Scientists have found that the fungus also triggers atrophy in its victim's muscles—specifically those around its mandibles. This atrophy is prompted by metabolites that purge the muscle cells of mitochondria and sarcoplasmic reticulum (which provide energy and signals), according to the BMC Ecology research. Perhaps counterintuitively, when the infected ant bites onto the leaf vein in it's so-called "death grip" this atrophy causes it to have lockjaw, leaving it there to die. This seemingly small detail is crucial to the fungus's success. "Without the death grip," Hughes explains, "the ant would fall to the ground," destroying the launching point for the fungus's spores.
By that stage, cells from the fungus have grown even more numerous in the ant's body. They have proliferated around the ant's brain and between surrounding muscle fibers but have not entered the brain, glands or muscles, making the manipulation all the more amazing.
Ants appear to die within six hours after their final bite. About two to three days later a fungal stalk will start to emerge from the back of the ant's head. After maturing over the course of weeks the stalk's head will shoot spores onto the soil below. Researchers have also discovered that this relatively slow-growing fungus can have its main stem broken off and regrow it later. Foraging worker ants can unwittingly pick up spores as they pass by.
The death of an ant outside of its colony and subsequent growth of the fungal stalk might be key adaptations of the fungus, researchers have hazarded. "Ants quickly remove dead nest mates so that dying in the nest would not allow sufficient time for stalk development and spore release" before the dead host ant was ejected, Hughes and his colleagues noted in their BMC Ecology paper.
The doomed ants do not wander too far afield, often ending up within meters of their familiar territory. And large groups of dead ants are commonly found near colonies. These graveyards can contain anywhere from 50 to hundreds of corpses, says Sandra Andersen, currently of the University of Oxford's Department of Zoology, who has studied many of these sites. If a nearby colony is new, there is not likely to be as large a graveyard—yet.
Ants' complex and large social groups are thought to be one of the keys to their global abundance. The fungus has capitalized on ants' social behavior. "Sociality can be thought of as evolution's winning lottery ticket," Hughes says. But "this zombie fungus is natural selection's tax man."
The zombie fungus, however, cannot live without the winning ants' continued success. It appears to be an obligate parasite, requiring a specific, local species of ant for it to inhabit, grow and propagate its spores.
A specialized but global threat
The ants best known for getting zombified by the Ophiocordyceps fungus are tree-dwelling carpenter ants found in Brazil and Thailand, but the fungus is thought to be broadly distributed in tropical areas around the globe. In fact, the full range of strange behavior— observed in Sulawesi, Indonesia—was first described in the scientific literature by Alfred Russell Wallace in the 19th century.
Although many ants in different areas are similarly infected and dispatched in this strange way, the species of fungus infecting them is not at all the same. "Instead of one variable species, there may be tens or even hundreds," says Harry Evans, a principal scientific officer at the Center for Agricultural Bioscience International. In a 2011 paper published in PLoS ONE, Evans, Hughes and Simon Elliot (of the Department of Animal Biology at the Federal University of Vicosa in Brazil) described four new species of the Ophiocordyceps fungus that were found in just a small section of rainforest in Brazil's southeastern state of Minas Gerais. Each of these species was associated with a different Camponotus ant species, denoting a high degree of specialization.