Everyone knows that zombies aren’t real, just the stuff of fictional Halloween movies, right?
Not so fast! There are numerous examples of parasites such as fungi, wasps and other organisms turning animals into zombies.
What exactly do we mean by zombies? Well, these parasites hijack the animals’ decision making ability and influence them to perform actions that support the parasites’ lifecycle. Unlike symbiotic relationships where both organisms cooperate and flourish, these interactions are to the detriment of the host animal and usually end in their death … spooky!
In this article we’ll cover some real-life examples of parasites and how they turn animals into zombies.
Jewel Wasps and Cockroaches
Have you ever been to a rodeo and seen a cowboy riding a bucking horse? That’s kind of like what the jewel wasp does to a cockroach, except the wasp has an extra trick that makes the ride a whole lot smoother. First, they sting the cockroach on the top of the head and inject a venomous, mind-controlling cocktail into their brain (Gal and Libersat, 2010). After the venom’s effect has settled in, the wasp chomps off one of the cockroach’s antennae, bites the remaining antennae stump, and guides the cockroach to a prepared burrow.
This isn’t just some innocuous fieldtrip; however, the jewel wasp has a specific purpose in mind for zombifying the cockroach. The wasp lays an egg inside the cockroach’s back. Then, the jewel wasp finds objects to block the cockroach in the burrow so that another predator won’t find and eat the cockroach incubator. Later, when the jewel wasp eggs hatch, the cockroach provides the developing wasp larvae with a nice meaty snack while they grow and eventually emerge leaving the cockroach carcass behind.
The injection of the mind-controlling cocktail is the vital step in this interaction and is teaching scientists about neurological networks (Libersat and Gal, 2014). The key here is that this cocktail doesn’t paralyze the cockroach, otherwise it would make the task of dragging the cockroach to the burrow a lot more strenuous! Rather, these mind-controlling substances induce a lethargic state in the cockroach and reduce their intrinsic motivation to initiate movement, thereby allowing the wasp to walk the cockroach, like a dog on a leash, to its eventual demise.
Wasps and Spiders
Cockroaches aren’t the only creatures that wasps coerce into doing their bidding. Wasps also target spiders! Ichneumonidae wasps sting Tetragnathidae spiders and induce a temporary paralysis of about 15 minutes during which the wasp glues an egg to the spider’s abdomen (Eberhard, 2000).
After the paralysis wears off, the spider resumes normal behavior and builds its regular webs for the next 7-14 days while the wasp egg hatches and the larva grows by drinking the spider’s hemolymph, which is the invertebrate equivalent of blood.
On the night that the developing wasp larvae will kill the spider, it chemically induces the spider to deviate from its normal web building style. The spider will repeat the first step of building its web, over and over, without proceeding to any of the subsequent steps. The end result is that instead of spinning a normal web, the spider spins a tiny web that provides silk starting material that the larva can easily use to spin its cocoon.
After the spider has spun this deviant web, the larva kills and eats the spider, then spins its silk cocoon to finish developing for another eleven days. Although it’s not known exactly how the wasp larva chemically controls the spider’s behavior, this is considered one of the most subtle perturbances of host behavior by just slightly modifying how it builds its web to generate cocoon starting material instead of a regular web (Eberhard, 2000).
Flatworms and California Killifish
So far, we’ve heard about two instances where killing the host provides food for the developing parasitic larvae to grow and complete its lifecycle. But sometimes, controlling a host’s mind is all about getting back to the right place so that the parasite can complete its lifecycle.
California killifish and their parasitic flatworms (Euhaplorchis californiensis), are an example of this relationship. The flatworms can live inside the killifish, but they cannot reproduce there. So, they influence the fish’s behavior to get back into birds where they are able to reproduce.
The flatworms adjust the killifish’s brain chemistry, altering serotonin and dopamine metabolism to encourage the fish to engage in riskier behavior (Shaw et al, 2009). This change means that the fish swim closer to the water’s surface where they are easily snatched up and eaten by avian predators. Back inside the birds, the flatworm reproduces and forms thousands of cysts, allowing their lifecycle to continue. Who says you can’t go home? (You just might need the help of a fish to get there.)
Intestinal Parasites and Wolves
Another example of parasites influencing a host’s behavior involves the single-celled parasite Toxoplasma gondii, and gray wolves. T. gondii can live in the intestines of wolves, but only reproduce in felines.
Researchers suspect that wolves are exposed to T. gondii after eating the feces of infected cougars. Infected wolves were more likely to display risky behavior such as leaving their pack to form a new one, or asserting themselves as pack leaders (Meyer et al, 2022).
