Impact of colony size on survival and sanitary strategies in fungus-infected ant colonies

Abstract

Group-living species have to deal with higher risks of exposure to pathogens and of disease propagation between group members. Insect societies have therefore evolved behaviours that contribute to the social immunity of the whole colony. Here, we investigate how the sanitary strategies displayed by ants depend on colony size, which is known to influence their social organisation. Myrmica rubra ant colonies of different sizes were faced with waste items that were infected or not by the entomopathogen fungus Metarhizium brunneum. By keeping nest parameters and pathogen loads proportional to colony size, we show that the largest colonies less suffered from exposure to life-threatening spores. Indeed, the largest colonies showed the lowest mortality rates and were the fastest to reject all the infected waste items out from their nest. Unexpectedly, small-sized colonies displayed distinct sanitary strategies according to the pathogenicity of the waste items. When challenged with fungus-bearing items, they opted out for an “emergency strategy” in which workers first moved out from their nest carrying brood and reintegrate it after sanitization by waste-transporting individuals. This demonstrates the behavioural plasticity of ant colonies of which both the size and the sanitary threat determine the efficiency and the type of hygienic responses to hygienic challenges. Insect societies have evolved several collective hygienic behaviours that contribute to their “social immunity”. Here, we found out that both internal and external factors determine the efficiency and the type of hygienic responses to waste items introduced in an ant colony. In particular, small-sized colonies demonstrated a behavioural plasticity with distinct sanitary strategies according to the pathogenicity of the waste items. Such a plasticity appears to be an adaptive way to limit exposure of larvae to pathogens and hence to preserve colony development. It also raises some questions about the origin of the differences in worker’s internal threshold allowing colonies to shift between sanitary strategies.