Science Friday
Hosted by Flora Lichtman
“Rise of the Zombie Bugs” explores how parasites create real-life zombies in the insect and invertebrate world.
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This is Science Friday. I'm Flora Lixman. Today on the podcast, we'll take a look at the real-life zombies that walk among us and the zombifiers that take over their brains.
Parasitism is, it's a great way to live. You have to do barely any work at all.
It's zombie season. At least if you're watching the fungal thriller The Last of Us airing right now on Max, which chronicles what happens after a fungus turns most of humanity into zombies.
Billions of puppets with poisoned minds permanently fixed on one unifying goal: to spread the infection to every last human alive by any means necessary.
It's fiction for us, but for some organisms on the planet, it's more like a documentary. The Fungus That Zombies Humanity in the show is based on Ophiocordyceps, a real fungal group that infects ants, takes over their brains and bodies, and turns them into spore factories. But this isn't the only example of real-life zombies. My next guest found a whole book's worth of stories about the real horrifying and creative zombie makers that inhabit the Earth. And she says that studying these zombies and zombifiers can teach us about ourselves. Mindy Weisberger is a science writer and author of the new book, Rise of the Zombie Bugs: The Surprising Science of Parasitic Mind Control. Mindy, welcome to Science Friday.
Hello. Thanks for having me.
You know, when most people write nature books, they tend to gravitate towards the majesty of nature or like the cute fuzzies in the natural world. You went right for the freaky side. Why? Why zombies?
Well, fungi is cute and fuzzy, you know, from a certain perspective.
Fuzzy, at least.
Yeah, right. Well, before I was a science writer, I was a filmmaker, and I was always attracted to the kind of low-budget filmmaking. And zombie movies are a great example of that. So, you know, very early on zombies and zombies in pop culture were very much on my radar. And so it wasn't until I was working on a project for the American Museum of Natural History where I first came across the zombie ant fungus Ophiocordyceps. And that just blew my mind a little because I did not realize that there were actual zombifying organisms in nature that could infect their hosts and actively change their behavior. And then, as a science journalist,
I learned about other types of behavior-manipulating organisms. And it was just, it's just so fascinating, just the idea that this parasite can not just infect its host, but actively change its behavior in a way that does not benefit the host at all, but makes it possible for the parasite to reproduce. And that's just something that I think is uniquely fascinating and wonderful, even if it's not cute and fuzzy.
I mean, I want to hear some examples of the zombies among us. Let's go on like a slow, stiff walk through zombie town. And maybe we should start with Ophiocordyceps? This is the fungus that inspired The Last of Us? Like, how does it work? What's its deal?
Okay, so with ophiocordyceps, what happens is an ant will pick up some of the fungal spores, you know, as it's wandering over the forest floor. So the fungus will penetrate the ant's cuticle and start to multiply. And initially, the ant's behavior doesn't change at all, it just goes about its business. But after a certain point, what happens is the fungus has multiplied to the point where it actively starts changing the way the ant behaves. And what it does is it directs the ant away from the colony, which is something that's very unusual for ants to do, and makes it climb up a plant stem or leaf, bite down on the underside of the leaf, and then it dies there. And after it dies, a fungal stalk extends from behind its head, grows a stroma, and spews its spores out onto the forest
floor, where they are then picked up by ants and the cycle continues. So that's the zombie ant fungus.
I just need to go a little deeper, because this is just unimaginably weird, right? Like, it's not just like it's changing its behavior and now the ant can't walk, right? It's directing the ant to do this specific thing. And I think, like, the thing that's really hard for me to wrap my moth-eaten brain around is, how does this evolve? Like, it feels like there are a lot of steps for this to happen. Do we know for this group or another zombifier how these steps came online?
Well, what's interesting specifically about ophiocordyceps is that there's actually fossil evidence of this relationship, specifically the manipulation of this relationship that's 48 million years old. So scientists found a preserved leaf from the Meßl Pit in Germany, which is a fossil site. And there were marks along the veins of the leaf that look nearly identical to bite marks that are left on leaves today by manipulated zombie ants. So it's a really, really interesting example of this indirect evidence that shows how old this particular relationship is and how far back it goes.
I mean, how many zombifiers are out there? Like, how common is this as a strategy for parasites?
Well, one of the scientists that I spoke with for the book, Dr. Kelly Weiner-Smith, who is a biologist at Rice University and also an author, and she studies parasites. And she mentioned to me that something that parasitologists say to each other jokingly is that the first form of animal life that evolved was free-living, and the second was a parasite. Because parasitism is, it's a great way to live. You have to do barely any work at all.
