Non-lethal fungal infection could reduce aggression towards strangers in ants

Abstract

Many parasites interfere with the behaviour of their hosts. In social animals, such as ants, parasitic interference can cause changes on the level of the individual and also on the level of the society. The ant-parasitic fungus Rickia wasmannii influences the behaviour of Myrmica ants by expanding the host's nestmate recognition template, thereby increasing the chance of the colony accepting infected non-nestmates. Infected ants consistently show an increase of the alkane tricosane (n-C23) in their cuticular hydrocarbon profiles. Although experimental application of single compounds often elicits aggression towards manipulated ants, we hypothesized that the increase of n-C23 might underlie the facilitated acceptance of infected non-nestmates. To test this, we mimicked fungal infection in M. scabrinodis by applying synthetic n-C23 to fresh ant corpses and observed the reaction of infected and uninfected workers to control and manipulated corpses. Infected ants appeared to be more peaceful towards infected but not uninfected non-nestmates. Adding n-C23 to uninfected corpses resulted in reduced aggression in uninfected ants. This supports the hypothesis that n-C23 acts as a 'pacifying' signal. Our study indicates that parasitic interference with the nestmate discrimination of host ants might eventually change colony structure by increasing genetic heterogeneity in infected colonies.

Fig. 1. Schematic representation of the overall experimental design.

The graph shows the five different behavioural experiments which were performed during our study. (1) Corpses were washed five times and the response of ants to hexane-washed dummies was observed. (2) The reactions of live workers were observed towards infected and uninfected dummies. (3) Corpses were washed (see 1 herein), and cuticular extracts from uninfected or infected workers were applied to them. The behavioural responses of live workers were observed towards these dummies, which were obtained from uninfected colonies. (4) The reaction of workers towards uninfected dummies treated with synthetic n-C23. (5) The reactions of workers to hexane-washed dummies onto which we had applied cuticular extracts from uninfected workers supplemented with synthetic n-C23. I CHC = cuticular hydrocarbon extracts from infected individuals. U CHC = cuticular hydrocarbon extracts from uninfected individuals. Hexane washed corpses (uninfected) are marked with an asterisk (*). Rel Ab = Relative Abundance. The majority of the elements (vial, Petri-dish, syringe) were created by the authors, using freely available elements on the internet (Petri-dish, syringe, vial). The Myrmica ant icon was created by Ms. Natalia Timuș, our previous co-author, who gave her consent to the free use of the icon within the manuscript.

Fig. 2. Aggression indices resulting from interaction assays between live uninfected and infected M. scabrinodis workers and different types of dummies.

The upper part of the graph shows the reactions of uninfected ants towards (a) Uninfected corpse; (b) Infected corpse; (c) Uninfected corpse + n-C23; (d) Dummy + uninfected CHC; (e) Dummy + infected CHC; (f) Dummy + Uninfected CHC + n-C23. The lower part of the graph shows the reactions of infected ants towards (g) Uninfected corpse; (h) Infected corpse; (i) Uninfected corpse + n-C23; (j) Dummy + uninfected CHC; (k) Dummy + infected CHC; (l) Dummy + Uninfected CHC + n-C23. Dummies are coloured in grey, while corpses are coloured in dark brown. Box-and-whisker plots with median, 25–75% quartiles, maximum/minimum range and outliers. The Myrmica ant icon was created by Ms. Natalia Timuș.