The scent of symbiosis: gut bacteria may affect social interactions in leaf-cutting ants

Animal gut microbiota affect host physiology and behaviour. In social insects, where colony level integrity is preserved via a nestmate discrimination system based on cuticular hydrocarbon mixtures, microorganismal effects may therefore influence social dynamics. Although nestmate recognition has undergone a thorough exploration during the last four decades, few studies have investigated the putative role of gut microbes. Here, we integrated 16S rRNA-based microbial community profiling, chemical and behavioural approaches to test whether gut microbes affect nestmate recognition in Acromyrmex echinatior leaf-cutting ants. Treating workers with a sterile diet or with antibiotics resulted in a substantial alteration of their gut microbial communities. In pairwise social interactions, untreated versus antibiotic-treated nestmates behaved more aggressively than other nestmate and non-nestmate pairs, suggesting that bacterial suppression may alter chemical social cues and trigger aggressive behaviour. Chemical analyses of treated individuals revealed a decrease in the abundance of two metapleural gland antifungal compounds, and confirmed the correspondence between aggression levels and chemical profile differences. Feeding microbiota-remodelled ants with conspecific faecal droplets partially restored the original bacterial communities. Ants fed with faecal droplets from different colonies were unusually aggressive compared to pairs fed with faecal droplets from the same colony. We cannot exclude confounding effects resulting from the potentially harmful action of antibiotics on ant hosts. However, our results suggest a correlation between chemical profiles and the presence of certain microbial species in the gut, which may affect nestmate recognition and division of labour. This opens novel questions about the role of symbiotic microorganisms in the evolution of social insect behaviour.