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Pupal cocoons affect sanitary brood care and limit fungal infections in ant colonies

Simon Tragust123, Line V Ugelvig12, Michel Chapuisat4, Jürgen Heinze2 and Sylvia Cremer12*

Author Affiliations

1 Evolutionary Biology, IST Austria (Institute of Science and Technology Austria), Am Campus 1, 3400 Klosterneuburg, Austria

2 Evolution, Behaviour and Genetics, Biology I, University of Regensburg, Universitätsstr. 31, 93040 Regensburg, Germany

3 Animal Ecology I, University of Bayreuth, 95440 Bayreuth, Germany

4 Department of Ecology and Evolution, Biophore, UNIL-Sorge, University of Lausanne, 1015 Lausanne, Switzerland

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BMC Evolutionary Biology 2013, 13:225  doi:10.1186/1471-2148-13-225

Published: 14 October 2013



The brood of ants and other social insects is highly susceptible to pathogens, particularly those that penetrate the soft larval and pupal cuticle. We here test whether the presence of a pupal cocoon, which occurs in some ant species but not in others, affects the sanitary brood care and fungal infection patterns after exposure to the entomopathogenic fungus Metarhizium brunneum. We use a) a comparative approach analysing four species with either naked or cocooned pupae and b) a within-species analysis of a single ant species, in which both pupal types co-exist in the same colony.


We found that the presence of a cocoon did not compromise fungal pathogen detection by the ants and that species with cocooned pupae increased brood grooming after pathogen exposure. All tested ant species further removed brood from their nests, which was predominantly expressed towards larvae and naked pupae treated with the live fungal pathogen. In contrast, cocooned pupae exposed to live fungus were not removed at higher rates than cocooned pupae exposed to dead fungus or a sham control. Consistent with this, exposure to the live fungus caused high numbers of infections and fungal outgrowth in larvae and naked pupae, but not in cocooned pupae. Moreover, the ants consistently removed the brood prior to fungal outgrowth, ensuring a clean brood chamber.


Our study suggests that the pupal cocoon has a protective effect against fungal infection, causing an adaptive change in sanitary behaviours by the ants. It further demonstrates that brood removal–originally described for honeybees as “hygienic behaviour”–is a widespread sanitary behaviour in ants, which likely has important implications on disease dynamics in social insect colonies.

Social immunity; Sanitary brood care; Grooming; Hygienic behaviour; Metarhizium fungus; Formicidae