Real-life zombies

Zombies might seem like the stuff of horror movies, but there are lots of examples of parasites taking over bugs’ bodies and bending them to their will. Guest: Mindy Weisberger, author of Rise of the Zombie Bugs: The Surprising Science of Parasitic Mind-Control For show transcripts, go to ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/unxtranscripts⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠For more, go to ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/unexplainable⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠And please email us! ⁠⁠⁠[email protected]⁠⁠⁠We read every email. Support Unexplainable (and get ad-free episodes) by becoming a Vox Member today: ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/members⁠⁠⁠⁠⁠⁠⁠⁠Thank you! Learn more about your ad choices. Visit podcastchoices.com/adchoices

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• 1:10Speaker 02

  All right, so we're going to fade up on a moonlit field.

• 1:16Speaker 02

  You can see tall grasses moving in the breeze, a little bit of moonlight. And you can see on one of the blades of grass, there's an insect with its wings spread wide.

• 1:25Speaker 03

  This is Mindy Weisberger, a science writer.

• 1:28Speaker 02

  It looks like it might be a bee because it seems to be striped with bands of yellow and black. But you get a little closer and you see that it is not a bee. It is in fact a fly. It is a dead fly. And the yellow bands that are striping its body are actually a fungus that has grown inside it and then burst through the seams in its exoskeleton.

• 1:51Speaker 03

  How did this fly become the star of a real-life horror movie? Well, several days ago, a spore from a fungus landed on it. And the spore buried inside of the fly and then started extending little filaments throughout its body. These fungal tendrils essentially that started digesting the fly from the inside out, consuming its fat cells.

• 2:21Speaker 03

  Now, the fly, as this was happening, behaved pretty normally, at least at first.

• 2:27Speaker 02

  But over the next few days, the fungus continued to grow. And it started congregating its cells first around the fly's heart, and then around, what is the spinal cord in insects, and then around its brain. And then, after it had pretty much consumed most of the fat reserves in this fly, it was ready for the next stage, reproduction. So, this is where the fly stops behaving like a fly and starts behaving like a fungus in a fly suit. It finds a place where it can climb. And it's thought that getting to a higher position is better for the fungus

• 3:10Speaker 02

  because it produces these spores that are released in kind of an explosive poof. And the higher they are, the more widely they are likely to disperse. So, the infected fly climbs up a blade of grass until it is literally no longer able to walk anymore. Again, stuffed with fungus. So, at a certain point, stops walking.

• 3:33Speaker 03

  It extends its proboscis, which is a little trunk-like mouth part, and oozes out a sticky substance to basically glue it into place. And then, the fly dies. Its abdomen cracks open, and the spores come spilling out, ready to land on new flies and infect them. And to increase the odds that a fly will get spored, the fungus turns into a kind of horrible fly fatale.

• 4:03Speaker 02

  These dead fungus striped flies with their big bloated abdomens are very attractive to male flies who are looking to mate.

• 4:14Speaker 03

  Apparently, male flies of this species find big, swollen abdomens very alluring. And they also find this scent that the fungus starts to pump out, this chemical signal alluring tube, like a sexy fungal love perfume.

• 4:32Speaker 02

  That also sends a signal to male flies, like, hey, you know, maybe here's something that you wanna, you know, you wanna hit that. I'm sorry.

• 4:43Speaker 03

  All of this draws in new victims who, if everything works out, will soon be turned into their own fungal fly puppets.

• 4:58Speaker 03

  Now, Mindy has a word for this kind of horror story: zombification. She calls these flies real-life zombies, and the fungus that takes them over is a real-life zombifier. And it turns out, there are a lot of examples of zombies and zombifiers out in nature. There are so many, in fact, that Mindy wound up writing a whole book about them, called Rise of the Zombie Bugs. So, this is Unexplainable. I'm Britt Pinkerton. And today on the show, Mindy will tell us what we know and what we don't know about zombie bugs.

• 5:57Speaker 03

  Okay, let's start by just defining some terms. What do we mean when we say, like, zombifier? Or zombie?

• 6:07Speaker 02

  Sure. So, a zombifier is an organism that manipulates the behavior of its host. And a zombie is an organism that is being manipulated to behave in a way that it normally would not, and which only benefits the parasite that's manipulating it. So, to a certain extent, like, let's say you catch a cold. You're going to change your behavior because you're feeling sick. You feel like you need to rest more, you need to drink more water. These are all things that help you fight off the infection. So, in a certain sense, that's the cold virus generating a change in behavior. But- It's a behavioral change that actually benefits you.

• 6:50Speaker 03

  Mm hmm.

• 6:51Speaker 02

  For a zombie, the changes to its behavior are not something that benefit the host. They only benefit the parasite. That's what makes it a zombie.

