How does a fungus turn an ant into a zombie?

A fungus called Ophiocordyceps unilateralis can steer an ant's behaviour so that the ant dies where the fungus can spread. It is an insect-pathogenic fungus, discovered by the British naturalist Alfred Russel Wallace in 1859, and it primarily targets ants of the tribe Camponotini, including carpenter ants. Popular media calls the infected victims zombie ants.

The sequence is precise. Spores land on an ant, attach to its exoskeleton and break through using mechanical pressure and enzymes. An infected ant then has irregularly timed full-body convulsions that dislodge it from its canopy nest to the forest floor. It climbs a plant stem and uses its jaws, with abnormal force, to secure itself to a leaf vein. Ants generally clamp on about 26 cm above the ground, in a humidity of 94 to 95 percent and temperatures of 20 to 30 degrees Celsius.

After the bite, the muscle fibres of the jaw atrophy and the ant can no longer let go. Fruiting bodies then grow out of the ant's head and rupture, releasing spores, in a process that takes 4 to 10 days. Scientists tested how much the position matters: when dead ants were moved elsewhere, the fungus's growth either failed or produced undersized, abnormal reproductive structures.

How the fungus does this is only partly understood. Large fungal cell populations sit beside the host's brain and within its muscles and secrete metabolites with behavioural effects, but more research is needed to understand exactly how they act. The label zombie ant has even been called catchy, yet misleading.