Daniel and Kelly’s Extraordinary Universe - How to become a parasite

Episode Date: July 10, 2025

Daniel and Kelly talk about whether or not dinosaurs had parasites, how often parasitism has popped up in the tree of life, and more. See omnystudio.com/listener for privacy information.

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You hear the spring peepers calling out to each other in a nearby pond. Birds are chirping in a nearby pond. Birds are chirping in. the trees preparing to turn into their nest for the night. Off in the distance, you see a deer walking with her fawn. It's a beautiful scene, reminding you how wonderful it is to live in Virginia. When I hear and see these denizens of the forest, though, I see something else as well. Walking ecosystems for parasites. You see, on average, a bird species harbors more than three species of

tapeworm. Nearly three nematode species probably call that frog. home. Those deer are harboring trematodes, tapeworms, nematodes, and more. Though hidden from sight, parasites are thought to make up over half of the biodiversity of multicellular organisms on our planet. Today, we're going to answer listener questions about the evolution of parasitism. Hope you're not listening to the show while eating dinner. Welcome to Daniel and Kelly's extraordinarily infected universe. Hi, I'm Daniel. I'm a particle physicist, and I'm looking forward to meeting alien parasites.

Hello, I'm Kelly Wienersmith. I study parasites and space, but not parasites in space. But maybe one day I'll study. Yeah, maybe one day. I'll be studying parasites in space. So my question for you today, Kelly, is what is your favorite depiction of parakeet? parasites in science fiction. Probably aliens. Don't need to think about it.

Aliens, it's a great parasitoid in there. I went through a phase where I was just loving all of the zombie movies, and I was actually taking notes on, like, what mechanism was producing the zombies. So for a while, you know, when there were big fears of nuclear bombs getting dropped on the U.S., for example, you know, nuclear material was the cause of zombies. But then it became, like, infectious diseases. And I've gotten off course. I was thinking about writing a whole book on this, actually.

And why didn't you? It sounds fascinating. You know, enough people told me that, like, the zombie craze is behind us and that this is a book nobody would want to read because it would be two of the time. Yeah, two of the moment, right? Like, when people weren't watching the zombie movies, like the Last of Us, or which I guess that's running still now. Yeah. But zombies aren't a moment. They're of movement, you know.

That's right. Well, and I gave a talk at the zombie apocalypse medicine meeting. Or I think of my exam. That's a meeting. Yeah. And it's happening in October of this year. I highly recommend folks go to it. These are legit scientists.

These are legit scientists. Yes. Wow. And I gave a talk on that topic. And it was like the most fun research I've ever done in my life. Well, I think I speak for the audience in saying that we all want to read your book on the science of zombies. That sounds fantastic. You know, I've got a link to the talk that I can share if anyone wants to see the talk.

But I don't think it's going to become a full-length book. Not with what I've got on my agenda right now. All right. Well, today we're not talking about space or science fiction or zombies. We're answering a really fascinating question from a listener about the context, the evolution, and maybe even the future of parasites in our world. Take it away, Zach. Hey, there, Extraordinaries. The recent episode on the history of Trichinella, I should say the recent history of Trichinella, got me wondering about the more ancient history of parasites.

I assume that being a parasite is a strategy that existed back in the time of the dinosaurs, maybe even further back. I'm curious if we have evidence of that, and if parasites tend to become extinct when their host species does as well, or if we're still living with the same kind of parasites the dinosaurs dealt with. I'm also curious if we know about how parasites evolve, how does a organism go from being not a parasite to a parasite? Do we have any idea of that evolutionary history? Looking forward to hearing more squishy, wormy biology from you guys. Thank you, Zach, for sending in this question. If you have questions about the nature of the universe or the history of parasites or anything in this extraordinary universe, we want to hear from you.

We want to answer your questions. We want to scratch your itch because everybody's curious about the universe. So don't be shy. Write to us to questions at danielandkelly.org. Everyone gets an answer. And I am currently researching why we scratch itches, which is a question sent by a listener. So you even get those kinds of questions answered. Amazing.

Yep. So dig into this, for us, Kelly. Tell us about the history of parasites and why we call something a parasite and what fraction of species we could classify as parasites. Well, Daniel, let's pretend that you didn't read the outline, which maybe you didn't, although you're usually very well prepared. If you had to guess what percent of multicellular organisms were parasites, What percent would you guess?

It doesn't seem to me like most things are parasites. Bunnies are not parasites. Flies are not parasites. Whales are not parasites. So, you know, it seems to me unusual. So I guess 1% is my absolutely no information guess. Are you playing along? Are you really guessing 1%?

I'm really guessing 1%. Oh, wow. That's fantastic. Okay, I love those moments where you can be like, you're totally wrong. So, Daniel, you're totally wrong. Yay. Yay! So if you look at that rabbit, so rabbits are mammals.

