Zombie ant fungi and their hosts’ final choices of where to die

Ian Will, Sara Linehan, David G. Jenkins, Charissa de Bekker

This is a plain language summary of a Functional Ecology research article which can be found here.

Ophiocordyceps “zombie ant fungi” infect and modify the behavior of carpenter ants. In the final moments before death, the infected ant wanders the forest vegetation in search of an elevated position that benefits production and spread of fungal spores. Once found, the ant bites down and the fungus consumes the ant’s insides to sprout a mushroom-like structure from the back of the cadaver’s head to release spores and infect new hosts. Due to reasons currently unknown, infected ants tend to die in dense clusters called “graveyards.”  We investigated ant cadavers in such graveyards in Florida to understand where and when manipulated ants bite, and what repercussions this might have for the fungal parasite. During a year-long field survey we captured natural “real-world” variation and biological patterns through the collection of high amounts of data from hundreds of naturally occurring zombie ant cadavers. With this large and dedicated sampling effort we were able to identify several factors that appear to shape zombie ant biology.

Healthy Ophiocordyceps growth and infection by two hyperparasite species (credit: Davide Dal Pos)

            An overwhelming majority of cadavers were found on epiphytic bromeliads – small plants with pliable, tangled stems growing on larger plants, such as trees. We suspect that such vegetation may catch sick and stumbling manipulated ants and/or provide hiding spots less easily noticed by scavenging animals. Additionally, more manipulated cadavers appeared during the warm and wet summer months, likely due to a more optimal combination of temperature, precipitation, and humidity. We also found evidence that Ophiocordyceps-infected ants seem to seek optimal light levels to determine their final biting positions. Finally, zombie ant fungi are not alone in the forest, and are threatened by parasites of their own. We observed two fungal species that act as hyperparasites (parasites of other parasites) of Ophiocordyceps and drastically reduced its ability to fully develop and release spores. Ophiocordyceps fungi are found around the globe. By learning how they interact with different environments, we can begin to understand their disease dynamics and unravel how these, and other parasitic manipulators, thrive.