Zombie-ant fungus feast: New research is uncovering how zombie-ant fungus might control its hosts. But this parasite also has its own fungal threats. Image: Wikimedia Commons/David Hughes/Maj-Britt Pontoppidan/PLoS ONE
An unsuspecting worker ant in Brazil's rainforest leaves its nest one morning. But instead of following the well-worn treetop paths of its nest mates, this ant stumbles along clumsily, walking in aimless circles, convulsing from time to time.
At high noon, as if programmed, the ant plunges its mandibles into the juicy main vein of a leaf and soon dies. Within days the stem of a fungus sprouts from the dead ant's head. After growing a stalk, the fungus casts spores to the ground below, where they can be picked up by other passing ants.
This strange cycle of undead life and death has been well documented and has earned the culprit the moniker: "zombie-ant" fungus—even in the scientific literature. But scientists are just learning the intricacies of this interplay between the Ophiocordyceps parasitic fungus and the Camponotini carpenter ants that it infects. Fossil evidence implies that this zombifying infection might have been happening for at least 48 million years. Recent research also suggests that different species of the fungus might specialize to infect different groups of ants across the globe. And close examination of the infected ant corpses has revealed an even newer level of spooky savagery—other fungi often parasitize the zombie-ant fungus parasite itself.
"We have advanced a great deal in understanding how the fungus controls ant behavior," David Hughes, an assistant professor of entomology and biology at The Pennsylvania State University, says. Every few months scientists are discovering yet another peculiar trait that, added together, make this parasite one of the most insidious infections—or perhaps that honor goes to the parasite that ultimately kills the killer parasite.
This clever Ophiocordyceps fungus depends on ants to reproduce and spread, but it has found an abundant host animal. As Hughes notes, ants have been incredibly successful, currently comprising an estimated half of all insect biomass worldwide.
One of the first clues that a tropical carpenter ant has become infected with Ophiocordyceps is that it will leave the dry tree canopy and descend to the humid forest floor, staggering over debris and plants. "Infected ants behave as zombies," Hughes and his colleagues wrote in a 2011 BMC Ecology paper describing some of the latest findings. The ant will walk randomly, displaying "convulsions that make them fall down and thus preclude them from returning to the canopy," they noted, comparing the stumbling gait with a "drunkard's walk."
The clumsiness cannot, however, be blamed on the ant. "While the manipulated individual may look like an ant, it represents a fungal genome expressing fungal behavior through the body of an ant," the researchers noted in the paper. Hence the zombie designation.
Evans suggests that a nerve toxin spurred on by the fungus is at least partly to blame, "judging from the uncoordinated movements and hyperactivity of the ants infected," he says. Ants that have been dissected at this stage of infection reveal heads already full of fungal cells.
Eventually, an affected ant will stop on the underside of one leaf, roughly 25 centimeters from the forest floor, and clamp down on the leaf's main vein. (This position appears to be optimal for the fungus's later stage in which it ejects spores onto the soil directly below.) Biting leaves is not normal ant behavior. The zombies' bites are synchronized near noon (possibly cued by clock genes in the fungus) and usually occur in a north-northwestern orientation.