And on average, mammals harbor one to two species of trematodes, one to two species of tapeworms, and four nematode species. Wow, I didn't want to eat rabbits before, but I certainly don't want to eat rabbits now. Well, then let's not talk about more typical food animals like cows and pigs. We'll stay away from those. But it's estimated that this was from a 2008 paper that was estimating about 44,000 host species with vertebrates and about 77,000 species of trematodes, tapeworms, nematodes, and acethecephalins. So this is four kinds of parasites, almost twice as many parasites as there are hosts, and that doesn't even include things like lice and ticks and stuff like that.

What? Hold on a second. You're telling me that like every species of critter out there has like two unique species of parasites. So there's like a two to one parasite to critter ratio? This is a great question. this is where it gets a little bit complicated. So, for example, if you have two different species of rabbit, they might share one of their trematode species, for example. So there are some parasites that are generalists,

and you can find them in more than one host species. But there are also a lot of parasites that are found in one species. And so in general, when we look at how many parasites are generalists and how many are specialists and are just found in one host, we get this estimate where we feel comfortable saying that probably more than 50% of the multicellular biodiversity on Earth is using the parasitic strategy in some way. Oh, my gosh, we're in the minority for not sucking on some other critter.

We are like unusual. We're total chumps, man. We got to rethink this. I know. There's a lot of interesting questions that arise from there. So we know that a bunch of species have this strategy of being a parasite, which essentially means that you rely on another organism where you're extracting their energy. It's not benefiting them.

and you need them to get the energy. Well, then can I ask a technical question? Like, is a cow a parasite because it relies on grass and, you know, it's extracting the energy from the grass? Why isn't a cow listed as a grass parasite? So it is eating lots of different blades of grass. And so this is usually an intimate and durable interaction between two individuals. Intimate. Wow. Intimate.

Well, you know, if you're living inside, for example, of something and sharing its dinner with it every night. That's pretty intimate. And so, yeah, cows are eating lots of different grass. We call mosquitoes and lampreys and ticks and stuff, micro-preditors. So they take little meals from lots of animals. They don't kill them, which predators usually do, but they just take little meals from lots of animals. And so the idea is that to be a parasite, you have to be on one organism for a long time. Okay, so mosquitoes technically not parasites then, right?

You would be amazing how much fur flies at parasitology conferences when the mosquito and the leech people are like these are parasites and then some of the community is like they're micro predators and the leech people are like we belong here and I'm like they belong here don't kick them out well they're parasites in spirit even if they don't qualify technically because otherwise like a giraffe which nibbles on this tree and then the other tree and the other trees basically has the same relationship with a tree as a mosquito does with people right so that's because it's not killing all of them yeah yeah and so So sometimes you can get into debates with some folks about to what extent herbivores can be thought of as parasites. So, you know, if like an insect lives on the same tree for a long time, is it a parasite or is it a different category altogether because it's a plant? And anyway, I guess at the end of the day, nature doesn't care that humans like to have very nice clear boxes to put things in. She's always giving us edge cases to make things complicated. And life is an ecosystem, right? It's a web and there's nothing isolated. We're all munching on each other and eating each other and being eaten by each other.

It's a big party, right? I don't want to go to the parties you go to, Daniel, based on what you just described. We have fun. We have fun. But how useful are these categorizations? I mean, is it matter what we call a parasite or not? So I think it does. And the main reason I think it does is because often folks will try to model the impact of parasites on populations.

or use some modeling to try to understand how to control a parasite population. And those models depend on the assumptions you make about what that organism is doing. How many organisms it's feeding on, how many babies it produces. Like having these details helps you create models that can describe different groups of strategies. And those models can be important for a lot of things, including epidemiology. I see. All right. And also, it's nice to get like-minded people together, right? And so you got people want to know which conference do I go to.

Do I go to the zombie conference or the parasite conference or the zombie parasite conference or whatever? And so we got to put names on things so that we can get ourselves organized, even if we admit that the names are fuzzy, you know, is Pluto a planet or not? Does it really matter? We're interested in studying this stuff floating in outer space. I once organized a symposium called How to Make a Zombie with my friend Zen Falks who has like the coolest name ever. And then it was me who has the least cool name ever. And it was a very interesting combo. Anyway, it was all about parasites that manipulate host behavior and how they do that.

Right, right. Yeah, parasites and zombies are connected in the end. Sure. So the bottom line is that there are lots and lots and lots of parasites out there. I was totally wrong. And there's lots of different kinds of parasites out there. But what about by body mass? Are parasites like a tiny fraction of the mass?

Or if you made a big pile of all the parasites on the planet, would it be bigger than the pile of non-parasites? This is a great question. So I didn't necessarily prepare for you to ask this question, so I don't have all the numbers that I want at my fingertips. But there was a system where this question was studied explicitly, and it was the estuaries in Southern California. And essentially, they took the biomass of all of the free-living organisms, and they extracted the parasites from them, and they took the biomass of them. And in these systems, the main predators are predatory birds. They eat the crustaceans. They're like the main predator out there.

and the biomass of trematode parasites was equal to, and maybe I think in some cases greater than, the biomass of the top predators in the system, which is to say greater than the biomass of the birds. Wow. And that's because this system has a lot of trematode parasites that infect the snails. And when the snails get infected, they get castrated. And all of the energy that would have gone to making baby snails now becomes parasite biomass. And so the ground, it's like a carpet of snails in these estuaries. There's just loads and loads of them.

