Zombie Ants

An extreme case of a parasite manipulating its host
The evolutionary arms race between parasites and their hosts is a never ending competition; each player constantly adapts in an attempt to overcome the other, […]

Art by Ilse Lee

The evolutionary arms race between parasites and their hosts is a never ending competition; each player constantly adapts in an attempt to overcome the other, resulting in ever more complex relationships between the two. It represents a struggle between competing sets of co-evolving genes that develop adaptations and counter-adaptations against each other. Observation of interactions between parasites and their hosts allows us to see a snapshot of evolution in progress.

Zombie ants, created by the fungus Ophiocordyceps unilateralis, provide one of the most dramatic examples found in nature. The ‘zombie–fungus’, as it is nicknamed, specifically infects the tropical rainforest dwelling carpenter ant, Camponotus leonardi and manipulates its behaviour to facilitate its own reproduction. It is a relative of the fungus from which LSD is derived, and uses a powerful combination of chemicals to control the ant brain.

Colonies of C. leonardi are found in the rainforest canopy, where climatic conditions are highly variable; but infected ants are directed by the fungus to climb down to a shrub closer to the forest floor where conditions are perfect for fungal replication. Using solar cues, the fungus steers the ant to the perfect position, forces it bite down on a leaf and then kills it. Over the next two weeks, the fungus grows a spore-dispersal structure from the base of the ant’s head, transforming the ant cadaver into a launch pad for missile-like spores. These are actively discharged over short distances, creating an infectious killing zone of approximately 1 m² below the dead host. To maximise the likelihood of spores landing on oblivious passing ants of the correct species, spore production needs to be constantly maintained. To feed it during this process the fungus must protect the resource pool provided by the ant’s body. Brown external fungal filaments, called hyphae, are grown to cover the carcass, which both protect the fungus’ food supply and act to further secure the body to the leaf. Chemicals with anti-malarial and cancer fighting properties have been isolated from these hyphae, suggesting that they also provide protection against invasive microbes that would compete with O. unilateralis.

The precision of ant positioning by the fungus is truly astounding. Bodies of infected ants are almost without exception found exactly 25 cm off the ground, clamped onto a major vein of the underside of a leaf. Incredibly, the chosen leaf is always on the north-northwest side of the plant, where it is protected from the wind. Here, the temperature is consistently low and humidity consistently high, perfect for fungal replication.

In response, carpenter ants have evolved defences against the fungus. The predominant way of avoiding infection appears to be by staying as far away from victim ants as possible. Studies have mapped the distribution of dead ants and found that they occur in high density aggregations known as ‘graveyards’. Remarkably, one graveyard was found to contain 2,243 dead ants and only 2 live ants. It is speculated that the ants make their nests high in the forest canopy as an adaptive response to avoid fungal breeding zones. The principal foraging routes of C. leonardi are aerial trails running between tree canopies. The ants only occasionally descend to ground level and when they do it is for no more than a few minutes. It is thought that C. leonardi ants avoid the forest floor as a defence mechanism and only descend when aerial trails cannot bridge gaps in the canopy.

This is just one intriguing example demonstrating the complexity of the parasite-host interaction, but many more exist in nature. A relationship in constant change, it provides a fascinating insight into the forces of evolution and triggers speculation about what the next move in the arms race will be. Indeed, there have been four new species of zombie-fungus identified in Brazilian rainforests just this year, so who knows what else may lie out there.

About Emma Houghton-Brown

Emma Houghton-Brown '11 is an undergraduate studying Biology at St. Hugh’s College.