What humans might learn from nature’s real-life zombies
Cicadas can be infected by a fungal parasite that turns them into zombies. | Chip Somodevilla/Getty Images Zombies, it turns out, are real — and science journalist Mindy Weisberger can give you plenty of examples of them. She’s read up on the fungi that take over flies’ bodies, partially digesting them from the inside out before forcing them to climb up blades of grass, so that fungal spores can explode out from their swollen corpses and claim new victims. She’s considered the hairworms that grow inside of crickets before inducing their hosts to toss themselves into a nearby body of water, where the worms emerge from the crickets’ exoskeleton in a miniature but all-too-real imitation of the alien in Alien. She’s even researched the snails that fall victim to certain flatworms. The flatworms’ larvae need to be eaten by birds to reach the next stage of their lifecycle, so broodsacs full of larvae take up residence in the snails’ eyestalks and turn them into pulsing, colorful, caterpillar-like bird-lures. The parasite also manipulates the snails into wandering into the open in order to increase the odds that a bird will spot the snails and devour both their eyestalks and the larvae within them. Weisberger dug into these specific nightmare-inducing examples of parasitic mind-control — and many others — as part of her effort to understand real-life “zombification” in her book, Rise of the Zombie Bugs. What she found was that these natural zombie stories are not only sources of inspiration for horrifying fiction — they could also inspire researchers who are trying to better understand everything from immune responses to pest control. So we spoke to Weisberger about research on real-life zombies for Unexplainable, Vox’s science podcast. What follows is a version of our conversation, edited for clarity and length. There’s much more in the full podcast, so listen to Unexplainable wherever you get podcasts, including Apple Podcasts and Spotify. Let’s start by just defining some terms. What do we mean when we say “zombifier,” or “zombie?” Sure. 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. Let’s say you catch a cold — you’re gonna 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 recover, 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. 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. 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 it. Yeah, exactly. There are zombifying viruses; there are zombifying fungi; there are insects that are able to zombify their hosts. There are worms that can zombify their hosts. Most of the organisms that they infect are arthropods — bugs. (I do have to apologize to entomologists, because as far as entomologists are concerned, bugs are only insects with sucking mouth parts. However, as we all know, colloquially, “bugs” covers a much broader range.) What are some of the biggest categories of mysteries about how [zombifiers do what they do]? 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. The first thing that a zombifier needs to do is to somehow get past that. That’s a big question for zombifiers, from viruses to wasps to fungi to worms: When they get inside an organism where they’re not supposed to be, how exactly are they telling their host immune system, “No, there’s nothing to see here! Just go about your business! You don’t need to worry about me!” Another one is, once it gets to the point of manipulation, what are the cues? How does it decide “OK, now’s the right time to get this host moving to a place where I need to be”? The third big question is obviously the nuts and bolts of: How is it manipulating behavior? 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 orb-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 larva essentially piggybacks on the spider and drinks from it like it’s a living juice box. And the spider just goes about its business until the larva 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 larva builds its cocoon and sets itself up in the spider’s final web to hang out until it becomes an adult wasp. What scientists found is that when spiders start making that final web, their little spider brains are being flooded with ecdysteroids, which is the hormone that the spider naturally releases when it’s ready to build a molting web. And scientists aren’t sure yet: Is the larvae actually producing the ecdysteroids? Is it somehow triggering its production in the spider through another compound? That’s something that they’re still figuring out. Why is it important to understand how this behavior manipulation works? In a lot of ways, this is looking at sort of really big questions about how behavior works, which is something that scientists are still piecing 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 biochemical; some of them have to do with environments; some of them have to do with social relationships. So, this is one way of trying to understand behavior writ large. You mentioned that these insects suppress the immune systems of their hosts. Is there stuff that we could learn from that about how immune systems work in general? 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 something that is just something that would be hugely beneficial to people. 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 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. 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 fight off pests? Pest control is definitely one avenue that scientists have explored. 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? 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. These types of viruses are 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 also 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. So what seems like 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 going full-on zombie warfare to get rid of our agricultural pests. Could humans be zombified this way? Like, are we also susceptible to this? Well, there are some types of pathogens that are known to manipulate behavior in mammals and indeed in humans too. So rabies, of course. There have been medical cases of rabies-infected humans that are thousands of years old with documentation of 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. Its definitive host is cats. 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 evidence of certain types of behavior: of being more risk-taking, of being bolder. And what’s interesting about it is that Toxoplasma gondii is known for manipulating behavior in rodents. And what it does is it makes them bolder and less afraid of cats. What? 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. 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. 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. What? Did I just blow your mind? So there could literally at this moment be zombifiers within us shaping us in some way? It’s entirely possible. There are so many things that make us who we are that shape how we behave. There are environmental factors; there are social factors. But, you know, there might also be zombifiers.