I'm imagining stepping on them, which is a crunch, crunch, crunch, crunch. Yeah, it's not crunch, crunch, crunch, because it's very muddy out there. So you're, like, squishing them underground, like, they're getting covered in mud. So you do feel bad about that. I did feel bad. It was great exercise working in this system, though. This is what I did for my PhD, because every step you take, you need to, like, steal your foot back from the mud. And, anyway, good exercise.

Point is, some systems have really, really high parasite biomass. But, for example, if you were to collect a deer, the biomass of parasites in a deer is probably a very small percent. Like 1 percent? Yeah, I'd guess probably like 1%. That's where your 1% came from, Daniel. That's what I meant. Yeah. Yeah, let's say that.

That's retcon that. Totally right. You hit the nail on the head. It was perfect. If we keep asking questions, we'll eventually find one that lines up in my answer. That's right. And we got there pretty quick. We got there pretty quick.

And this biomass question that you ask depends a lot on what the strategy of the parasite is. And in our next segment, we're going to talk about different strategies. But basically, when the strategy involves killing your host, you can take up more of that host biomass because the host is going to die anyway. But now I want to talk about, you know, so we've determined that maybe as much as 50% or more of the number of species on the planet that are multicellular are parasites. But how many times did you get a transition from free living to a parasitic strategy. Right. Like, did this evolve one time and now we have a huge explosion to parasites? Or is it a

common thing that's been developed independently multiple times like flight, for example? I mean, do you want to give a like guess at how many times parasitism has evolved? Well, this time I think... You're such a good sport. I would guess that it's evolved multiple times because it seems like kind of an obvious strategy and it doesn't require like some huge technological innovation like photosynthesis or the engineering a flight even. So I would guess it's pretty common. I don't know, a dozen times. When we had Joe Wolf on the show, she pointed out that the crab body plan has popped up, I think, eight times. I think you undersold the difficulty of parasitism a little bit. So if you

are trying to infect a host, you need to find that host. You need to survive its immune system. You're now in it completely foreign environment that is actively trying to kill you all the time. But despite those challenges, we think that parasitism has shown up at least 223 times in Kingdom Animalia. I'm sorry, 223 times? 223 times. Like you can identify individual moments when something went from non-parasite to parasite in the evolutionary record hundreds of times.

Hundreds of times, yes. Wow. Why aren't we all parasites? Well, you know, there's got to be some hosts. Oh, I see. For the grifters to keep grifting, we need some suckers. That's what you're saying. And you and I are the suckers, I guess. But so sometimes you get this transition to parasitism that results in a lot of additional species. So I think when you ask people about parasites, they usually think about things like tapeworms and trematodes and nematodes and stuff like that. Unfortunately, that is my mental image, and that's why it gives me the willies.

Yeah, I'm sorry. It's intimate, though. It's very intimate. So in phylamylamylamythes, so that's where you get the trematodes and the tapeworms and stuff, there were about 13 transitions to parasitism last time we counted. But in arthropata, so these are invertebrates with exoskeletons, so we're thinking about like wasps and crustaceans and stuff like that. It evolved 143 times. Wow. So lots of transitions, and lots of times it happened in insects, and it happened 60 times in flies.

So flies, you often get a transition from free-living to parasitic strategies. How do we trace these transitions? Like, I understand we can look at the fossil record and see how whales return to the ocean, this kind of stuff. But insects, we don't have fossils. How do we know this history? We do have some insect fossils. Often they get trapped in things like Amber. And Amber actually does a really great job of preserving things and making it so you can still see their features.

But we probably are undercounting because they're probably. are a lot of like transitions that happened and then were lost. Those species went extinct. Or a lot of just instances where there was like a free living host that we just never found. And so these, you should imagine that all of these estimates are very likely to change over time. Yeah. But we have been lucky to get a fair number of fossils in the record. And the fossils are usually not beautiful exactly what you want fossils. But for example, there's a lot of tapeworms and monogenians. So these are monogenians are parasites

that you often find on the outside of fish. They have these hooks that they use to hold on to the fish and then they suck like the mucus. Sometimes they suck blood, but they live on the outside. The less intimate side, we'll say.

