Corpse Removal as Simple Defense Mechanism that Keep Pandora’s Box Closed in an Ant Nest System Parasitized by a Highly Pathogenic Fungus

Erős, Katalin and Markó, Bálint and Rákosy, László (2015) Corpse Removal as Simple Defense Mechanism that Keep Pandora’s Box Closed in an Ant Nest System Parasitized by a Highly Pathogenic Fungus. (Unpublished)

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Abstract

Social systems are attractive targets for parasites due to the high abundance of potential hosts and the spatial stability of colonies. Among social insects, ants in particular have developed various defense mechanisms in order to fight parasites, for instance nest cleaning, allogrooming, and corpse handling. The ultimate organizational form of the colonies of specific ant species is so-called supercolony: a system made up of a high number of related nests, which are connected through permanent worker and information exchange. These interconnections can facilitate the dispersal of parasites. However, in the Formica exsecta polydomial system that we studied the lethal endoparasitic fungus Pandora myrmecophaga had very low prevalence. The fungus manipulates infected workers that climb and die on grass blades near the nest, which then ensures the efficient distribution of the parasite by covering the mound surface with the conidia that is then produced. This strategy predicts a high prevalence of the fungi. We tested a hypothesis regarding the existence of simple defense mechanisms in ants: workers would dispose of every corpse appearing on grass blades as potential source of infection, thus lowering the chances of spore dispersal. We imitated the appearance of infected ants by fixing fresh corpses on grass blades near the nests at two different distance classes, while using dummies as control objects. The results confirmed our hypothesis: ants discovered and disposed of corpses efficiently, primarily those that were close to the nest, whereas dummies were dealt with less frequently. While the discovery rate was independent of the mound size and the activity of ants, the disposal rate was positively influence by the activity of ants around their mounds. This simple defense mechanism does not require any specific adaptation on the part of the ant and it ensures the low occurrence of a lethal pathogen in a supercolony.

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