Zombies are real and everywhere we look. Not human zombies, of course, but animal ones: insects, arachnids, and countless other species.
In her new book, Rise of the Zombie Bugs: The Surprising Science of Parasitic Mind-Control, author and science writer Mindy Weisberger delves into the world of zombifying parasites, aka the various lifeforms that have evolved to nudge or outright force their hosts into doing their bidding.
The average person probably knows about a few of these parasites, such as the fungi that take over an ant’s body so it can climb up blades of grass and release a new generation of infectious spores—an act so horrifically spell-binding that a fictional version of it sparked the human apocalypse seen in the popular game and HBO TV show The Last of Us. But Weisberger details a whole litany of zombie-making bugs (microscopic germs, insects, and other creepy crawlies) for readers to be grossed out by, including a few that might even be able to influence human behavior.
Gizmodo spoke to Weisberger about her fascination with these bugs, the evolution of parasitism in general, and disco-lighted snails.
Ed Cara, Gizmodo: We here at Gizmodo are no strangers to covering all sorts of zombie bugs. But what drew you specifically to spend so much time digging into these parasites and their gruesome way of life?
Mindy Weisberger: For starters, they’re inherently fascinating.
And like a lot of people, my first introduction to these zombifying agents was the cordyceps fungus in The Last of Us, which of course is inspired by an actual zombie ant fungus in the genus Ophiocordyceps. So this is something that people maybe are a little more aware of now.
But over time, as a science reporter, I came across more examples of different types of zombifying organisms, and I just started to get a sense of the scope of how many different kinds there were and all of these different mechanisms they had for zombification. So, of course, there are zombifying fungi, but there are also zombifying viruses. There are zombifying insects. There are many, many species of wasps that have evolved to manipulate their hosts. There are zombifying worms. So, it seemed like the deeper I went, the more that I uncovered and the more I just wanted to learn about all of these different types of zombifiers. And that just led me down the rabbit hole.
And, of course, I wrote a whole book about them, but there are still so many examples that just didn’t even make it into the book. So, these parasites are a very, very rich source of study, and they tell us a lot about the natural world and how different relationships work and the different kinds of strategies that have evolved for different forms of life to survive.
Gizmodo: Throughout the book, you talk about how abundant and ancient these parasites really are. That raises the question of why. Why have so many organisms evolved to have this sort of freeloading lifestyle, despite the very real risks of being wholly dependent on another living thing?
Weisberger: So, just the fact that you use freeloading is very telling. People in general have a negative view towards parasites because many parasites cause disease. And I also think, from a capitalistic perspective, the idea of being a freeloader is like, “Oh my God, there’s nothing worse than that.” But in fact, this is obviously a very successful strategy, and parasitism has been around for about as long as there has been life on Earth.
One of the researchers that I spoke to for the book was Kelly Weinersmith, who studies parasitic wasps. And she said that parasitologists like to joke amongst themselves that the first form of life that emerged on earth was free-living, and the second was parasitic. Because it is actually a very attractive strategy. I mean, if you had to worry about going out and looking for food, finding a safe place for you to reproduce and raise your young, and potentially exposing yourself to all different kinds of threats and predators, would it not be profitable to find yourself a place where everything you need is there, all the nutrients are there, the place where you can reproduce is there? Once you’ve established yourself in a way that you can evade the host’s immune system and just do your thing, that is actually a safer, better option for you. And this is why parasitism has evolved so many times across not just the animal kingdom, but plants and fungi as well.
By some estimates, there are roughly about 8 million known animal species, and at least 40% of those are estimated to be parasitic. And this is something that goes back hundreds of millions of years. The earliest direct fossil evidence of parasitism is found in the shells of marine organisms called brachiopods, from a site in China dating to about 512 million years ago. These parasites were probably worms that built these little mineralized cylinders for themselves on the shells of these brachiopods. And they were thought to be kleptoparasites, which means that they stole their host’s food. And the way that scientists figured that out was when they looked at the brachiopod fossils, the ones that were carrying a greater load of these parasites were smaller, which seemed to suggest that they were not getting enough to eat.
So parasitism goes back a very long time. But of all the known parasite-host associations on the planet, only a tiny fraction to date are known to involve behavioral manipulation.
Gizmodo: You detail so many different examples of zombie parasitism that the average person might wonder; is this something I should ever be worried about? Are there any bugs out there that can or possibly could zombify people someday?
Weisberger: Well, it’s natural to be concerned about how this might affect you personally. And the fact is that there are some pathogens that are known to affect mammal behavior, and you probably know them already.
Rabies, of course, is a very common one. Cases of rabies are recorded in texts that go back thousands of years, and it’s known to affect its host’s behavior very dramatically. This usually involves behavioral changes that make them more aggressive, and there’s also excessive salivation involved. The thinking is that this benefits the parasite, because aggressive animals are more likely to fight. And the virus particles are shed in their saliva. So the combination of changing aggressive behavior and a lot of drool means that the rabies virus is able to increase its chances of successful reproduction.
Another example you might know about is Toxoplasma gondii, which causes the disease toxoplasmosis.
T. gondii’s definitive hosts are cats, which means that it only can reproduce in cats. But it can live in lots of different species of birds and mammals, and that includes people. And so there’s robust evidence that T. gondii changes the behavior in infected rodents. What it does in rats and mice is it reduces their fear of cats. It makes them attracted to cat urine, which is something that’s normally, for good reason, a deterrent for them. It makes them bolder around cats, which means they’re more likely to be eaten by cats, which means the T. gondii they are carrying will then get inside a cat where it needs to be to reproduce. But there’s also evidence starting to come out in papers within the last decade or so showing that there seem to be similar types of behavioral changes in animals that are not rodents. In hyena cubs, for example, that are infected with T. gondii, they seem to be bolder around lions. And there are studies of captive chimps infected with T. gondii that seem to lose their fear of leopards, which are a natural prey of theirs.