• 6:59Speaker 03

  So it'd be like if I got sick and instead of going into my room and trying to sleep it off, I went and I licked everybody that I could lick in order to spread the cold.

• 7:10Speaker 02

  Exactly. And so there are zombifying viruses, there are zombifying fungi, there are insects that that are able to zombify their hosts, there are worms that can zombify their hosts. So most of the organisms that they infect are arthropods. So.

• 7:32Speaker 03

  Bugs, but the colloquial version of bugs.

• 7:34Speaker 02

  Bugs. Exactly. I do have to apologize to entomologists because as far as entomologists are concerned, bugs are only insects with sucking mouthparts. However, as we all know, colloquially bugs covers a much broader range.

• 7:51Speaker 03

  Poor entomologists. Do you have, um, do you have personal favorite zombifiers or ones that make you squirm?

• 8:00Speaker 02

  Um, let's see. So hairworms infect terrestrial insects, but hairworms reproduce in water. And let's say a cricket is infected by a hairworm. Now, the worm grows inside this cricket. It grows to the point where it is many times the length of the cricket itself, like to the point where if you look at a cricket that is infected, you might be able to see it moving around the abdomen. This actually spoke to me because when I was pregnant, during the last month of my pregnancy, the fetus is getting really big at this point. So, you know, it's kind of like the movie Alien, right? Like you could see a foot, you could see an elbow. You could, it's like, you know, there's something, there's, there's a something

• 8:40Speaker 02

  alive moving around inside your body. That is actually kind of what these, uh, these infected crickets look like. Uh, but the worm has to get out eventually. So what it does is it manipulates the cricket's behavior to get it to find water and then jump in.

• 8:58Speaker 02

  And once it's in the water, the worm starts to emerge. So watching a cricket trying to swim in a pool of water while this long thread-like worm is slowly, over minutes, spooling out from its backside is something I highly recommend watching if you really want to quick yourself out.

• 9:18Speaker 04

  Oh my god, it's like, oh, sick.

• 9:21Speaker 02

  There are many, many videos of this on YouTube. That is so friggin nasty.

• 9:26Speaker 05

  This is so gross. Tyler's getting changed. This is so gross.

• 9:31Speaker 03

  Oh, okay. But so there are all these different approaches, you're telling me, that fungi and viruses and insects use to do this zombifying.

• 9:47Speaker 03

  What are the, what are the sort of biggest categories of mysteries, I guess, about how they do this?

• 9:54Speaker 02

  Hmm, okay.

• 9:58Speaker 02

  Some of the biggest mysteries start with the moment that the host is infected. Because obviously, a body's first response to any kind of infection is going to be an immune response. So the first thing that a zombifier needs to do is to somehow get past that. And so that's a big, that's a big question is for zombifiers, from viruses to wasps to fungi to worms, when they get inside an organism where they are not supposed to be, how exactly are they telling their host immune system, no, there's nothing to see here. You can just go about your business. You don't need to, you don't need to worry about me.

• 10:42Speaker 02

  So that's, that's a good question.

• 10:44Speaker 03

  Mystery number one.

• 10:44Speaker 02

  Yeah, that's a big one. Then, of course, another one is, once it gets to the point of manipulation, first of all, what are, what are the cues? How does it decide, okay, now is the right time to, you know, to get this host moving to a place that where I need to be? So the description that I gave you earlier of the fly in a moonlit field, there seems to be a circadian rhythm element to the fungus that infects flies specifically because they start their climbing at night.

• 11:14Speaker 03

  Got it. So mystery number two would basically be, are there things out in the environment that that trigger a fungal takeover? And how does the zombifier kind of recognize those cues, I guess it sounds like?

• 11:27Speaker 02

  Yeah. And the third big question is obviously the nuts and bolts of how is it manipulating behavior?

• 11:33Speaker 03

  Right.

• 11:34Speaker 02

  The thing about this field is that there is still so much that scientists are piecing together about the precise mechanisms of how this works. Behavior is something that is just super complicated, even in insects. So when we look at, for example, the wasp that parasitizes or weaving spiders, scientists have found that in the spiders that are zombified, what the wasp does, it lays an egg on the spider. The egg hatches and the wasp larvae essentially piggybacks on the spider and drinks from it like it's a living juice box.

• 12:10Speaker 02

  And the spider just goes about its business until the larvae is ready to reproduce. And then somehow the wasp larvae is manipulating the spider to think that it's time to molt so that the spider makes a different type of web than it normally does, something called a resting web. It's reinforced and it's meant to support the spider and protect the spider while it's molting. And then, once that web is done, the wasp larvae drains the spider dry. The spider's empty husk of a corpse drops to the ground and the wasp larvae builds its cocoon and sets itself up in the spider's final web

• 12:53Speaker 02

  to hang out until it becomes an adult wasp.

