The suicide bite of zombie ants

infected ant

There’s lots of weird and wonderful life cycles in the natural world but they probably don’t come much stranger than a parasitic fungus in the tropical rain-forest. This beast of a fungus infects ants, destroying their brains before finally forcing the ant to take a suicide bite of a leaf.

A newly published paper (Behavioral mechanisms and morphological symptoms of zombie ants dying from fungal infection David P Hughes, Sandra Andersen, Nigel L Hywel-Jones, Winanda Himaman, Johan Billen and Jacobus J Boomsma BMC Ecology) describes the life cycle of Ophiocordyceps unilateralis, a parasitic fungus. The fungus lives in the tropical carpenter ant (Camponotus leonardi) that live high in the Thai rain-forest canopy.

The fungus attacks the ants on two fronts. Firstly by using the ant as a walking food source, and secondly by damaging muscle and the ant’s central nervous system, resulting in zombie walking and the death bite.

The fungus feeds on the living ant.

When the fungus infects the ant it starts to grow finally filling the ants body and head. The growing fungus forces the muscle fibres apart and breaks the fibres down. As it grows the fungus invades the central nervous system changing the way that the ant behaves. Normal ants in colonies will follow clear pathways between foraging areas and the nest, rarely leaving the trail. Infected ants though wander aimlessly around off the track and are unable to find their way back to the nest.

The ants will also suffer fits and will fall from the canopy to the ground. The impacts of the growing fungus on the nervous system though means that the ants are unable to find their way back to the top of the canopy. The fallen ants will stay in the lower story of the forest at about 25cm above the floor. This new level which is cooler and more moist than the top of the canopy provides ideal conditions for the fungus to thrive.

Noon is the deadly hour for the ant.

Noon is the deadly hour for infected ants. As the sun rises to the solar noon, the fungus forces the ants to take a bite out of the underside of a leaf. The fungus within the head of the ant causes the muscles of the mandible to break and the ants suffers a type of lockjaw. This causes the ant to just hang from the leaf even after death.

Because the ant is now hanging in the ideal conditions for the fungus within a few days the fungi will sprout it’s fruiting body from the head of the ant and release spores which are picked up by another wandering ant and so the fungus life cycle starts over again. The newly released spores enter the next victim ant through the respiratory spiracles (openings or ducts on the surface of the ant).

Dr David Hughes, from Penn State University, said, “The fungus attacks the ants on two fronts. Firstly by using the ant as a walking food source, and secondly by damaging muscle and the ant’s central nervous system, resulting in zombie walking and the death bite, which place the ant in the cool damp understory. Together these provide the perfect environment for fungal growth and reproduction. This behaviour of infected ants is essentially an extended phenotype of the fungus (fungal behaviour through the ant’s body) as non-infected ants never behave in this way.”

While an infection of the fungus is capable of destroying an entire ant colony previous research has shown that the carpenter ants have developed a sort of defence mechanism. Healthy ants are able to sense when another ant has become infected and will carry the infected ant away from the colony to prevent spores from infecting others. The healthy ants also go to lengths to ensure that their foraging paths do not go underneath areas that have infected ants in.

The fungus could have beneficial medical properties.

The fungus may not be good news for the ants but as with so many organisms with extreme lifestyle it could provide benefits to people. The fungus is known to secrete anti-microbial agents as it grows and fruits in order to suppress competition and this anti-microbial action is being looked at as a source of new natural drugs. The chemicals have already been shown to have anti malaria properties.