The answer, his team found, is that the ants have an unwitting ally: a fungus that "castrates" the zombie-ant fungus.
Ant zombification begins when an Ophiocordyceps fungus shoots spores onto an insect. The parasitic fungus gradually takes over the ant's brain and directs the insect to a cool, moist location. The fungus then kills the ant, and fruiting bodies erupt from the ant's head and spread more spores.
"When you go into the forest, you find graveyards of these [infected] cadavers," said study leader Hughes, of Penn State.
"That would suggest that, for the ants running around the forest floor, it's terribly precarious—it must be festooned with spores of these fungi."
Not so, Hughes and his team discovered.
Combining new data from Brazilian zombie-ant graveyards with from previous studies of Thai graveyards, the scientists realized that an as yet unnamed fungus keeps the zombie-ant fungus in check.
"The vast majority [of zombie-ant spores] have been taken out of the game" by the other fungus, Hughes said.
(See pictures: "Photos: 'Zombie' Ants Found With New Mind-Control Fungi.")
The fungus-killing fungus chemically "castrates" its zombie-making cousin, Hughes explained—and highly effectively, at that.
The team's analyses showed that only 6.5 percent of zombie-ant fungus specimens were able to produce spores—meaning that the unnamed fungus largely limits Ophiocordyceps' spread.
Hughes likens the situation to oak-tree reproduction. "Of all those little acorns, the vast majority die—only a few get to be mature," he said.
"There are lots of these really cool interactions going on daily in the forest," Hughes added, "and I think we should be studying them in more detail."
The fungus-versus-fungus study appears in the May 2 issue of the journal PLoS ONE.