• 12:59Speaker 03

  So basically, the spider is convinced by this wasp larvae in some way to build a cocoon for it. It uses all of its final webbery to like build a really nice little house for the wasp to grow up in.

• 13:19Speaker 02

  I like final webbery. Yeah. So what scientists found is that when spiders start making that final web, their little spider brains are being flooded with ectosteroïdes, which is the hormone that the spider naturally releases when it's ready to build a molting web. So it's actually, it's almost like it's taking a page out of the spider's existing biochemistry. And scientists aren't sure yet if what the wasp larvae is doing is, like, is the larvae actually producing the ectosteroide? Is it somehow triggering its production in the spider through another compound? That's something that they're still figuring out. But clearly, it's something that is already sort of baked into the spider's biology, that this is, this is a chemical trigger that lets it know, now it's time to build this kind of web. Except this time, it's not going to survive the process.

• 14:04Speaker 03

  So it is sort of a like, you couldn't necessarily get the spider to do something that it wouldn't be able to do under any other circumstances, but you can like puppet it to do things that it might do otherwise.

• 14:22Speaker 02

  That, that seems to be the much more common strategy among zombifiers. But exactly how they're being activated is still, is still a question.

• 14:30Speaker 03

  Why is it important to understand sort of more about how this behavior manipulation works?

• 14:40Speaker 02

  Yeah, that's, that's a really good question.

• 14:42Speaker 03

  And it is one that we will answer after the break.

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• 17:25Speaker 03

  Scientists still have a lot of questions about the precise mechanics of zombification. But if they could learn more about how zombifiers take over their victims, it turns out they could learn a lot that might benefit all of us.

• 17:41Speaker 02

  This, in a lot of ways, is looking at really big questions about how behavior works, which is something that is, like, scientists are still piecing this together on so many levels for all different types of organisms, because there are so many factors that shape behavior. Some of them are genetics, some of them are biochemicals, some of them have to do with environment, some of them have to do with social relationships. So this is one way of trying to understand behavior writ large.

• 18:18Speaker 03

  You mentioned that these insects suppress the immune systems of their hosts. Is there stuff that we could learn from that about, like, how immune systems work in general?

• 18:31Speaker 02

  Oh, yeah. Looking at the immunosuppressive aspect of zombifiers is definitely something that is a huge area of interest, because that could inform the development of immunosuppressive drugs, which is just something that would be hugely beneficial to people.

• 18:47Speaker 03

  Right.

• 18:47Speaker 02

  Not that this should be all about what's in it for me, but that is usually a consideration for scientific research. Could there potentially be applications for this that have medical applications? And so there is not yet a direct line between any research into how zombifiers evade their host's immune system and...

• 19:08Speaker 02

  The development of some kind of pharmaceutical immunosuppressive, but that's definitely something that is part of the mix when scientists are following that line of investigation.

• 19:18Speaker 03

  I think about all the insects that invade homes, some of which are beneficial, some of which are less so. Could we potentially borrow from this to like, fight off pests?

• 19:33Speaker 02

  Pest control is definitely one avenue that scientists have explored as a way of, okay, well, you know, is there some way that we can take what we're seeing these zombifiers do to insects and apply it to insects that we don't like?

• 19:48Speaker 02

  So baculoviruses, which are these viruses that infect caterpillars and make them climb and then dissolve their bodies into goo. This is something that has been deployed as a strategy for pest control in China and in Europe, in the US, in Brazil. So these types of viruses, they're an interesting alternative to traditional insecticides because they are very targeted. They're less toxic to the environment. They're not harmful to insects that are not their host species and they're not toxic to people. But they're not as quick as I think the insecticides that people have gotten used to and people like things to be quick and they like them to be absolute.

• 20:32Speaker 02

  So what seems like is the best way is perhaps to incorporate this alongside insecticides and use this along with other approaches because there are a lot of benefits to just, you know, going, going full on zombie warfare to get rid of our agricultural pests.

• 20:48Speaker 03

  Is there any attempt to like, design our own to be like, all right, we really want to get rid of American cockroaches? Like-

• 20:56Speaker 02

  You are describing the plot of a zombie movie right now where a team of scientists are working with, they're studying zombie bugs and they're like, oh, hey, wouldn't it be great if we could manufacture an artificial version of this zombie virus and release it into the wild? What could possibly go wrong?

• 21:13Speaker 03

  Oh, no, the hubris.

• 21:17Speaker 03

  Well, this does kind of get to, to a question that I have around all of this, which is maybe a silly question, but like-

• 21:26Speaker 03

  Could humans be zombified this way? Are we also susceptible to this?