Now with humans, they’re dead-end hosts. More than 2 billion people worldwide are thought to carry this pathogen, even if they don’t show any symptoms or have any signs of toxoplasmosis. And there’s also a growing body of evidence hinting that T. gondii can change human behavior, even if the person doesn’t show any other symptoms, and in similar kinds of ways where the person with T. gondii will be bolder or more aggressive. But figuring out what actually makes a specific behavioral change is very complicated. And it’s even more complicated in people compared to figuring out what changes behavior in an ant, for example. So there is still, at this point, a lot of work to be done to be certain that you can separate out these specific changes and link them to T. gondii, rather than there being other factors involved. But it’s definitely an interesting area of study.
Gizmodo: Speaking of unresolved questions, what are some of the biggest mysteries left to be solved about these zombifying parasites?
Weisberger: Well, if you look at the history of how scientists have studied behavior manipulation and zombification, some of the first records of these are centuries old. And usually it just starts out with the scientist observing that an insect is either behaving in an unexpected way or that it seems to be sprouting things that are not normal. But it’s only really been in the last 20 years or so that scientists have been able to drill down and look at the neurochemistry of what’s going on.
We’re finally at that point we can start to figure out questions like: What are the proteins that are being changed? What are the genes that are being expressed? What is the parasite actually doing to its host? And one of the big questions is; is the parasite itself producing the compounds that are causing the change, or is it producing compounds that then get the insect to produce chemicals that affect its behavior?
For example, there is a type of wasp that zombifies spiders. And what it does is it lays an egg on the spider, the egg hatches, and the wasp larvae essentially just piggybacks on the spider. It just sits there discretely sipping the spider’s hemolymph [the invertebrate version of blood], almost like a juice box, until it’s ready to pupate. And when that happens, there is a very dramatic behavioral change in the spider. The spider starts to build a web that is completely different from the normal web it makes. You can probably picture the Charlotte’s web type of web, which is a series of concentric circles with spokes. And that’s a typical prey catching web. But the zombified spider builds a web that’s usually used to keep it safe and secure as it molts. Once the spider is done with this web, its job is done. The wasp larvae drains it dry, the spider corpse drops to the ground, the wasp builds itself a little cocoon and then it hangs out in the wasp web—the last web that the spider ever built.
So what the wasp is doing is it’s stimulating in the spiders a massive amount of hormones called ecdysteroids. And the big unanswered question right now is, is the wasp producing this itself or is it stimulating it in the spider? Because spiders naturally produce these hormones just before they molt; it’s actually the trigger that starts the whole process of them building this web in preparation for them having this big physical change.
So in this and in many, many other examples of zombification, there is still so much to be unpacked about what the specific pathways are between the parasite and its host, about the small nudges that it’s doing to cause these dramatic changes in behavior.
Gizmodo: So to close things out, what’s your favorite zombie bug that you learned about in writing this book?
Weisberger: I was originally a filmmaker before I was a science journalist, so I’m naturally attracted to things that are very visual. And one of the most dramatic examples that caught my attention are the discofied zombie snails.
So these are land snails that are infected by worms in the genus Leucochloridium. What these worms do is they infect the snails using these broodsacs, which is like these little sausages full of worm larvae. And these broodsacs are very colorful, they’re usually striped, patterned in shades of brown and green depending on the species. The sacs migrate into the snails’ eye stalks, and once there, they pulse, making the stalks look very much like the undulation of a crawling caterpillar. Now, the definitive hosts of these worms are birds; they have to be in a bird to reproduce. So this display, which looks like a caterpillar, is something that is uniquely attractive to hungry birds. The worm also manipulates the snail’s behavior so that it will wander out into exposed spaces, rather than hunkering down in the undergrowth where it normally stays. So they’re now out in the open and they have these caterpillar-looking eye stalks, making the broodsacs an enticing meal. But the eyestalks split very easily, so the broodsacs will often just pop right out, and the snail will often heal its eyestalks and be fine afterward.
That’s my favorite species example, but I also have a favorite specific individual zombie bug.
There was a zombie ladybug that became TikTok famous in 2021, which became known as Lady Berry. There’s this content creator named Tiana Gayton, who’s very enamored of insects and spiders. And one day, she was in a grocery store when she looked at a head of lettuce and saw a ladybug that looked like it was hugging something. It looked like it was hugging a small cocoon. And she was like, ‘Oh, this is weird. I’m going to take this ladybug home with me and see what’s happening.’ She took it home and she tried to pry the ladybug’s legs away from the silk around the cocoon, but the ladybug refused to let go.
It turned out that the ladybug was parasitized by a species of wasp that manipulates its behavior. It will lay an egg inside the host’s abdomen, the egg hatches out of the ladybug and forms into a pupa, and the host then becomes the pupa’s bodyguard. So the ladybug was guarding the cocoon. But Tiana Gayton was determined to save it. She pried it off the cocoon, separated it from the cocoon, and put the ladybug in a little jar. She gave it water, gave it food, and nursed it back to health. And eventually she took Lady Berry to the park and returned it to the wild.
And so there’s an example of a zombie that got something most zombies don’t: a second chance.
Rise of the Zombie Bugs: The Surprising Science of Parasitic Mind-Control, published by Johns Hopkins University Press, is now available in hardcover and as a e-book.