The Marching Dead

A pleasant stroll through the countryside from Romero's "Night of the Living Dead" (1968)

The dead take a pleasant stroll through the countryside in Romero’s Night of the Living Dead (1968)

Ever since Romero’s 1968 classic Night of the Living Dead revolutionized the zombie film, popular fiction has been flooded with stories of a world brought to its knees by the undead. Whether at the hands of the slow re-animated corpses of The Walking Dead or the lightning-fast, virus-infected cannibals of 28 Days Later, the way in which a zombie apocalypse could start holds a strange fascination for fans worldwide. The existence of zombies is so improbable that most believe they can’t exist outside the worlds of science fiction and fantasy. Fortunately, the real world is a very weird place.

The reality is much more complex than the mindless, shuffling hordes would have you believe. Here and now – outside the world of science fiction – zombie ants actually exist, driven not by unconscious cravings, but by parasites. Through millennia of adaptation and evolution, these parasites have honed their survival skills into an art form, developing intricate life cycles at the ants’ expense. The most amazing part? No single parasite is responsible for this zombie ant phenomenon; many diverse species, including fungi, flies, and worms, have independently evolved the ability to control ant behavior in order to survive.

One notable zombifying parasite is Dicrocoelium dendriticum, more commonly known as the “lancet river fluke.” Flukes are a group of parasitic flatworms (although not all are of the mind-control variety) whose life cycles include a vertebrate definitive host, such as fish or mammals, and a snail. Lancet river flukes were first identified in 1819, but their life cycle is so complex that it wasn’t fully unraveled until the early 1950’s.

The lancet river fluke is thought to be endemic in countries on every continent, living in grazing animals and causing such a mild infection that it usually goes unnoticed. That seems harmless enough, but here’s where things get interesting: once the grazer excretes the fluke’s eggs, land snails eat them and get infected.

"BRAAAIIINS!" Image: Shutterstock

Image: Shutterstock

As a self-defense mechanism, the snail goes into overdrive on slime production in order to wall off the flukes, which have since grown into larvae called “miracidia,” from the rest of the snail’s digestive tract. The snail drops the miracidia-filled slime balls on the grass, and continues on its way. Fortunately for the fluke, a much sought-after ant delicacy is – you guessed it – snail slime balls. The ant follows the slime trail snails leave behind, munching on slime balls along the way.

Most flukes that have made their way into the ant stay in the ant’s haemocoel, where blood bathes its organs, and wait until they can find their way into the next host. However, one adventurous fluke works its way up to the ant’s brain. From there, the fluke goes into mind-control mode, waiting for evening.

When the sun begins to set and the air cools off, the fluke manipulates the ant’s nerves, compelling it to move away from its colony and up to the top of a blade of grass, where it firmly clamps down its mandibles. The ant stays there, paralyzed, until dawn. When the sun begins to rise again, the fluke allows the ant to return to the colony and go about its daily formic business; none in the colony suspects there is anything wrong with their zombified neighbor.

While this series of events seems weirdly specific, the lancet river fluke isn’t simply making a power play. By forcing the ant to move to the top of the blade of grass, it makes it much more vulnerable to consumption by a cow or sheep, which would complete the fluke’s life cycle. Prolonged direct sunlight would kill both the ant and parasite, however, so the fluke ensures that they are only exposed during the coolest parts of the day.

Zombie ants are found all over the world, but human infections with the lancet river fluke are exceedingly rare and only cause mild, non-neurological symptoms, if any at all. In other words, you won’t be seeing human zombies climb trees to watch the sunrise any time soon.

For those who get excited at the possibility of a zombie apocalypse – don’t be too disappointed. Modern science may not be able to support mindless reanimated corpses, but nature has provided us with the next best thing: a horde of zombie animals.

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