Yeah, slightly less intimate but no less durable. And so they use these hooks. And then tapeworms also tend to have these hooks and they use them to hold on inside. So now we've got intimate and durable. But these hooks have, very distinctive shapes, and sometimes we can find in fossilized animal poops called coprolites,

which we've talked about before, these hooks in like the pairings that you would expect and in the shapes that you would expect and inside of or like very adjacent to organisms that you would expect to find them in. And we found evidence of these sorts of relationships from the Mesozoic area, which was 66 to about 250 million years ago. This includes like the Jurassic period where, you know, Jurassic Park, and the Paleozoic period, which is even later, I think Paleozoic means something like old life. And we found some evidence of these parasites from back then, too. So it's totally reasonable to assume that, like, T-Rex was probably carrying around some nematodes. These are ancient interactions. I love thinking about when things arose, because

it's a way to figure out, like, how hard is it to happen? And also, is it like they happen on alien planets. Like if we think about how quickly did we get life after there were the conditions for life on Earth pretty quickly. How quickly did we get intelligence after there were their conditions for intelligence? Not that quickly. And it's only n equals one, but it's like a hint as to like, is this likely to happen elsewhere? And so how far back is the evidence of parasitism? How quickly did people start to slurp and munch intimately on each other? Well, so we are mostly focusing on multicellular organisms going after other multicellular organisms, I think it would be really hard to know if there was, like,

a bacterial parasite going after other organisms in the, like, primordial soup era of life. I guess bacteriophages could be considered parasites, like viruses that infect bacteria, right? Yeah. I mean, at conferences, we like to joke that the first organism was free living and the second one found some way to exploit it. But, you know, we don't know that for sure. In this economy, that's just how it works. Yeah, that's right.

And so, you know, I think pretty early on when you start getting multicellular organisms, you get parasites also. And remind us how early did we get multicellular organisms for those of us who are not evolutionary biologists? So multicellular life is probably something like 700 million years ago. And I think we have decent evidence of parasites in multicellular organisms from at least 250 million years ago. And I'm guessing that before that, it gets even harder to find evidence of parasites. And you have to be super careful that you're not tricking yourself. So, for example, there are these isopods that attach to fish. And sometimes they'll, like, live on top of their gill arches.

So, like, underneath their gill flap. And then other times they'll live on their tongue. And so if you find an isopod next to a fish, you might be like, oh, there's a parasitic isopod. But how do you know that it wasn't just, like, fossilized next to the fish? How do you know it was actually a parasitic relationship? So this stuff can be really difficult. These glimpses into deep time are super fascinating.

When I favor things in science, you know, to think that, like, Earth had life on it for billions of years before it had multicellular life. You know, like so many years, so many seasons, so many days passed with just, like, frothing bacteria and stuff floating in the oceans. Not really very interesting, which is also kind of depressing because it means, like, if you find a planet with life on it, there's a good chance. it's just a bunch of single-cell organisms, nobody really to talk to. It's incredible how rapidly things have changed. Like 700 million years ago, we get multicellular life and then, you know, things explode. And only recently have we had intelligence and technology and podcasts. I promise to not rat you out because you essentially said there's just bacteria and that's not interesting. I promise I won't tell Katrina that you said that. But I agree. Well, this is our

way to find out if she listens to the podcast or not. If I get in trouble over here. That's right. We'll keep me posted. All right. So when we come back, let's talk about categories that parasites fall into. So we were talking about how do you define and categorize these things? Let's talk about the categories that we think make the most sense.

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how this transition has happened probably more than 223 times. And there's this long-running belief in the community that what usually happens is that an organism starts specializing on dead stuff. So you go from a decomposer to a parasite. And the idea is you get good at living off of flesh when it's dead. And that sort of lowers the threshold to becoming a parasite and living on that thing and taking its energy while it's still alive. So first you identify like, hey, these corpses are rich in nutrients, I might as well eat them. And then you start eating the living versions of them. Yeah.

And so my friend Dan Metz and I wrote a paper. He was leading this paper. I was just on the support team. But we wanted to find like the papers that showed this because this is a thing that you can see found in the literature for like a hundred years or more. But when you look into the literature, it actually hasn't been tested that often. So I think a lot of us have this gut feeling that, you know, being a decomposer is. the step before becoming a parasite, but folks really haven't done the nice kind of analyses you'd need where you'd need a tree of life. You'd need to know who the free living organisms

and the parasites are. And you'd need to know what that free living organism was doing before the transition to parasitism and then look to see if certain kinds of free living organisms were more likely to transition to parasites. And you need some control groups too. And that requires a lot of natural history data. And so you and I have talked in the past about, you know, the value of taxonomy and the value of going out and collecting information. And I think it's critical because a lot of times we'll want to do these sorts of analyses and then you'll get your tree of life together and you'll be like, oh, we have no idea what any of these species are doing in the wild. And so you can't ask these questions that really help us understand, you know,

the paths that life take. You need that natural history data. And you always need data, right, especially to check your intuitions. Yeah. Because, you know, science is a story and we're putting together a history of what happened. And when we don't know, we're inclined to like fill in with our intuition. I'm saying, well, here's an idea. Maybe this is reasonable. And if it sounds natural to us, we accept it. We're like, okay, yeah, I believe that. Yeah. But sometimes the universe is not intuitive. So you always got to go out there and check, especially when you're telling a reasonable sounding story that doesn't have any actual data to support it. So more science, people, right? So much more science. Yes. So much more science. Okay. So we were talking a little bit earlier about

why you would want to try to create boxes to describe what parasites are doing, what different animals are doing. And I was arguing that you need those boxes so that you can create models to help understand what's happening. And there's a couple different folks who have come up with categories that they think keep popping up as strategies that parasites take. And I saw a quote in one of these papers that made me think, oh man, this is for Daniel. Oh, oh. So here it is. This is from Poulin, 2011. It seems safe to speculate that if Life exists on other planets, as long as alien ecosystems show some basic similarities to ours, then parasitism will not only have evolved, but alien parasites would be using exploitation strategies very close to the ones to find here.

