Parasitic fungus makes zombie ants

A parasitic fungus turns carpenter ants into walking zombies, altering their behaviour and forcing them to die at a spot ideal for the fungus’ reproduction.

This is a dead carpenter ant attached to a leaf in the understory of a Thai forest. Before killing the ant, the fungus growing from ant's head changed the ant's behaviour, causing it to bite into the leaf vein. Credit: David Hughes, Penn State University

Tropical carpenter ants – Camponotus leonardi – live high up in the rainforest canopy in Thailand, and a group of researchers from Penn State University used transmission-electron and light microscopes to study what happens to ants infected with the fungus.

“The fungus attacks the ant on two fronts: first by using the ant as a walking food source, and second by damaging muscle and the ant’s central nervous system,” said David P Hughes, assistant professor of entomology and biology.

The growing fungus fills the ant’s body and head, causing muscles to atrophy and forcing muscle fibres to spread apart. The fungus disrupts the CNS, causing the ants to walk in a random manner, straying from the trail – something unaffected worker ants would never do, say the scientists.

The ants also suffered convulsions which caused them to fall to the ground. Once there, the ants were unable to find their way back to the canopy and remained in a lower, leafy understory – a cooler, moister area than the canopy – ideal fungus growing conditions.

“The behaviour of these infected zombie ants essentially causes their bodies to become an extension of the fungus’ own phenotype, as non-infected ants never behave in this way,” said Hughes.

At solar noon, the fungus synchronises the ant’s behaviour forcing affected ants to bite the main vein on the underside of the leaf. Multiplying fungal cells in the ant’s head cause lock jaw – muscle fibres controlling the mandibles to become detached. The infected ant is unable to release the leaf – even after death. Within a few days the fungus grows through the ant’s head, releasing spores to be picked up by other ants.

“The result for the ant is zombie walking and the death bite, which place the ant in the cool, damp understory,” said Hughes. “Together these events provide the perfect environment for fungal growth and reproduction.”