• 21:34Speaker 02

  Well, there are some types of pathogens that are known to manipulate behavior in mammals and indeed in humans too. So rabies, of course, has been around and has been, there have been medical cases of rabies-infected humans that are thousands of years old with documentation of the heightened aggression. So there is already a virus among us that can manipulate human behavior. And recently, there have been studies into Toxoplasma gondii, which is the pathogen that causes toxoplasmosis. So this is an organism that its definitive host is cats.

• 22:18Speaker 02

  So it's, it's very entrenched amongst human populations. And in fact, many, many people, millions of people, carry Toxoplasma gondii, but it doesn't cause any symptoms. It tends to be dangerous in people that are pregnant or in immunocompromised people. Most of the people who are carrying Toxoplasma gondii have no symptoms. However, there have been studies recently, in the last 10, 15 years or so, that have looked at people who are carrying the parasite and have found that there does seem to be

• 22:54Speaker 02

  evidence of certain types of behavior, of being more risk-taking, of being bolder. In some groups, women being more nurturing. And what's interesting about it is that Toxoplasma gondii is known for manipulating behavior in rodents.

• 23:16Speaker 02

  And what it does is it makes them bolder and less afraid of cats. Because Toxoplasma gondii needs to reproduce inside cats. So it infects rodents. And then to get back into a cat, it makes the rodent less afraid of and attracted to the smell of cat pee. And that brings the rodent closer to a cat than it would normally go. And then, once it's eaten, then the parasite is back inside the cat. And scientists have found that this is true for other animals, too. So hyena cubs that are infected with Toxoplasma gondii are bolder around lions and are more likely to be eaten by lions. Chimpanzees that are infected with Toxoplasma gondii lose their fear of jaguars.

• 24:00Speaker 02

  And some studies found that people who are infected with Toxoplasma gondii are more likely to make risky business decisions or be bolder in traffic. So there's still a lot of work to be done because, obviously, human behavior is its own form of complicated. But there is some evidence that seems to suggest that Toxoplasma gondii can shape human behavior, too.

• 24:23Speaker 03

  What?

• 24:24Speaker 02

  Yeah, just blow your mind.

• 24:26Speaker 03

  Yes. That's so...

• 24:30Speaker 02

  I can send you links to the studies.

• 24:32Speaker 03

  I think I was expecting you to be like, no, unless you have rabies, you're definitely not.

• 24:39Speaker 02

  Well, do you have a cat?

• 24:41Speaker 03

  No, I don't.

• 24:45Speaker 03

  So there could, literally at this moment, be zombifiers within us, shaping us in some way.

• 24:53Speaker 02

  It's entirely possible.

• 24:57Speaker 02

  But there are so many things that make us who we are, that shape how we behave. There are environmental factors, there are social factors.

• 25:07Speaker 03

  The responsible answer.

• 25:09Speaker 02

  Oh, sure. But, you know, there might also be zombifiers.

• 25:14Speaker 07

  In a world where a team of scientists, they're working with, they're studying zombie bugs, and they're like...

• 25:21Speaker 02

  Oh, hey, wouldn't it be great if we could manufacture an artificial version of this zombie virus and release it into the wild? What could possibly go wrong?

• 25:30Speaker 07

  Everything goes wrong. Kill them. Kill the cockroaches. You can't kill a cockroach if it's already un-

• 25:41Speaker 07

  Dead. Guys, I think Gregor might have just turned into a cockroach.

• 25:48Speaker 07

  Coming soon, under the kitchen sink near you.

• 26:00Speaker 03

  There are so many more weird, wonderful zombie stories in Mindy Weisberger's book, that is, Rise of the Zombie Bugs. So if you want to read about zombified cicadas or fungi that will remind you of the show The Last of Us, or any number of other skin-crawling, mildly horrifying things, please check it out. This episode was produced by me, Bert Pinkerton. It was edited by Sarah K. Kramer. Meredith Hodnot runs our show. Noam Hassenfeld makes the music. Christian Ayala did the spooky sound design and the mixing on this episode. Melissa Hirsch did not enjoy all the gross

• 26:41Speaker 03

  bug stuff, but she powered through and checked the facts anyway. Thanks, Melissa. And the fact is that Julia Longoria and Jorge Just are just lovely human beings. Thanks to Brian Resnick, always, for co-creating the show. Thanks to Jonah for helping. Thanks to Charlie Harding, Claire White, Sergio Scardino, and Angela Chen for their voice work. And thanks to you for listening. If you have thoughts about this episode, we are at [email protected]. I love hearing from you all, so please email us. If you want to support the show and help us keep making it, please join our membership program. It is at vox.com/members and you'll

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