Zombies Are Real… In The Animal World

If you made it through the zombie movie craze of the early 2000s, you might have done so by whispering to yourself that zombies aren’t real. Well, Haitian Voodoo-style zombies might have a basis in fact, and there have been various points in history where other people were convinced that the walking undead were a thing — just ask a medieval peasant about zombies on your next time-traveling trip. But, come on, we all know that there’s no such thing as the living dead shambling around, biting others, and spreading a terrible disease through an entire population. Right?

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Maybe not, if you’re an ant. As “The Last of Us” so dramatically demonstrated, first in video game form and then as a hit HBO series, there are some real pathogens in the animal world that induce very zombie-like behavior. While the jury is very definitely out on whether or not a cordyceps (shortened form of Ophiocordyceps) fungus can leap into human bodies, it’s definitely able to wreak havoc on rainforest ant colonies. It’s not alone, either, as different pathogens take on the creepy task of controlling the quasi-dead bodies of insects, fish, crabs, and even a few mammals.

In “The Last of Us,” humans are typically infected with a variant of the cordyceps fungus in classic zombie fashion. That is, if they are bit, they quickly turn into very hungry undead. Within the world of the show, some characters speculate that the pandemic got started via spores infesting the food supply, which is a bit more in line with how the real Ophiocordyceps fungi species works its way into ants. It’s not a perfect one-to-one comparison, as ants are hardly shopping at the grocery store. What really happens is that an unfortunate ant comes into contact with Ophiocordyceps spores, perhaps from a former host now raining spores down from a high leaf. Those spores then latch onto an ant’s hard exoskeleton and, using strands of material called hyphae, make their way through into the rest of the animal’s body.

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Unlike other infectious agents that might induce zombie-like behavior, an Ophiocordyceps problem progresses agonizingly slowly from there. While other fungi and parasites can turn an animal around in hours (in “The Last of Us,” exposed humans have a mere one or two days before they start attacking others), Ophiocordyceps-infected ants can linger on for weeks as the fungus takes root. This stretch of time gives the fungus plenty of space to fully establish itself in the ant’s tissues and begin to manipulate its behavior.

Once the Ophiocordyceps fungus really has its hyphae in an ant, it begins to take over the muscle tissue of the creature, both by growing fungal bodies around those muscles and releasing chemicals that facilitate this control through muscle contractions. This is key in allowing the fungus to direct ant behavior, especially when it comes to walking, climbing, and clamping mandibles down on a leaf in what will be one of the ant’s very last acts. This also affects how the ant is able to use its antennae, as some Ophiocordyceps-infected species, fairly far along in the process, have been observed with their antennae stuck in an L shape. This clearly hampers how the ant is able to communicate with fellow members of its colony or move through its environment, leading to a wandering movement that typically drives the ant from its home base.

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Curiously, while Ophiocordyceps is busy invading the muscles and other tissues inside an ant, it doesn’t seem able to get into the insect’s brain. A 2017 paper published in PNAS found that a fungus tailored to the Camponotus cataneus species of carpenter ant was able to get into practically all parts of the host species, but the brain.

When the fungus reaches critical mass — estimated to be around 40% of what used to be just an ant minding its own business — it’s time for the final stage of the infection to begin. It’s not entirely unlike what you may see in “The Last of Us,” where end-stage infection and a lack of human victims have completely exhausted the host, and what’s left becomes a stage for generating more spores. Only this time, it’s on a much, much smaller scale.

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At this point, the fungus has taken over the ant’s muscles, including the ones that control its legs and mandibles. It begins wandering, but only kind of, as the ant is actually being manipulated to find a place that’s just right for the still-growing fungus. That generally means a more humid area that’s relatively high off the ground, at least as far as a carpenter ant is concerned. Scientists have found that ants infected by Ophiocordyceps tend to find leaves that are about 10 inches up from the forest floor and facing north. Ants find a spot on the underside of the leaf, clamp their jaws down, and die. The bodies remain affixed to the leaf, with the fungus chowing down on what’s left. Days later, a thread-like fruiting body sprouts from the ant’s head and begins distributing spores — perhaps onto yet more unlucky ants somewhere below.

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“The Last of Us” makes a cordyceps infection seem possible, but is it really? Not at all, say scientists. Like many other parasites, Ophiocordyceps unilateralis, the species that inspired “The Last of Us,” is tailor-made to carpenter ants in a process that’s taken millennia (fossils hint at it doing its thing as far back as 48 million years ago). There’s not much room for the fungus to make a leap, as major differences in biology (note that you don’t have an exoskeleton) present an almost impossible task. What’s more, it’s really difficult for humans to spread fungi person-to-person, so unless someone is very improbably sneezing out mushrooms, you’re fine. Also, humans have been happily chomping away on cordyceps for a while now, and in some places it’s become a trendy wellness ingredient that’s actually cribbed from old-school Chinese medicine (where Ophiocordyceps sinensis is common).

