The Zombie Apocalypse: Already Underway

Common Name: The Zombie-Ant Fungus

A.K.A.: Ophiocordyceps unilateralis

Vital Stats:

• Whole “graveyards” of 20-30 ants may be found within a single square metre
• Telltale bitemarks on fossil plants suggest this fungus, or a related species, may have been in operation for the last 48 million years
• Host species is the carpenter ant Camponotus leonardi

Found: Tropical forests throughout the world

It Does What?!

Despite all the advances of modern neuroscience, the fact is, human understanding of brain chemistry and its manipulation still has a long way to go. Much to the chagrin of those plotting world domination, we won’t be chemically controlling each other’s minds any time soon. How embarrassing then, that a mere fungus seems to have perfected this technique. Almost fifty million years ago. Scooped again, humanity.

It begins with an ant walking along the ground, deep in a tropical forest somewhere. This ant, Camponotus leonardi, lives high in the trees, but must occasionally come down to cross from one tree to another when there is a break in the canopy. As it walks, a minute fungal spore drifts down from above and lands in its back, unnoticed. The unseen spore springs into action, producing an enzyme which breaks down the ant’s exoskeleton just enough to allow a fungal hypha, like a tiny root, to enter. The host’s fate is now sealed.

While the ant climbs back up into the canopy and goes about its business, the fungus grows through its insides, breaking down and consuming the non-vital soft tissues as it goes, keeping the animal alive even as it is being eaten. Soon, the fungal tendrils reach the brain and begin to produce chemicals which affect the host’s behaviour in very specific ways. First, it will experience convulsions that cause it to fall out of its tree. These will continue periodically, preventing it from returning to its colony. Over a period of hours, the ant will then wander, erratically and aimlessly, over the ground and low-growing plants.

This is where the precision of the fungus’ mind control becomes truly impressive. At solar noon, when the sun is highest in the sky, the infected ant will abruptly climb the stem of a small plant and find a leaf pointing north by northwest at a height of 20-30cm above the ground. Yes, really. No one knows how this jaw-dropping specificity is accomplished, but it’s what the fungus wants, providing a temperature of 20-30 degrees Celcius (68-86F) and a relative humidity of around 95%. In cases where ants were experimentally moved to different heights or orientations, the fungus was unable to reproduce properly.

Having found the perfect leaf, the zombified ant will go to its underside, find a major leaf vein, and just bite down on it as hard as it can. The fungus has already destroyed the muscles required to release this grip, and so there the ant stays, slowly dying over the course of the afternoon. Once its victim has been dispatched, the fungus grows toward the leaf, further anchoring itself to the plant. Around a week later, the parasite completes its horrifying circle of life by growing a fruiting body, similar to a mushroom, from the back of the dead ant’s head. This will open to release thousands of tiny spores, raining down over any potential hosts which may be walking below.

While the fungus is able to infect other, closely related, species of carpenter ant, it has less precise control over these hosts and isn’t always successful in getting the ant to do its bidding, suggesting that even minor variations in brain structure can stump it. So we’re probably safe from the fungal zombie apocalypse. At least for the time being…

Says Who?

• Andersen et al. (2009) American Naturalist 174(3): 424-433
• Hughes et al. (2011) Biology Letters 7: 67-70
• Hughes et al. (2011) BMC Ecology 11: 13
• Pontoppidan et al. (2009) PloS ONE 4(3): e4835