Wow. Parasites in the universe. In space, space, space. So he was really going out on a limb here. Very strongly worded, he believes these categories explained, like, truths about paths taken by life. I think it's fascinating when scientists do that. When they find something here, they understand it, and then they say, this must be universal. We found it here, and therefore it's everywhere. It seems like it could be the only way.

And that's so often just reveals like a lack of the broader context. Like you're within a certain set of box of ways of thinking about the universe, and it's hard to imagine outside that box doesn't mean that outside the box doesn't exist. And like, as I was just saying, like, this is why we got to go and find alien parasites and blow earthly. parasitologist's minds about ways that you can suck on other people, right? There may be even more intimate ways to be a parasite we never even imagined. Totally true. As you know, and that's why I wrote this whole book about how aliens might do science trying to think outside the box. Daniel, what is that book called? It's called Do Aliens Speak Physics? And it's for sale in November. Check it out. At all fine bookstores. Yeah, no, it is a fantastic book. I got to read it.

So I love that this parasitologist is thinking about aliens, but I'm a little skeptical. that our earthly categories of parasites are the only ones in the universe. But anyway, walk us through the kinds of parasites we have found here on Earth. And everybody out there, if you're eating dinner, maybe finish up before we dig into all these intimate details. Okay. So one of the exciting things about these strategies is that organisms from very different locations on the tree of life seem to have ended up with very similar strategies,

which suggests that there's something critical about these bodies. that we're creating. Yeah. So one of the strategies, which we've already talked about, is parasitoids. So the idea here is that you infect one host species. You use all of its energy. You have to kill it as part of your life cycle. And so we get parasitic wasps that do this.

We get nematomorph. So these are sort of nematode-looking things that live in cricket. So have you ever seen that video of a cricket that jumps into a pool and a worm that's like 10 times the length of the cricket, maybe even more, glides? out of its back end. Nightmare fuel, yes. Absolute nightmare fuel. There's one with a praying mantis also, I think.

Yeah, yeah, yeah, yeah. So sometimes those animals survive, actually, after they jump into the water, which blows my mind, because you would think something that big comes out of you, you're a goner. Yeah. I think that some mornings, yes.

Oh, God. There's an intimate detail for everybody. That's right. Intimate, I hope not durable. I hope we can all forget that. Okay. So you get those like wormy things that eat almost everything inside of the organism. And it's kind of amazing because they've managed to take everything from the organism without killing it. So they're able to like avoid the critical organs. But all the like fat has been slurped out. Anything that's not absolutely critical has been slurped out and turned into parasite. And those zombie ants that you see with the cortiseps fungus, they also are exhibiting a parasitoid strategy. They kill the ant and then they use all of that energy to make their own. babies. So explain to me the word here, parasitoid, because as a non-expert, I hear that and I think that's sort of like a quasi-parasite or a semi-parasite or something. Oid makes me think like,

not quite. What does the word mean parasitoid? This is a great question. I think the original phrase came from a German scientist, and that doesn't necessarily mean that, like, their prefixes and suffixes meant something different. You're right. I'm looking up Merriam-Webster and the first definition of parasitoid is resembling a parasite, which must be the oid thing. The way that, like, our jokes resemble humor, but aren't always right on the money. You know, it's humoroid. That's right. That's right. We're a humoroid kind of podcast, which sounds maybe too close to hemorrhoid. A lot of intimate jokes today. Yes, right. So many. My guess would be, and I, this is total spitball. Okay, so there are no parasitoids that infect humans. And so I think that

when we think of parasites, we think of things that are like living in your guts or, you know, like we think of the kind of parasites that we have. And so it's possible that when parasitoid was first named, they were like, oh, like, it's living in another thing. We're familiar with that. But then it, like, kills them and bursts out of them. And it's, this is something different. So let's call it parasitoid. And I'm totally spitballing. You have asked me yet another question. I don't know the answer, too. But I'm going to double down on my answer. All right. So these guys don't live in humans, they infect their host, slurp up all the crucial bits, and then eventually kill them. Yep. That's a strategy. All right. And so if that wasn't gross enough for you,

let's hear about the next category, which I shiver to even say, parasitic castrators. Yes, lovely. So they don't kill the hosts, but they do reduce their evolutionary fitness to zero. So essentially they make it so that that organism will never be able to pass down its genes again. It won't be able to have any more of its babies. Oh, how sad. I don't know. But now the parasites get to live. Cute little parasite babies. You know, the parasite I studied for my PhD has these two little eye spots and it has like a little suckery kind of mouth that opens and closes. And by the end of my very long PhD, I took a really long time to finish. I kind of thought that they were cute. So I am capable of thinking some parasites are cute. They've infected your brain.