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However, there is some bad news. Climate change and rising global temperatures have made it easier for some fungi to proliferate. So has the overuse of antibiotics, which can clear out helpful bacteria that could otherwise out-compete a fungus. All of this has, in turn, increased the chances of fungal infection in some parts of the world, especially for immunocompromised people.

If Ophiocordyceps sounds like an unstoppable baddie, take a breath. While ants have no reason to like the fungus, assuming they can have an opinion, we might reserve a little sympathy. That’s because Ophiocordyceps has to face predators of its own, including the colorfully-named specter of “hyperparasites.” For Ophiocordyceps, these include Niveomyces coronatus and Torrubiellomyces zombiae, which can essentially hop into the cordyceps reproductive cycle and interrupt it with their own fungus-growing behavior. The Ophiocordyceps fruiting body can be significantly damaged by hyperparasites, preventing it from spreading spores. It’s possible that, in a way, these hyperparasites are actually helping Ophiocordyceps out. If they weren’t occasionally hampering the activity of Ophiocordyceps, it’s possible that the original fungus would go hog-wild on an ant colony, burn through its own host supply, and extinct itself.

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Of course, even if hyperparasites aren’t so bad from the Ophiocordyceps perspective, it doesn’t do much for the ant that still has a fruiting body sprouting from its empty husk of a body. But ants have their own defenses here, including social grooming behaviors and antimicrobial secretions that can prevent the spread of fungal disease within a colony (though already-infected individuals can be out of luck, as they’re often forced out of the group). Carpenter ants have also been observed adapting their behavior in Ophiocordyceps-rich environments, such as moving through trees instead of the forest floor, limiting their exposure to spores.

Ants aren’t the only creatures that have to be concerned about parasites taking control of their minds and bodies. The California killifish has a formidable opponent in the form of a seemingly easy-to-defeat flatworm. The parasite, Euhaplorchis californiensis, can infest a fish en masse, leaving a killifish with thousands of cysts in and around their brains. But it doesn’t just give a fish brainworms. Instead, the flatworm parasite modifies the killifish’s behavior, much to the larger animal’s detriment.

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A 1996 paper published in Ecology found that infected killifish become real show-offy swimmers, making themselves so obvious that they’re between 10 to 30 times as likely to be consumed by predatory birds as their uninfected brethren. But that’s all in the flatworm’s plan, as this allows the parasite to make its way through the bird’s digestive tract and continue on with its reproductive cycle. Still, the fish try to fight back in a way, as a 2020 paper in Functional Ecology demonstrated. There, killifish that had been exposed to the presence of the parasite demonstrated increased metabolic activity, whether or not they were actually infested with the flatworm. That indicates some way in which their bodies are attempting to evade or fight off the tiny invader.

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For many animals, a fungal infection that changes behavior is pretty bad news. But one fungus-infested animal actually makes quite a lot of noise … and they might even like it that way. Meet Batrachochytrium dendrobatidis, a fungus that appears to help male frogs produce louder, longer calls in an effort to link up with female frogs. As research published in a 2016 edition of Biology Letters showed, that’s just what happened to a group of Japanese tree frogs (Hyla japonica) studied in the wild. Precisely why the fungus is getting the males to put forth extra effort isn’t entirely clear, though it could be a way for the fungus to increase transmission, given how successful males are more likely to make physical contact. Another tree frog species, Litoria rheocola, has been observed undergoing a very similar effect under the same fungus (via Ecology and Evolution).

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It’s not a perfect system, however. The fungus can lead to a condition known as chytridiomycosis, which affects hundreds of different amphibian species across the world. Affected animals may stop eating, have convulsions, and begin shedding copious amounts of skin. Some species — more than 200 or so — have been so hard-hit that their populations are perilously low or they’ve just gone extinct. Some frogs have developed a defense in the form of anti-fungal bacteria growing on their skin, but decreased temperatures can lower those defenses and allow the fungus to proliferate.

Some fungi really have it out for the animal kingdom, as a few species have evolved to really, really mess up an animal’s day. One of the most striking may be Massospora cicadina, which, as close readers of the scientific name have already guessed, strikes cicadas. It’s a bit cruel when you consider that periodical cicada broods spend well over a decade doing their thing underground before emerging to speed run through the reproductive cycle and then die. But, so too does Massospora cicadina, waiting quietly in the soil right along its pending insect hosts.

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If a cicada comes into contact with the fungal spores in the soil, things seem fine at first. But once the cicada morphs into its adult phase, things get rough. For males, part of their abdominal exoskeleton falls off, revealing the fungus that’s been growing underneath the whole time. And, yes, that shedding also includes the male reproductive organs. Yet, despite the devastating damage, the fungus has a chemical trick up its sleeve. According to research published in a 2019 edition of Fungal Ecology, different Massospora species can be dosing the bugs with psilocybin — the mind-altering compound in admittedly strange magic mushrooms — or an amphetamine known as cathinone. The juiced-up insects are less hungry, more energetic, and feel rather frisky. In their fruitless attempts to mate, they dust fellow insects with spores and encourage the spread of the fungus.

