The Bond Between a Parasite’s Parasite and a Zombie Ant

Jun 5, 2012 | Jason Hayes | Research & Policy

After taking control of the ant’s brain and forcing it to act on its behalf, the fungus erupts out of the ant’s head to reproduce. The fungal fruiting body, or stalk, that emerges from the ant’s head does not do so with pyroclastic force, nor does it creepily ooze from the exoskeleton like a miasma. Instead, the slender tower, sometimes taller than the host insect, breaks the calcium carbonate plate armor with the tedious pressure of water building in a closed space. It seems to work with the same assuredness as well; after all, at this point, the fungus already won. The stalk may grow for weeks and manifest from other joints before it sheds spores into the air to be inhaled by other ants. The thing that I remember most from every photograph and video of an ant’s useless body under an otherworldly fungal flagpole is the eyes. You don’t normally notice an ant’s eyes. Typically, they are camouflaged and vacant looking glasses that seem to reflect a shimmer of light, much like a shark’s. After the fungus has plied its trade, they seem different; they no longer share the same color as the body or produce any sheen. It seems like after the ant lost control of its brain the eyes turned to lead.

The first time I heard about Cordyceps fungus was on the television series Planet Earth and, like many other phenomena on that panorama of mind-boggling biology, it seemed impossible. Each Cordyceps fungus, of which there are hundreds, infects only one other species of insect; they are, indeed, specialists. Often, Cordyceps fungi wipe out entire ant colonies as they control the brains of their hosts to position their fruiting bodies high up on surrounding plants where the spores can distribute to the most susceptible ants. However, in the Amazon, scientists found a fungus that would not consume colonies, but instead cause a steady and surmountable number of individual ant deaths. This fungus acts like tuberculosis in the United States, it is contagious and present, but most of the time people don’t think about it. The specific fungus for those still-surviving ant colonies is called Ophiocordyceps.

The Ophiocordyceps and the ants that it infects forge an intriguing bond in nature. It is one that potentially existed millions of years ago, before the Himalayas were formed. However, as is often the case in ecology, their relationship does not occur in a vacuum and there are other callers knocking on the door. So, why doesn’t Ophiocordyceps decimate ant colonies with the fury of other Cordyceps species? Scientists recently discovered a new character in this web and the reason that the entire colony was not dying off: a hyperparasitic fungus. The term hyperparasite refers to a parasite whose host is another parasite.  In this case, the hyperparasite lives off the Ophiocordyceps fungus. Scientists at the University of Copenhagen identified the new fungus while studying ants afflicted with the mind-controlling Ophiocordyceps fungus in Brazil.  After noticing mass graves of dead ants with fungal fruiting bodies erected from their heads, scientists took samples to figure out why many were dying over time, but the colony remained safe.

Once in their labs, the ants mired in Petri dishes and Vaseline, it became clear that many Ophiocordyceps fruiting bodies failed to produce spores; in fact, most did not at all. The reason, scientists discovered, is the hyperparasite attacks Ophiocordyceps during its fragile fruiting stage and effectively castrates the stem. As a result, many fruiting bodies do not mature to the point where they produce spores. Of the spores that did reach maturity, one ant controlled by Ophiocordyceps fungus would yield only one new ant with a fruiting body that survived long enough to reproduce. One mind-controlled, lead-eyed ant yielded one more, which yielded one more, and so on. The attack rate was low enough for the colony to survive. Scientists theorized in an article in PLoS ONE that the hyperparasite and Ophiocordyceps make each other sustainable. Effectively, the hyperparasite ensures that Ophiocordyceps, and its fragile, UV-sensitive, juvenile stage, do not survive to adulthood often enough to deplete its source of hosts. What the hyperparasite provides is balance: it ensures that Ophiocordyceps reproduces slowly by killing off most of the fruiting bodies and the ant colony continues to thrive. The idea that something that bears the title “hyperparasite” could have a seemingly altruistic impact on two other species is odd, but in doing so it also secures its own food supply. It is selfish altruism, and it is only another fold in this true story of mind control and hyperparasites.

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