Zombie-Ant Fungus

The Zombie-Ant Fungus (Ophiocordyceps unilateralis) is an entomopathogenic ascomycete in the Ophiocordycipitaceae, famous for the precision with which it alters its host's behaviour. What is treated as a single name is in fact a species complex, with many closely related forms each specialising on a particular carpenter ant species (Camponotus and relatives) in a particular region.

An infected worker leaves its colony, climbs the understorey to a characteristic height, and locks its mandibles onto a leaf vein or twig, a posture known as the death grip. It dies there, anchored, and days later a stalk emerges from the back of its head.

What the field observer sees is therefore not a mushroom on the ground but an insect fixed to vegetation with a dark, wiry projection growing out of it, often with a pale swollen patch partway along. Dead ants accumulate in dense clusters called graveyards on the vegetation above ant trails.

Stroma

A single (rarely two or three) dark brown to blackish, wiry, cylindrical stalk 0.5-3 cm long emerging from the dorsal surface of the ant's head, sometimes flexed or curved. It is tough and fibrous rather than fleshy, and tapers toward a sterile tip.

Ascoma and fertile plate

Partway along the stalk, usually on one side, sits a paler, often cushion- or plate-like swelling in which the perithecia are embedded. The one-sided placement of this fertile cushion is what gave the species its epithet. The perithecia appear as fine dark dots or bumps in the surface of the cushion.

Host and posture

The host is a carpenter ant, still recognisable, its mandibles clamped onto a leaf midrib, leaf edge or small stem. Fine white to pale brown mycelium often coats the ant's joints and undersurface, cementing it in place. Internally the ant is filled with fungal tissue, with the muscles consumed and the body cavity packed.

Spores

No spore print. Ascospores are long, thin and multi-septate, forcibly discharged from the perithecia; they can produce secondary spores on the leaf surface below to extend the infection zone.

The fungus is found in humid tropical and subtropical forest across South and Central America, Southeast Asia, Africa and parts of Australia, with related members of the complex in warm-temperate woodland. It requires the year-round high humidity and stable temperature of the forest understorey.

Infected ants are almost always found in the understorey, typically within a metre or two of the ground and on the undersides of leaves or on small stems, in shaded, humid conditions. Because the fungus needs these conditions to fruit, dead ants cluster in patches where microclimate is suitable, and these graveyards can hold dozens of hosts within a few square metres. Fruiting continues throughout the year rather than in a defined season.

This is an obligate parasite with a highly specialised life cycle. A spore lands on a worker ant out on the trail, germinates, and penetrates the cuticle. The fungus then grows as yeast-like cells through the body cavity, building a network that surrounds the muscles controlling the mandibles while leaving the brain itself largely intact.

At a species-specific time of day the fungus induces the ant to leave the trail, climb, and bite down, anchoring the body at a height and humidity that suit fruiting. The ant dies within hours, and over the following days the fungus consumes the soft tissue, hardens the cuticle from within, and pushes the stroma out through the head capsule. Ascospores fall onto the ant trails below, where they may infect further ants, and ant colonies respond by avoiding or removing infected nestmates.

Other ant-infecting fungi differ in the structure produced. Beauveria bassiana covers a dead insect in a dense white powdery bloom of conidia with no organised stalk. Akanthomyces and Gibellula species on spiders and other arthropods form pale, felted, club- or brush-like synnemata rather than a single dark wiry stroma. Ophiocordyceps myrmecophila and other ant-associated species in the genus produce stromata from the thorax or from between abdominal segments rather than from the back of the head, and often lack the one-sided fertile cushion.

Superficially, an ant bearing a mite or a plant hair can be mistaken for an early infection; the difference is that the stroma is continuous with tissue emerging from inside the head capsule and the ant is fixed in a death grip with mycelium cementing its feet.