Zombies - Not Just For TV.

Zombie-me, c/o Joseph Hewitt, who happens to have
a cool RPG game open source thing, if you're curious.

You may have noticed that ScienceBlogs has gotten a little... strange today. That's because it's ZOMBIE DAY! There are a ton of posts around the site about the various biological, philosophical, physical, chemical and overall nature of zombies. What I've decided to talk about, though, might be the eeriest and creepiest of it all. That's because I'm going to talk about real zombies.

No, seriously. Real zombies. Things that walk around aimlessly, living dead, that kind of thing. In the real world, not just in horror movies.

Yes, they do exist.

While most think of the brain-hungry undead portrayed in horror films, zombies originated in the traditions of Voodoo. Powerful sorcerers, called Bokors, are said to have the power to kill and then raise whomever they wish from the dead, turning the person into a zombi, a mindless slave with no free will (here's a good post on this by Scibling Mo). Hollywood has added flesh-eating and transmissible to the true, original zombie.

Real zombies, like those of voodoo traditions, do exist. They walk the Earth every day, alive but dead, serving their sorcerer masters. What creature have this kind of horrific power? Why, of course, some of my favorites: parasites.

Parasites are the Victor Frankensteins, the bokors of the the natural world. They are mini-neuroscientists, constantly inventing new mind control techniques. They do that voodoo better than we do, and they have for eons.

Take the wasp Cotesia glomerata, which turns its victims into decaying bodyguards, protecting the growing wasp larvae even after they have feasted on its flesh. These wasps lay eggs in their victims, in this case caterpillars. When the larvae hatch, they begin feeding on the caterpillar's tissues. The larvae, which eat the caterpillar from the inside out, target the organs in such an order that the caterpillar lives as long as possible. Once they've had their fill, they exit the caterpillar and pupate, forming cocoons so they transform into adult wasps.

But the poor little caterpillar's job isn't over yet. The larvae are at their most vulnerable, and though scientists are clueless as to how, they have convinced the caterpillar to bend to their will so that they have a better shot at survival. Before it dies, the caterpillar spins a web of silk to protect the little babies that just ate its insides, and acts like a bodyguard, vehemently defending the metamorphosizing wasps until they emerge. Only then does it get the sweet release of death.

In general, there are a number of zombifying wasps. The emerald cockroach wasp attacks, as its name suggests, cockroaches. With carefully placed stings it renders the bug into a zombie-like state where it happily follow the wasp as it is lead to a dark chamber underground. The wasp then lays her eggs in the complacent roach, and seals it in to its tomb, where the eggs hatch, eat, pupate, and emerge while the cockroach sits and waits for its body to be consumed.

But wasps don't monopolize the creepy, zombie-inducing parasites.

Phorid flies, too, do their own kind of voodoo. These flies, from the genus Pseudacteon, are small and resemble fruit flies. But they ain't just pesky little insects - at least not to the ants which serve as hosts for their young. Phorid flies are the perfect zombifying parasite, including a healthy appetite for neural tissues. As parasitic larvae, phorid flies eat the brains of ants while turning them into living zombies for a bit before decapitating the unfortunate insects and moving onward.

The poor ants don't stand a chance. One a phorid female uses her hypodermic-needle-like ovipositor to inject her eggs into the ant's body cavity, it's all over. The larvae hatch and make their way to the ant's brain, where they feed on hemolymph (ant blood), muscle and nervous tissue (aka brraaaaaiiiiiinnnnssss). The fly-to-be completely devours everything inside the ant's head. The ant, meanwhile, wanders around as if it still has its brains for a couple of weeks, in a zombie-like state. When the larva decides it is ready to pupate, it releases an enzyme which decapitates the ant by dissolving the membrane which attaches its head to its body. The fly larvae pupates in the ant's disembodied head for about two more weeks, then emerges as a full-grown adult fly.

Ants are also the target of Dicroelium dentriticum, a trematode which lives in the livers of sheep. How does a parasitic fluke get from an ant to a sheep? Well, first it infects a snail, where it develops from one stage into another, and is excreted in the snail's slime to be eaten by an ant. Once infected, the ant continues about its business as normal for the most part, but when night sets in, the parasite takes over. The ant climbs to the top of a blade of grass, bites down, and waits. It will do this every time the temperature drops until it gets accidentally eaten by a sheep, thus completing the parasite's life cycle.

The nematode Spinochordodes tellinii uses crickets as a host for its young. As adults, the nematodes live freely in water. So when the worm is old enough to emerge, it needs to be in a wet place. What better way to ensure that than to drive the unwitting cricket host to suicide by drowning?

Even barnacles get in on the zombifying fun. The barnacles in the genus Sacculina aren't your typical, small, encrusting barnacles. Instead, they live a parasitic life, infecting crabs. A female barnacle will infect a crab, and as it grows, intertwines with the crabs nervous system. The crabs begin to act very strangely. As the parasite develops eggs, the crab acts as if they are its own, even if it's a male crab. The crab nurtures the eggs, and eventually, aids in their release in the water column just like it would its own. The crab lives only to serve its parasite master, until it eventually dies.

Perhaps the best example of total zombification, though, is the total take-over of ants by the fungus Ophiocordyceps unilateralis. Ants which are infected with this mind-controlling fungus from Thailand are compelled to amble far from their homes, climb trees or sprouts, and end up on the bottom side of a northwest-facing leaf, where they clamp down their jaws and wait while the fungus eats them from the inside out. After a few weeks, spores fall from the now fully eaten corpse, to infect other ants and continue the fungus' life cycle.

In all of these cases, once the host is infected, they are the living dead. They have no hope of recovery, no chance for redemption. They will perform their acts as needed by the parasite, and only once their bodies have served their function will the animal be released into the sweet arms of eternity.