there are other options too. Yes. So, okay, we're thinking some kind of boat and we don't have the ancient boats from that time.

25:53We can't know for sure what kind of technology they had, what kind of boats they might've been able to make, but we can guess based on the tools that they had available, based on later technology. And so this team, they have been trying themselves to physically travel from Taiwan to the Ryukyu Islands since 2013. And the first kind of things they tried were rafts made of reeds or like long pieces of bamboo. And that didn't work. So this new paper is the success story, which is a 7.5 meter long dugout canoe. They chopped down with Paleolithic style axes, a Japanese cedar, hollowed it out, again, with all sort of authentic tools, polished and burnt the outer and inner surfaces.

26:46And five of them got in this boat one day with a lot of prep and training and set out to try and reach Japan. I mean, that is a Herculean effort. Firstly, how long did it take to make this boat, do we know? Days, yeah. Because, again, you can't use any machines, just a lot of person people power and then yeah the actual journey just over 45 hours do you know that's maybe not as long as i thought you were gonna say i thought you're gonna say it took weeks but i mean that's still plenty of time to be in open sea yeah in a hollowed out overnight and again like you can't necessarily see the islands until you're really quite close to them so navigating by the stars navigating by the direction of the swells and having to contend with this big current, which actually requires rowing really hard at a particular angle to it in order to get across it.

27:40And so success then, they made it. And their work suggests that potentially, obviously, in the absence of a time machine, potentially, this is the way that humans could have made this journey. Yeah, yeah. I mean, what has this taught them other than this is potentially one of the ways that humans could have made this journey? Well, it does really highlight how impressive this feat would have been, both the navigation, the physical rowing. This would have probably been, they think, a one-way trip. This was very much exploration. And a researcher who wasn't involved in the work said about it, these journeys are definitely much more epic than we give them credit for.

28:17And I think the people who actually did the really hard work rowing probably would agree with that, having tried it out themselves. I think they're doing some work with models as well. That's not part of this paper, looking at sea currents and what the currents would have been like then and sort of different times of year and trying to find out how easy would it have been in different circumstances to make this same crossing that they managed. But there is certainly something to be said for this kind of experimental archaeology where they've actually set out to try it themselves to give you a real insight into the possibilities and what that might have been like at the time.

28:53Well hats off to them and sticking at it for all that amount of time to try and work out how that journey could have been made. Let's leave it there for this week's Briefing Chat. And listeners, for more on those stories and where you can sign up for the Nature Briefing to get even more like them delivered directly to your inbox every weekday, check out the show notes for some links. And that is all from us for now. We will, of course, be back next week with more stories from the world of science. And if you can't wait a whole week, why not reach out? You can chat to us. We're on Blue Sky or X.

29:24Or you can even send us a good old-fashioned email. It's podcast at nature.com. I'm Sharmini Bandel. And I'm Benjamin Thompson. Thanks for listening.

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