See, they're working their magic on you. That's right. That's right. So tell us about what these guys do. All right. So at the top of the show, we talked about those snails where a bunch of the biomass of those snails is actually going towards the production of parasite babies. That's an example of castration. All of the energy that would have gone towards making snail babies is now making parasite babies. And those snails can live for like, I think it's like a decade after getting infected. And sometimes when the tide comes in on a warm day, they can release up to 2,000 of those parasites into the water column. And so we think of them as essentially like extended versions of parasites at that point. They're not working towards their own evolutionary goals

anymore. Also, side note, the parasites that live in the snails have social behaviors. Like, do you know how ant communities have different jobs for the different ants and the ants have different shapes? Yeah. You've got like a million miles. stair right now. Is this too much? No, I'm thinking about ants. Oh, all right, okay. I was thinking about that time they poured aluminum into an ant farm. I was trying to remember like the shape of it. That's a mean but informative way of getting that information. Okay, so ants have individuals who have different jobs and they have different shapes. And they have some individuals who never have their own offspring. Their job is just to support the colony. The same thing is happening inside of these

snails. So there are attacker morphs that are essentially just like giant mouths that go around and when a parasite tries to invade the snail, because sometimes a new parasite species will come in, invade a snail that already has parasites living in there. And it could kick out the parasites that are living in there and then take over the gonad. So it's like parasite versus parasite battle inside the snail. Yes. Wow. Yes. And some kinds of trematode parasites have essentially these mouths that never reproduce on their own, their whole job, to just patrol the snail, and if a parasite comes in, they go, and they suck a hole into the side of the tree with toad and, like, suck out the insides and kill it that way.

Wow, this is like gang violence. This is our territory. Yes. Yeah, they have, like, more interesting social lives than I do. They're fascinating. How high bar is that, Kelly, tell me. Oh, my God. It's like, it's like on the ground.

You don't have to lift your foot up much at all. I don't go out much. But anyway, okay, parasitic castrators, trematodes. Let's do one other example. There are barnacles. And so, like, if you've been to the ocean, you've probably seen barnacles. There are barnacles that stick to the sides of, like, whales and turtles. There are also barnacles and isopods.

These are these little crustacean things that are both able to castrate crustaceans like crabs. So the barnacle in particular does this very interesting thing where it has a sack where female crabs would usually have their babies. And it sets up shop there. and females usually will use their claws. So this is on like the underside of what you'd think of as like the crab's belly. They use their claws to like pass water over it to make sure that their eggs stay oxygenated.

They will do that when the barnacles down there to keep the barnacle oxygenated. Wow. And males will start doing that too. So it looks like not only do they castrate male and female crabs, but they get the males to start acting like females to take care of the barnacles.

We call this feminization. And it's kind of poorly understood. But anyway, so barnacles are castrating crabs, trematodes are castrating snails, and there's insects castrating other insects. This is another very common strategy. You just said the word castrating way too many times from my comfort. Let's move on. There's a total of six categories, but rather than making you all sit through all six, let's pick one more to give you a flavor of the amazing strategies that parasites have taken on. Let's do vector transmitted parasites because that sounds physics-y. Vectors.

Ooh, all right. Well, you can try to connect this with physics at the end. We'll see. But so what we mean by vector transmitted is that you have one animal that is bringing a parasite to another animal and essentially injecting it into that other animal. Probably the most famous vector transmitted parasite is malaria. So mosquitoes will take a blood meal from a person who has malaria. they suck the parasite up. The parasite does some developing inside of the mosquito and eventually goes to live in the mosquito's salivary glands.

Wow. And then when a mosquito goes to take a blood meal from another human who isn't infected yet, the parasite ends up in that person's blood, and then that person gets infected and the cycle starts again. I think it's fascinating. You call this a blood meal.

It makes it sound so wholesome, you know, like it's just going home to have dinner after a long day of work. You know, it deserves it. But like, this thing is sucking my blood. You know, I've read stories from the early 1900s in the Journal of Parasitology where they sort of mentioned in an offhanded way that like the mosquitoes and the lice in the colonies were fed blood meals by volunteers, which means somebody was like sticking their arms in the mosquito cage. Anna Phillips was telling us in the Leach episode that now you can like fill medical gloves up with blood from, you know, a hospital or something and feed them that way. But I love the faces you make when we talk about parasites.

It's so entertaining for me. But if I can tell you my big goal here, I feel like when you start learning about parasites, you can't get over the disgust hump. It's just like so upsetting. But the more you learn about it, this doesn't happen for everybody, but maybe you'll get there. I feel like the more you learn about it, the more kind of amazing it is that evolution has resulted in this super complicated strategy. Like, I mean, imagine being able to live in environments as different as a mass. and a human, like, to conquer both of those hosts in a way that, like, human ingenuity has just not been able to outsmart. There's good evidence that malaria is able to essentially, like,

change the proteins on the outside so that our immune system can't recognize it. Like, we start to get a handle on it. And then it shows up again in, like, a whole different outfit. And our body's like, where did the malaria go? But it's still there. And it's, like, racing us. And, you know, we have, we've created all of these vaccines that keep, you know, not quite keeping up. These are kind of incredible creatures. You can hate them, and I wish there was no malaria because I don't want to see any child with malaria ever again. But you kind of have to respect sort of the amazing strategies

that they've been able to come up with with the help of natural selection. Yeah, I totally appreciate you leaning into the wonders of the universe. And, you know, that's our whole brand. And the universe is extraordinary, and these things are incredible. And I guess I'll try to think about that. Next time I see a mosquito biting me, I'll be like, wow, think about all the amazing.

