This hint at such vast diversity and specialization also contains broader implications for assumptions about fungus numbers in general. "We may need to increase fungal diversity estimates by these sorts of factors," Evans notes.
The zombifying fungus's vast geographic distribution also hints at the possibility that it has been possessing ants at least since before many of the continents split apart. But these delicate organisms have not been discovered in fossils. Other examples of parasitism have been found preserved in amber dating back 150 million years, and they reveal the parasite and host as a common arrangement—but not any evidence of actual behavior manipulation.
Research published in Biology Letters in 2010 describes a 48-million-year-old fossilized leaf from Germany that bears the distinctive scars of a bite from an ant's mandible on its main vein. The researchers, led by Hughes, describe the find as perhaps "the first example of behavioral manipulation in the fossil record." During that time period the region of Germany would have been similar in climate to the areas of Thailand where contemporary zombie-ant fungus has been documented.
A parasite's parasite
The zombie-ant fungus is not the end of the parasitizing line, however—it meets its own death at the work of yet another parasite.
Andersen and her colleagues have found that a different breed of fungi grow over the ant corpse and the emerging fungus stalk. By covering the original fungus and its stalk, this secondary fungus—or hyperparasite—effectively prevents the zombie-ant fungus from ejecting its spores. "It looks like they completely sterilize it," Andersen says of the second-level parasite.
Even these hyperparasites seem to be specialized for growing on specific parasitizing fungi. "They're not really growing on anything else" in the area, Andersen says. This makes the hyperparasite another obligate parasite, which depends on the zombie-ant fungus, which depends, in turn, on the carpenter ant colony. "Once you're very successful, something else will take advantage of it," she notes. "It's really a little ecosystem in its own [right]."
The zombie-ant fungus's doom, of course, is little consolation for the infected ant. But the castration of the ant-killing fungus means that it will not go on to turn other local ants into zombies. This hobble might, in fact, be one of the reasons the zombie-ant fungus has been so successful over the long term. As a deadly infection, it could severely damage an ant colony. But, if another parasite renders more than half of its mature spores infertile (and more still failing to reproduce due to other interferences), that creates a sort of equilibrium with a colony. According to the research by Andersen and her colleagues (published in May 2012 in PLoS ONE), the actual reproduction rate for each mature zombie-ant fungus organism is a little more than one new mature organism, thus allowing the species and local population to sustain itself as long as there are ants nearby to infect.
In addition to the fungicidal fungi, scientists have also seen small bugs laying their eggs in the infected ant corpse, where their larvae can then eat the growing fungus. These bugs include specialized gall midges (in the Cecidomyiidae family) and a species that appears to be new to science, Hughes says. "It seems their entire nutrition comes from eating the fungus that manipulates ant behavior."
Are such hyper-specialized hyperparasites a freak occurrence? Apparently not: "We have found it all over the world, and most [ant] cadavers have hyperparasites exploiting the zombie-ant fungus at some stage," Hughes notes. And Anderson suspects "the more people study parasites, the more examples we'll find." Even if they are hiding in darker corners.