It makes sense, like it's a good strategy.
Yeah, yeah, exactly. So by some estimates, looking at however many millions of animal species are around today, and these estimates vary widely, by some estimates, about 40% of those species are parasitic.
40% of all species on Earth?
Yes.
Wow, okay.
Yes. However, that does not mean that they manipulate behavior. The number that actively, they're known to actively manipulate behavior is much lower. So I think it's, you know, less than 1,000 species.
Okay, so it's quite small. I need a few other creepy examples of zombies.
A few, just a few, okay.
Well, I know, I mean, we could do this for hours. And I'm like particularly interested in these parasites that change behavior. Will you give me a couple of your favorites?
Just a couple, okay. Well, if you're a terrestrial insect, you probably are not likely to jump into water because you can't swim that well, and that's likely not going to work out so well for you. But if you are a cricket, for example, that is infected by a hairworm, hairworms need to reproduce in water. So what they do is they grow inside their insect hosts, and then when they are ready to exit, they manipulate the host, cricket, mantis, other types of insects, and sometimes spiders, to find a body of water and jump in. There are quite a few videos of this on YouTube, so if you want to see this in real time,
it's available to you. So as the drowning insect is, you know, is thrashing around in the water, you can start to see this thread-like worm start peeking out of the rear end of the insect, and then slowly start spooling out and looping and spooling and looping and spooling. It's going to take 10 minutes for it to come out, because it is coiled up inside the insect's body, it is many times the body length of the insect. And then, once it exits, it swims off to reproduce. And sometimes, if the insect hasn't drowned yet, it could conceivably get itself out of the water, but that is really not any of the hairworm's concern at this point. So.
I'm watching the hairworms right now and it is way worse than you made it sound. It is too mu- those are not small worms.
No, no.
They're really big.
No, they're not. I actually, as I was writing that chapter, I was thinking of because there, there, some of the reports of, you know, scientists that study these insects that have been parasitized by these worms, they say that it, towards the later part of infection, the later stages, when the worm is really big, you can actually see it moving through the exoskeleton. And it was reminding me a little bit of the, of my ninth month of pregnancy where.
Yeah, the most famous parasite of them all, the baby.
Exactly. It was a very personal connection. Another one, another one that is also very well represented on YouTube is the disco-eyed zombie snails. So these are snails that are infected also by a worm called Leucochloridium. And so what this worm does is it multiplies inside an infected snail and it forms these kind of sausage-shaped brood sacs. These brood sacs migrate into the eye stalks of the snail and they pulsate there. And as they do, you can see this on the videos, it looks very much remarkably like the pulsations, the undulations of a crawling caterpillar. But that's not all that the worms do. They also modify the snail's behavior to make it
wander out from the undergrowth where they usually stay, out into exposed areas where they are more likely to be spotted by hungry predatory birds that are just all too eager to swoop down and snap up what looks like a caterpillar snack. And in experiments, scientists found that the eye stalks that are stretched this way will rupture quite easily with just a touch. So it would be comparatively easy for a bird to just swoop down, snap up one of these brood sacs and swallow it. Now, birds are the definitive host of this parasite. So over many millions of years, the parasite has evolved this life cycle that involves reproduction in birds. Then the osysts are pooped out by the birds, eaten by the snails.
The larvae grow inside the snails and then they have to get back into a bird, which, this all seems like a very complicated strategy for reproduction, but it works for Leucochloridium.
I can hear the delight in your voice as you describe this.
It is. I have to say, I think because of my film background, I'm really, one of the things that attracted me to a lot of these zombie stories is how visual they are. And disco-eyed zombie snails, it doesn't get much more visual than that.
After the break, how studying nature's zombies could help us.
This is something that could potentially feed into medicines that are able to be used for people.
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Okay, do we understand how these parasites change the behavior of their hosts? Like, what are they doing to the brain?
That is something where it really isn't just the past 20 years that scientists really have started piecing this together. And the fact of the matter is, there is no one answer, there is no one silver bullet. There are some similar types of behaviors that are seen across different types of zombifiers, such as making their hosts wander, making their hosts climb, a behavior that's called summiting, having an attraction to light. But as far as the specific mechanisms, it's very hard to break these things out because behavior is something that is just, there are so many, there are so many different factors that are involved in producing behavior, and so many different compounds that are
involved, and so many different neurotransmitters. So being able to pinpoint these is a little tricky. That said, there are some examples where scientists are a little closer to finding these specific mechanisms. So for example, we are about to see an emergence of periodical cicadas that is one of the biggest in North America. And these cicadas, periodical cicadas, are zombified by a specific type of fungus that only affects periodical cicadas, called Mesosporicidina. And scientists recently found that the fungus, it floods them with a type of plant-based amphetamine called cathenone. And what scientists believe is that this promotes hyper-sexualized behavior.
