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How Ant Colonies Make Peace with Their Zombie Fungus Invaders

An ant-zombie fungus armistice.
August 18, 2014, 9:30pm

The zombie ant fungus is the stuff of parasitic lore. Its mission typically goes something like this: an ant just out doing ant things happens to get dusted with the fungus's spores, which punch through the ant's exoskeleton using a combination of enzyme action and mechanical force (a kind of "drilling" using hair-like protrusions).

Once inside the ant's guts, the fungus, known properly as Ophiocordyceps camponoti-rufipedis, grows as "free-living yeast cells," until after a couple of days, when the colonizing fungus begins producing nerve toxins. These toxins "instruct" the ant to find the fungus-friendly underside of some leaf, where the ant latches on.

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Finally, a phallic stalk of fungus punches through the back of the ant's neck area (if an ant had a neck), and a new dusting of spores is released, quite possibly onto the exoskeleton of a new ant host. The process repeats. Yes, it's grisly.

According to open-access research published this week in the journal PLOS One, ant colonies are actually quite good at defending themselves from this sort of parasitic attack. Ants demonstrate something called "social immunity," which the paper describes as, "mounted collective actions to prevent and control diseases spread in benefit of themselves and the others."

3D maps of foraging trail and monthly-infected ants surrounding ant colonies in Atlantic rainforest, Brazil/PLOS One

This keeps the fungus limited in its attack range, forcing it to deploy its spores outside of the ant colony threshold rather than inside, where those spores might do a lot more harm and possibly lead to colony collapse.

In long-term field observations of ant colonies—the first examination of this particular fungus/ant interaction outside of the lab setting—the researchers found that while 100 percent of the populations showed some fungal infection, no colonies failed.

The study involved placing ants infected with the zombie fungus inside of healthy ant colonies, with the goal being new observations of how the colony as a whole responds to their zombified kin.

"For the nest containing live ants, 9 (64 percent) out of 14 [infected] cadavers were removed from the leaf they were attached to and it was not possible to find them," the study reports. "The leaves were recovered but not the cadavers, suggesting they had been broken up by the live ants. The fungus in the remaining 5 cadavers (36 percent) did not grow the stroma correctly. Within the 14 samples placed in the bucket containing only nest material (absence of live ants), 8 (53 percent) of them did not grow at all."

The number of dead ants observed at four nests. While deaths weren't eliminated, they were controlled. Image: PLOS One

The conclusion: "the fungal parasite was incapable of reaching the infective stage inside ant nests, whether ants were present or not." The suggestion is that ants build their nests to be resistant to the proper development of the zombifying fungus' spore-producing stalk (the thing that erupts from the ant's carcass).

"The growth inside the nest may be constrained, primarily, by the physical limitations," the paper explains. "To successfully develop, the stroma must grow roughly one body length of the ant and this growth is perpendicular to the posterior-anterior axis of the ant to reach the successful development of the fungus. Secondly, social insects are known to control the nest climate, and O. unilateralis s.l. has been shown to be very sensitive to microclimate."

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It appears that the zombie ant fungus acts as something of a chronic infection for ant colonies, something that burdens but doesn't destroy outright. It's almost like how we've come to approach HIV infection in humans—it's an illness that can possibly be controlled but not cured.

"Although we did not observe colony collapse, we also did not observe any colonies clearing the infection," the study says. "For each month we conducted the census, we found new dead ants surrounding the colonies."

If that isn't yet a storyline within the human-zombie canon, consider it a freebie.

It's been thought that the parasite/ant colony relationship above has been something of an arms race, with colonies getting better at defending and the fungus getting better at spreading its spores. A final note in the study is that this might not be the case at all, and perhaps the two would-be antagonists have more of a mutually beneficial relationship.

The ants that are most likely to get infected by the fungus are, after all, the oldest and most expendable in the colony. (In the "assembly line" structure of an ant colony, the oldest get sent out the farthest.) And so by infecting a colony's weakest members, the fungus keeps the colony strong.

Rather than going to war against the fungus—which would likely mean pushing its anti-fungal abilities outside of the colony—"they may both be following quite stable evolutionary strategies in which the parasite is tolerated by the host." And if that isn't yet a storyline within the human-zombie canon, consider it a freebie.