The evolution of alarm pheromones in fungus-growing ants

Fungus-growing ants (FGAs: tribe Attini) have a fascinating chemically-mediated symbiosis with their fungus garden. The mutualistic garden serves as an obligate food source for the ants and their brood. In exchange, FGAs provide the fungus with nourishment, vertical transmission, and protection. To protect their valuable garden resource, FGAs communicate threats using volatile alarm pheromones and their defensive responses (e.g., ‘playing dead’ vs swarming and biting) vary across species. Although this communication directly impacts colony survival, the evolutionary origins and trajectory of attine alarm pheromones remain unknown. FGAs are ideal for comparative research investigating alarm compound evolution and communicative function due to varying levels of social complexity (e.g., caste system and colony size) found across the >250 FGA species. We hypothesize that chemical alarm mixtures shift depending on ant defensive behaviors and phylogeny. To test this, we compiled new and published alarm compound data and mapped chemical profiles to the FGA phylogeny. We also investigated defensive behaviors by measuring mandible opening responses, where restrained ants are exposed to 1) mandibular extracts, 2) solvent control, and 3) 3-octanol (positive control). This comparative study demonstrates the evolutionary patterns of alarm pheromone chemistry and how mandibular extracts prompt variable responses across species. We conclude that although species may face similar threats, alarm responses differ. Furthermore, despite broader evidence that alarm pheromones are rarely species-specific and distant lineages convergently evolve the same compounds, we have found phylogenetic signal suggesting only close relatives are likely to have similar alarm compound profiles.