Collectively, these behaviors increase the range of territory that wolves from infected packs travel, increasing the likelihood for further bidirectional transmission from a wolf to a cougar and from cougars back into wolves.
Keep in mind that leaving their pack or challenging the existing pack leader are both risky behaviors that put the individual wolf at increased likelihood of death. So, this is another example where a parasite skews a host’s decision making to prioritize the parasitic lifecycle over the host’s health.
Intestinal Parasites and Mice
T. gondii can also infect mice, but, as we discussed previously, it only reproduces inside felines. T. gondii influences mice in at least two ways that make it more likely for them to get eaten by cats – an event that allows the parasite to reproduce once back inside the cats.
First, the parasite increases testosterone production in mice, making them less afraid of cats (Ingram et al, 2013). While sometimes it’s important to be brave and less afraid in life, in this case it is much more beneficial for the mice to be appropriately afraid of cats!
Second, mice are naturally averse to cat urine. The scent of cat urine makes them avoid that area, making them less likely to come into contact with a cat and be eaten. T. gondii reverses this natural reaction and instead makes mice attracted to the scent of cat urine (House et al, 2011). This makes the infected mouse more likely to come into contact with a cat, and of functional importance, in contact with the cat’s mouth!
Interestingly, this behavior is also observed in T. gondii-infected chimpanzees who become attracted to the urine of leopards, which are a chimpanzee’s only natural predator (Poirotte et al, 2016).
Fungi and Ants
Have you read or seen The Last of Us (Druckmann & Staley, 2023)? In that fictional story, fungi colonize humans, turning them into zombies that bite one another to spread the fungal infection. The plot isn’t totally implausible as there are fungal infections that parasitize insects and animals for their benefit.
In fact, the actual fungus from that story, Ophiocordyceps unilateralis, is parasitic in insects. For example, this fungus infects carpenter ants and directs them to leave their colony and move towards high ground by following light. Usually this means that the carpenter ant climbs a tree or otherwise reaches a high point.
Then the fungus produces a spore that projects out of the ant’s head, killing the insect and releasing spores. Since the ant is at a high point, the stalk can distribute spores across a broader area, thereby more efficiently promoting fungal spread.
Fungi and Cicadas
A different type of fungus, Massoospora cicadina attacks cicadas emerging from hibernation and deteriorates their abdomen and butt, leaving a clump of yellow spores in their place (Cooley et al, 2018). The fungus also ramps up the cicada’s sex drive which facilitates the transfer of fungal spores to other cicadas, and the fungal life cycle continues in that manner. Apparently, the yellow rear-end does repel potential cicada suitors!
Fungi and Frogs
It’s not just cicadas whose libido is ramped up by fungi, but frogs too. The fungus Batrachochytrium dendrobatidis infects multiple types of frogs and increases their likelihood of finding a mate.
Infected male common mist (Litoria rheocola) frogs called more for a female mate during the night compared to uninfected frogs. Their efforts paid off, as on average the infected frogs were more successful at finding a mate (Greenspan et al, 2016).
Other researchers found that male Japanese tree frogs (Hyla japonica) infected with the same fungus called for a mate more rapidly and produced longer mating calls compared to uninfected frogs. Again, the infected male frog’s enhanced efforts paid off because they more frequently mated compared to their uninfected counterparts (An & Waldman, 2016).
For both of these types of frogs, it is thought that enhanced reproduction rates are coupled with increased fungal transmission, which is why the fungus activates these frogs’ nighttime activities.
Barnacles and Crabs
Our last example is of a barnacle (Sacculina carcina) tricking a female crab into taking care of it as she would her own babies.
First, the barnacle finds the soft spot near joints on the underbelly of a crab where she normally carries her babies. The barnacle then sheds her shell and burrows into the crab, projecting tendrils throughout the crab to steal nutrients and deliver commands to the pseudo-maternal caretaker. The barnacle hijacks the maternal instincts of the crab such that the crab will protect, groom, and flush the barnacle with oxygenated water, just as it would do for its own offspring (Powell & Rowley, 2008).
Now that the female barnacle is well situated, it bores a hole through the crab’s shell and invites a male barnacle to come mate, and then continues using the crab’s forced goodwill to care for its own offspring. Talk about a toxic relationship!
Mechanisms of Influence
While the exact details of each of these zombie examples are known to differing extents, what is clear in all of the cases is that the control the parasite exerts over the host is executed through chemical communication that alters brain chemistry.
Hopefully, you would notice if you had a jewel wasp on your back tugging at your hair, or a barnacle burrowed into your underbelly. In the absence of any such obvious hitchhikers, it may still be worth examining if the external influences in your life – such as the food, substances, or media that you consume – are adjusting your brain chemistry and causing you to act against your best interests, also known as turning you into a zombie!