science going into this itch that I'm going to be suffering for the next three days. Well, that's a micro-preditor, Daniel. I'm not interested. Just kidding. All right, so let's take another break and we come back. Let's hear some good news about how parasites can go extinct. Imagine that you're on an airplane

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Listen to America's Crime Lab on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Okay, we're back, and Kelly is grossing me out with intimate details of how parasites infect their hosts, castrate them, and pass on diseases. Yay, we're all supposed to think. So let's turn to some good news. We heard about how parasites have evolved many, many times in the record. Do parasites also go extinct? Do we have evidence for parasites disappearing? How does that happen?

They do go extinct. And so, you know, we talked about how parasitism as a strategy popped up, you know, over 200 times in the tree of life. It appears it's also been lost a few times. Oh. I know. So sometimes it's hard to get a handle on those losses because if an animal loses it and then it doesn't end up in the fossil record.

It can just be kind of complicated. But yeah, parasites go extinct. And in fact, some parasites are more susceptible to extinction than their host. And I know that's not sad for you, but we're losing diversity. So one parasite strategy that we didn't talk about in the last section are trophically transmitted parasites. Hmm. What does trophic mean? So, like, think about trophic levels in a food web.

And so trophic transmission is essentially a predator consumes an animal that was infected. and the parasite gets into the predator. So the trophically transmitted system I studied for my PhD was California Killyfish. They're these small fish in Southern California estuaries. And they get eaten by predatory birds like egrets. And the parasite goes from living on top of the fish's brain to living in the egrets gut.

I wouldn't want to live in either of those places. I mean, you're in Southern California. There's lots of beautiful places to live. Why would you choose a fish brain? But hey, you do you. I mean, I'd rather live in Virginia. But that's all right. So everyone knew that was coming.

But this parasite actually has an even more complicated life cycle. So parasites can have different strategies depending on where they are in the life cycle. So there you've got trophic transmission from fish to bird. The parasites then find love in the bird's gut and they make eggs that pass with the bird's feces into the salt marsh where they're accidentally consumed by those snails that we talked about that get castrated. And then this is such a horror movie. this whole thing is a horror movie. I love it. I eat it up. My mom loved horror movies. Maybe that's what set me up for all of this.

Yes, it's her fault, yes. Yeah, yeah. So anyway, in order for this parasite, this Tremitode parasite, to complete its life cycle, you need to have the fish, you need to have the birds, and you need to have the snail. If you lose any of those hosts in the cycle or any of them falls to numbers that are small enough where it's hard for the parasite to find them, then that animal can go extinct. And so parasites have all of these extra requirements that depend, on what's happening with their hosts. And so in some cases, we think that parasites might be more susceptible to extinction than

a lot of free-living animals because there's so many different components that need to work well for the parasite to complete the life cycle. It's kind of a fragile system. A bit of a fragile system. And so parasites can be used as what we call ecological indicators. So if you go to an estuary and you find a lot of parasites, that probably means a lot of host species are there, and they're there in pretty high numbers. And so a lot of parasites can be indicative of a healthy ecosystem, which I think is counterintuitive to some people, but makes perfect sense to my parasitologist's heart.

Like if you go to a new neighborhood and you discover there's lots of gangs there, you're like, oh, wow, this must have a thriving economy for the gangs to be siphoning money off of. Wow, where do I go with that? I mean, I guess it means that the police aren't really doing their job, which maybe means there's immune systems and hosts that are not. compromise, perhaps by pollutants, but maybe we're pulling this analogy a little bit too tight. So let's move on. All right. So they do go extinct and maybe even more often than their host because they rely on like a complex system and getting passed from one to the other.

But then they're everywhere and they're very common and they've evolved many, many times. How do we understand that all together? I mean, there's a lot of hosts also and you can be losing a lot of hosts, but you still look around and there's a lot of birds outside and stuff. It's just, you know, hard to keep track of all of this stuff. But if I can just, for one second, try to convince you that you should care about parasites going extinct. And I won't. You can try.

So parasites are important parts of ecosystem. So we mentioned that those snails that are castrated can release up to 2,000 parasites into the water. Those parasites are free-swimming bags of energy that get eaten by a lot of organisms. So they provide food to the system. Also, you know, we talked about those crickets that have those long worms in them and the crickets are forced. into jumping into the river, and then the worms, the nematomorphs, come out the back of them. Those crickets are terrestrial organisms that jump into creeks. And in Japan, there's this endangered trout where a lot of the food that it gets comes from crickets that are jumping into the creek that they wouldn't have access to otherwise.

