Garden substrate preparation behaviours in fungus-growing ants

Abstract

AbstractFungus-growing ants (Hymenoptera: Formicidae: tribe Attini) engage in mutually beneficial symbioses with fungi (Basidiomycota) that serve as their main food source. The leaf-cutters (genera Acromyrmex Mayr and Atta Fabricius), the most derived attine ants, employ elaborate nest-hygiene behaviours, including substrate preparation. By preparing substrate prior to its incorporation into the fungus garden, workers facilitate the physical breakdown of leaf material while reducing the abundance of potentially harmful microbes that contact their fungal mutualist. Despite its importance in ant fungiculture, substrate preparation has not been investigated in other genera of fungus-growing ants. We examined substrate-preparation procedures used by five genera of fungus-growing ants (Apterostigma Mayr, Cyphomyrmex Mayr, Trachymyrmex Forel, Acromyrmex, and Atta) representing most of the phylogenetic range of the Attini. Behavioural observations revealed that all five genera engage in substrate-preparation behaviours. Furthermore, these behaviours vary by genus, with Trachymyrmex, Acromyrmex, and Atta engaging in more elaborate preparation behaviours than the other genera. Additionally, we found that during substrate preparation, leaf-cutting ants inoculate leaf fragments with actinomycetous bacteria. These filamentous bacteria are known to produce antibiotics that suppress fungal pathogens, which suggests that inoculation with the bacterial mutualist during substrate preparation helps protect the fungus gardens of leaf-cutter ants from these parasites. Our finding that substrate-preparation behaviours occur across the phylogenetic range of attine ants suggests that these behaviours are a critical component of successful fungiculture by ants.