zombie-ant fungus

zombie-ant fungus, (Ophiocordyceps unilateralis), species of parasitic fungus of the order Hypocreales that infects ants and alters their behaviour before killing them. The fungus is largely pantropical and primarily infects carpenter ants (genus Camponotus). Its taxonomy is contentious, with some research suggesting that each species of carpenter ant is infected by its own species of the fungus; it is treated here in the broadest sense (sensu lato). The zombie-ant fungus was formerly placed in the genus Cordyceps and is still often referred to simply as “cordyceps.” It does not infect humans.

The zombie-ant fungus begins its life cycle as a sticky spore on the forest floor. If the spore attaches to the body of a passing carpenter ant, it germinates infective hyphae (threadlike strands) to breach the insect’s exoskeleton. Once successfully inside the ant body, the fungus grows and manipulates the ant’s behaviour, eventually positioning the ant in a warm, humid location near the ground that will best serve to distribute a new crop of fungal spores. After the ant dies, the fungus finishes digesting the cadaver from the inside out and, as its final act, erects a triumphant stalk from the base of its conquest’s head. The stalk releases spores from characteristic lateral cushions, and the cycle continues.

The moniker “zombie-ant fungus” arises from the organism’s uncanny manipulation of its host to generate a number of wholly novel behaviours, reminiscent of a zombie. Once the fungal mass inside an infected ant reaches a critical size (usually about 16–24 days following infection), the ant is induced to leave its nest at different times than its regular foraging habits, and it fails to walk along established ant trails. Infected ants are generally unresponsive to external stimuli, including other ants. Sporadic convulsions eventually make the ant fall from the tree canopy back to the forest floor, where its movements become seemingly nondirected and aimless. However, apparently triggered by the time of day, the moribund ant is soon directed to climb a low-growing plant, orient its body specifically on the vegetation, and permanently lock its mandibles to firmly attach itself to the plant. This “death grip” is closely synchronized with the extent of infection and is the final act of the ant; death follows very shortly thereafter. The place of biting is characteristic to the ecosystem in which the infection takes place: infected ants in warm temperate forests are found mortally attached to twigs, while those in rainforests are found on the main veins or margins of leaves. Graveyards of dozens of infected ants, all with fungal stalks protruding from their bodies and anchored to leaves or twigs at roughly the same distance from the ground, are sometimes found and indicate just how precisely the fungus can control the dying behaviours of its victims.

Research into the mechanisms behind the zombie-ant fungus suggests that the organism activates and suppresses certain ant genes throughout the infection to alter and control its behaviour. For example, the fungus suppresses a number of ant genes that are likely involved with immune- and stress-responses, thus creating more favourable conditions within the insect’s body. Fungal cells are known to invade the muscle fibers of the host’s body and may affect the expression of genes involved in tryptophan metabolism, causing the tremors and biting behaviour. Although fungal cells do not seem to enter the ant brain directly, the fungus secretes a number of neuromodulatory agents, particularly guanobutyric acid and sphingosine, near both the central and peripheral nervous systems of its host. The fungus also contains a slew of unusual bacteria-like toxins, which may impair the chemical signals that allow the ant to communicate with other members of its nest.

Carpenter ants in areas with zombie-ant fungus have evolved a number of adaptations to limit their exposure to the parasite. Carpenter ants in infested areas tend to be more arboreal than those without the fungus threat, suggesting that they avoid the forest floor to limit exposure to the spores. They also practice social grooming and can remove spores from each other’s bodies before infection can occur. The ants have also been observed to carry infected members of the colony away from the nest and from their usual ant trails on the forest floor to reduce the spread of spores to high traffic areas, and they are known to actively avoid the graveyards of infected dead.

In popular culture, the zombie-ant fungus is the organism behind the human zombies in the video game and subsequent HBO show The Last of Us (2013; 2023– )—the latter of which starred Pedro Pascal and Bella Ramsey—and the book and film, both written by M.R. Carey, The Girl with All the Gifts (2014; 2016).