And so these parasites often make lots of food available to other animals or make that food easier to catch. They play important roles in ecosystems that we don't always understand very well. And if you lose them, that could be bad in some cases. Yeah, I totally appreciate that. And it's sort of curious to me, like, why do parasites have this ick factor that, like, predators don't, you know, like a jaguar, it's still, like, finding another critter and taking it and eating its resources? Why is this somehow grosser to be, like, living inside as you consume somebody?

For some reason, it is definitely grosser, though, like, morally, it's not any better or worse. So, I don't know. There's just some sort of, like, human ick factor there. And maybe when we meet the aliens, they'll be like, yay, parasites. And Kelly would be like, I found my people. That's right. That's right. I understand you guys.

You know, I agree. There's definitely a judgment call. People think of parasites as like these super degenerate organisms. They have fewer sort of abilities to like sense the exterior environment. But they do manage to thrive in really complicated systems where they're being attacked. And yeah, anyway, people do tend to find parasites to be much brokered than lions. I get that some people could look at a lion and be like, that looks, much nicer than a tapeworm. But if you know what to look for in a tapeworm, some of them are

quite beautiful. I've got my tapeworm picture right here. Oh, wow. Do you know anybody who, like, has a coat made out of tapeworms or something like that? You know, tapeworms, maybe we need a fashion industry, you know, to replace fur with parasites somehow. I think that any parasite-themed fashion that is available is found at the American Society of Parasitologist's student travel auction. We bring it in and we all pay way too much for it to try to pay for student travel so that they can come to our conferences. But anyway, we're getting off track. We've got one more question from the listener and the question is, do parasites go extinct with hosts? So there's a number of different things that can happen when hosts go extinct or when hosts sort of split

into multiple species. And so, yes, often when a host goes extinct, the parasite can go extinct also. If the parasite was infecting lots of other host species, then maybe it will be fine. But we already talked about complex life cycles, where if you lose one host in the system, you can lose the parasite too. But there's also examples where you get these splits in the tree of life, and as you get those splits in the hosts, the parasites also split into new species over time because you now get like separations of parasite populations, and over enough time, they become their own species. And so there's a sort of famous example where pelicans, as they sort of speciated over time, they're lice speciated with them.

So pelicans, if you, like, create the tree of life for pelicans and then you create the tree of life for their lice, you could almost lay them on top of each other. They follow very similar paths. But you can also get cases where the parasites don't seem to be tracking what's happening with their hosts. So for example, guppies, these tiny little fish, they get infected by monogenians. These are these external parasites we were talking about that have the hooks and they hold on to the outside of the fish. They don't seem to be following the evolutionary trajectory of their hosts. And right now we don't have a really good framework for when you should expect the tree of life for parasites and species to essentially mirror one another and when you shouldn't. But we get all of these different options because nature is amazing and it depends. But we don't always know on what. An incredible variety of strategies we see helps us explore like the space of possibilities and maybe even helps us think outside the box to like what might be happening elsewhere in the universe. Absolutely. Yes. I would love to find a planet that was rich with free-living hosts and dissect a bunch of them to see if they have parasites.

But only if it was humane and ethical. All right, aliens, if you're listening, come on by. Kelly wants to look inside you. Kelly wants to say hi. In an intimate sort of way. All right, well, thank you so much to Zach for sending in his questions. Let's see if Zach feels like we've answered the many questions he posed about the evolution of parasitism. Hey there, Extraordinaries. Thank you so much for this incredible primer on parasites and their history. I feel like I got way more than I bargained for, which is awesome. You definitely answered my essential question that, yes, T-Rex probably had tapeworms or something like it that we can't be totally sure. I also really enjoyed hearing about all the different categories of parasite strategies and really appreciated hearing that there are no parasitoids or castrating parasites for humans, so I can sleep a little better at night.

I was also definitely convinced that parasites have an important role to play in our ecosystem, so I'll try to be a little bit more appreciative and a little less grossed out. I am still a little curious. Seems like it's still an open question how something goes from being a free-living organism to being a parasite, even if we can count when it happens. I'm also curious if anything has ever gone from being a parasite back to being a free-living organism. In any case, you guys rock. Thank you again so much.

All right. Thank you so much to Zach for all of his many questions. If you would like to send us questions, we would love to hear from you. Send them to Questions at Danielankelly.org. And I hope that you enjoy your meal or you ate it before we began this conversation. And before you heard Kelly say the word castrate, uncountable number of times. I don't think there's a limit on how many times you can say castrate. It's a great word. All right, everyone, thanks for joining us on this tour of one more aspect of our extraordinary universe. Daniel and Kelly's Extraordinary Universe is produced by IHeart Radio. We would love to hear from you. We really would. We want to know what questions you have about this extraordinary universe. We want to know your thoughts on recent shows, suggestions for future shows.

If you contact us, we will get back to you. We really mean it. We answer every message. Email us at Questions at Daniel and Kelly. Or you can find us on social media. We have accounts on X, Instagram, Blue Sky, and on all of those platforms, you can find us at D&K Universe. Don't be shy.

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