So what the fungus does when it infects these cicadas, it spreads within their bodies, weakens their exoskeletons, so that the last third of their abdomen drops off and is replaced by a fungal spore plug. Then the cathenone kicks their mating sense into overdrive, and they try to mate with as many cicadas as possible, and the infection spreads that way. So that's one example of just one, of one very specific compound that was isolated from this particular relationship between a type of fungus and its insect host.
You know, we've been talking a lot about insects. Are there mammals that have been zombified? And is zombified a term of art? Like, is that the technical term?
It depends on who you're talking to. I think it's become accepted to use zombified when talking about a specific kind of behavior manipulation. And that's manipulation that's performed by a parasite on a host in a way that does not benefit the host in any way, but is essential for the parasite's reproduction. And so, yes, there in fact is a virus that does manipulate mammal behavior that you probably know. It's called rabies. If you're if you're familiar with what a rabid animal looks like, there's usually a lot of salivation and aggressiveness. And this particular combination benefits the virus because viral particles are shed very copiously in drool.
And an animal that is infected and is very aggressive will be very bitey. And so that will spread more infection more widely.
What about scientifically? What can we learn from zombification?
There is so much we can learn from zombification. For decades now, scientists have looked to some of these zombifying relationships as options for things like pest control. Because this way, if there are pest species that have a zombifying organism that targets them very specifically, that gives you an option for controlling that pest that is potentially less destructive than pesticides or other kinds of strategies. And then, of course, obviously, there's the possibility of once things get to the point where scientists are able to isolate compounds within the systems of these arthropods that are being zombified to show how they're being manipulated, there are possibilities that this can be used for developing new kinds of medicines.
One of the first things that these zombifying organisms do when they invade a host is they have to suppress its immune system. Because, you know, they need to be able to grow and they need to be able to multiply. So they're going to need to shut down whatever the natural immune response is. And so, so this is something that could potentially feed into medicines that are able to be used for people. So there are a lot of opportunities here. And I know it's tempting to look at this and have the first question on your mind be, You know, are any of them going to evolve to zombify people? That is less likely to happen, but I think there are so many more interesting questions that speak to who we are as a species and where we're going as a species and how we understand the world around us in the world of zombies.
I mean, you're interested in film before this and you wrote this book. Why do you think this phenomenon, zombies, like it's its own genre, right? Like, why are we obsessed with this? Why do we like thinking about zombies as a phenomenon?
Yeah, I mean, zombie movies and zombies in pop culture have been around for many decades, but they do seem to be having a bit of a moment right now, don't they? So I think we have a unique fascination with zombies because the issue of free will and bodily autonomy and being able to choose what you do and make your own decisions. This is something that I think is kind of intrinsic to the human condition. And it's also something that I think people are especially aware of in this moment in time, because we're at a point where for many people, they do not have the ability to have their own agency, to make their own decisions about their bodies. And so I think that looking at zombies is a way of addressing that question of what
does it mean to have agency? What does it mean to make your own decisions? And what is it about that that makes us uniquely human?
It's philosophical. It's really like it's existential.
Yeah, yeah. And I think there's also something that zombie stories in general are very, very good at, and The Last of Us does a great job of this, is they're a way of teaching us about how humans behave towards each other in crisis situations, under the most dire circumstances, which tend to bring out the worst in us and the best in us. And so I think that zombie stories and zombies in general are a way for us to kind of take a step back and in a type of world that is unlike the world we are living in day to day, still lets us address these bigger questions about how we treat each other, how we relate
to each other, how we can be better. So I think that there's something that is universal and everlasting about that. And I don't think that's going to go away anytime soon.
I love that. Mindy, thanks for taking the time.
Thank you. Thank you so much for having me.
Mindy Weisberger is a science writer and author of the new book, Rise of the Zombie Bugs: The Surprising Science of Parasitic Mind Control. And you can read an excerpt from the book at sciencefriday.com/zombie.
And that is about all we have time for. Lots of folks helped make this show happen, including
Sandy Roberts.
Robin Casmer.
Charles Burquest. George Harper.
I'm Flora Lichtman. Thanks for listening.
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