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For some people, wasps and their stingers can already be pretty terrifying. If you count yourself amongst that group, at least be grateful that you’re not a cockroach. More specifically, we should all be thankful that we’re not a cockroach encountering an emerald jewel wasp. This particular relationship is extra-tough because the wasp uses a special venom to turn its cockroach victims into willing hosts for its hungry young.

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It starts when a tiny female wasp drops in and stings the cockroach’s mid-body with a paralyzing venom. She’s then free to sting the insect yet again, this time with the paralytic venom cocktail shot straight into the roach’s brain. Once the paralytic wears off, the cockroach begins grooming itself, which may be a mere side-effect or could provide a relatively clean host for the eggs. Meanwhile, the wasp prepares a cozy little burrow in which she leads the roach — now calm and incapable of fighting back or running away — and an egg stuck to its leg. Well, not before she breaks off its antennae and drinks a bit of insect blood as a quick pick-me-up.

That egg will, of course, hatch in just under a week, at which point things get even worse for the more or less brain-dead roach. The newly emerged larval wasp crawls into the roach and begins eating it, specifically targeting the roach’s trachea to breach the organ and provide air to the larva chowing down inside. It takes about 48 hours for the roach to finally die.

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One parasite that’s made a bit of a name for itself is Toxoplasma gondii. This single-celled protozoan really punches above its weight, at least when it makes its way into a mammal’s nervous system. When that mammal is a rat, things can get pretty dire for the furry little guys. You see, Toxoplasma needs to get into a cat’s digestive system to continue its reproductive cycle. So, what’s a simple parasite to do when it finds itself inside a rodent? Make said rodent want to get eaten.

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Toxoplasma makes rats begin to prefer the scent of cat urine. According to a 2011 paper in PLOS One, the parasite appears to use the brain pathways that push a rat to reproduce, making the prospect of milling about in cat-heavy areas all the more appealing … at least, until an actual cat appears. At that point, Toxoplasma can get into the cat’s digestive system and continue making new parasite protozoa. Another PLOS One paper, published in 2013, found a very similar effect in mice that seemed to be permanent, even after the parasite was removed.

It’s not just rodents, either. As a 2016 paper in Current Biology noted, chimpanzees infected by Toxoplasma behaved quite boldly when infected. Captive chimps were exposed to different varieties of predator urine, with infected individuals appearing to show less fear around the leavings of leopards, a natural predator in their homeland of Gabon. In fact, Toxoplasma might just be able to control your brain, too.

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While the risk-taking behavior that’s encouraged by Toxoplasma gondii parasites can land some small creatures in big trouble, larger mammals can be affected, too. What’s more, these animals might find some benefit from their parasite infestation, assuming the chips fall just so. In Yellowstone National Park, the wolves that typically operate within park boundaries might have found just such an effect. To understand what Toxoplasma is doing to these wild canines, you first need to know that wolf packs are generally a family group led by a female-male pair. Occasionally, an adult wolf will break away from this group to start a pack of their own, though operating as a literal lone wolf can be a dangerous proposition.

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But when Toxoplasma gets into a wolf’s system, that individual is 11 times more likely to strike out on its own, according to a 2022 study published in Communications Biology. What’s more, wolves that carried the parasite were a staggering 46 times more likely to become pack leaders than their Toxoplasma-free brethren. As for how exactly wolves are picking up the parasite, researchers suggest that it has something to do with the territory crossover between wolves and cougars. Given how pack leaders, well, lead a pack of animals, further research suggests that Toxoplasma infection can ultimately dictate the behavior not just of an individual wolf, but their entire group.

Why should fungi and protozoans have all the fun? As it turns out, some species of barnacles have turned their evolutionary hand to zombie-style parasitism. Sacculina barnacles take root in crabs … almost literally. These parasitic barnacles latch on to crabs while the invaders are in the larval stage, then start growing a structure much like roots that infiltrates the victim’s body. Over time, the parasite takes over so much of the crab that it begins to peek out as large growths at the bottom of the crab’s body.

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At this point, the crab is in a pretty bad state, but it doesn’t quite know it. Instead, the barnacle is able to trick the crab into taking care of the parasite, much like the crustacean would for its own brood pouch of eggs. That means attentively cleaning the parasite and making sure it’s washed over with plenty of oxygen-rich water to help it grow, even though the growth has destroyed the crab’s capability of making its own offspring. When it’s time for larvae to emerge, the crab will climb to a conveniently high spot and even help to agitate the water in order to more fully disperse the parasite to begin its lifecycle anew.