Zombies, it turns out, are real — and science journalist Mindy Weisberger can give you plenty of examples of them. She’s read up on the fungi that take over flies’ bodies, partially digesting them from the inside out before forcing them to climb up blades of grass, so that fungal spores can explode out from […]
Zombies, it turns out, are real — and science journalist Mindy Weisberger can give you plenty of examples of them.
She’s read up on the fungi that take over flies’ bodies, partially digesting them from the inside out before forcing them to climb up blades of grass, so that fungal spores can explode out from their swollen corpses and claim new victims.
She’s considered the hairworms that grow inside of crickets before inducing their hosts to toss themselves into a nearby body of water, where the worms emerge from the crickets’ exoskeleton in a miniature but all-too-real imitation of the alien in Alien.
She’s even researched the snails that fall victim to certain flatworms. The flatworms’ larvae need to be eaten by birds to reach the next stage of their lifecycle, so broodsacs full of larvae take up residence in the snails’ eyestalks and turn them into pulsing, colorful, caterpillar-like bird-lures. The parasite also manipulates the snails into wandering into the open in order to increase the odds that a bird will spot the snails and devour both their eyestalks and the larvae within them.
Weisberger dug into these specific nightmare-inducing examples of parasitic mind-control — and many others — as part of her effort to understand real-life “zombification” in her book, Rise of the Zombie Bugs. What she found was that these natural zombie stories are not only sources of inspiration for horrifying fiction — they could also inspire researchers who are trying to better understand everything from immune responses to pest control.
So we spoke to Weisberger about research on real-life zombies for Unexplainable, Vox’s science podcast. What follows is a version of our conversation, edited for clarity and length. There’s much more in the full podcast, so listen to Unexplainable wherever you get podcasts, including Apple Podcasts and Spotify.
Let’s start by just defining some terms. What do we mean when we say “zombifier,” or “zombie?”
Sure. 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.
Let’s say you catch a cold — you’re gonna 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 recover, 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.
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.
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 it.
Yeah, exactly. There are zombifying viruses; there are zombifying fungi; there are insects that are able to zombify their hosts. There are worms that can zombify their hosts. Most of the organisms that they infect are arthropods — bugs. (I do have to apologize to entomologists, because as far as entomologists are concerned, bugs are only insects with sucking mouth parts. However, as we all know, colloquially, “bugs” covers a much broader range.)
What are some of the biggest categories of mysteries about how [zombifiers do what they do]?
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. The first thing that a zombifier needs to do is to somehow get past that. That’s a big question for zombifiers, from viruses to wasps to fungi to worms: When they get inside an organism where they’re not supposed to be, how exactly are they telling their host immune system, “No, there’s nothing to see here! Just go about your business! You don’t need to worry about me!”
Another one is, once it gets to the point of manipulation, what are the cues? How does it decide “OK, now’s the right time to get this host moving to a place where I need to be”?
The third big question is obviously the nuts and bolts of: How is it manipulating behavior? 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 orb-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 larva essentially piggybacks on the spider and drinks from it like it’s a living juice box.
And the spider just goes about its business until the larva 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 larva builds its cocoon and sets itself up in the spider’s final web to hang out until it becomes an adult wasp.
What scientists found is that when spiders start making that final web, their little spider brains are being flooded with ecdysteroids, which is the hormone that the spider naturally releases when it’s ready to build a molting web. And scientists aren’t sure yet: Is the larvae actually producing the ecdysteroids? Is it somehow triggering its production in the spider through another compound? That’s something that they’re still figuring out.
Why is it important to understand how this behavior manipulation works?
In a lot of ways, this is looking at sort of really big questions about how behavior works, which is something that scientists are still piecing 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 biochemical; some of them have to do with environments; some of them have to do with social relationships. So, this is one way of trying to understand behavior writ large.
You mentioned that these insects suppress the immune systems of their hosts. Is there stuff that we could learn from that about how immune systems work in general?
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 something that is just something that would be hugely beneficial to people.
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 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.
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 fight off pests?
Pest control is definitely one avenue that scientists have explored. 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?
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.
These types of viruses are 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 also 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.
So what seems like 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 going full-on zombie warfare to get rid of our agricultural pests.
Could humans be zombified this way? Like, are we also susceptible to this?
Well, there are some types of pathogens that are known to manipulate behavior in mammals and indeed in humans too. So rabies, of course. There have been medical cases of rabies-infected humans that are thousands of years old with documentation of 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. Its definitive host is cats. 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 evidence of certain types of behavior: of being more risk-taking, of being bolder. And what’s interesting about it is that Toxoplasma gondii is known for manipulating behavior in rodents. And what it does is it makes them bolder and less afraid of cats.
What?
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. 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.
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.
What?
Did I just blow your mind?
So there could literally at this moment be zombifiers within us shaping us in some way?
It’s entirely possible. There are so many things that make us who we are that shape how we behave. There are environmental factors; there are social factors. But, you know, there might also be zombifiers.