Fungi turn ants into zombies. (need I say more?)

A stroma, or spore-releasing body, of a killer fungus grown out of the head of a victim ant. Image courtesy David Hughes and with thanks to Science News.
A stroma, or spore-releasing body, of a killer fungus grows out of the head of a victim ant. Image courtesy David Hughes and with thanks to Science News.

As much as Hollywood might want you to think they exist, zombies are fictitious. But a study out today claims that actually, they kind of do exist — if your undead is an ant and your possessive reviving sorcerer a deadly and clever species of fungus.

Imagine you’re a carpenter ant minding your business hanging around your nest high in a tropical forest canopy in Thailand.  Suddenly you begin to feel faint. Your body feels bizarre – possessed even – and in an eerie twist reminiscent of Being John Malkovich, your six legs turn around and begin to take you down the tree. Your will is too weak to turn around and return to your nest, and as it fades into the ethereal distance and the world begins to darken, you feel an odd compulsion to use those big mandibles of yours to latch onto a leaf. Within a few minutes you find one that seems suitable, and with your last bit of dying energy, you climb onto its underside and clamp down, biting with all your might before the rest of your body goes limp.

And there you hang. This is the story (with a little dramatization) as told by David Hughes of Harvard in the September issue of American Naturalist. The fungus, a species in the genus Ophiocordyceps, is a parasite that scientists have known infects ants and eventually kills them for more than 100 years. But Hughes’ work shows that this fungus has evolved also to precisely control the ants so much that the ant moves to an area that’s convenient for the fungus before it dies. The researchers found that ants predominantly died on the north underside of leaves about 25 cm above the ground, where conditions are optimal for fungal growth. How do they know this?  They moved leaves with dead ants to various heights from the forest floor; sure enough, the fungi grew most successfully at around 25 cm high. Hughes said in a statement:

The fungus accurately manipulates the infected ants into dying where the parasite prefers to be, by making the ants travel a long way during the last hours of their lives. The fungus has evolved a suite of novel strategies to retain possession of its precious resource.

So, let’s get back to your dead corpse. As you absorb your last wisps of oxygen, the fungus begins its growth, cementing body crevices to prevent other fungi from attacking, but, amazingly, leaving your mandible muscles alone so you don’t lose your grip and drop off of the prime-habitat leaf. Within a few days, a fungus fruiting body emerges from your head, sending spores down to infect unsuspecting passersby just feet below on the forest floor. The fungus life cycle then begins anew.

The one apparent loophole of this system is how the fungus gets back up into the tree to reinfect more carpenter ants. Does the carpenter ant life cycle include a stage on the forest floor? Or does the fungus hitch a ride on another organism from the forest floor to the treetrops, where it jumps ship and infects these canopy ants? Of course the fungus probably has many hosts, but it must have some way to reinfect this one, lest the cycle breaks.

See also Susan Milius’s Science News report.

Andersen, S., Gerritsma, S., Yusah, K., Mayntz, D., Hywel‐Jones, N., Billen, J., Boomsma, J., & Hughes, D. (2009). The Life of a Dead Ant: The Expression of an Adaptive Extended Phenotype The American Naturalist, 174 (3), 424-433 DOI: 10.1086/603640