Trophallaxis: the functions and evolution of social fluid exchange in ant colonies (Hymenoptera: Formicidae)

Trophallaxis is a complex social fluid exchange emblematic of social insects and of ants in particular. Trophallaxis behaviors are present in approximately half of all ant genera, distributed over 11 subfamilies. Across biological life, intra- and inter-species exchanged fluids tend to occur in only the most fitness-relevant behavioral contexts, typically transmitting endogenously produced molecules adapted to exert influence on the receiver's physiology or behavior. Despite this, many aspects of trophallaxis remain poorly understood, such as the prevalence of the different forms of trophallaxis, the components transmitted, their roles in colony physiology and how these behaviors have evolved. With this review, we define the forms of trophallaxis observed in ants and bring together current knowledge on the mechanics of trophallaxis, the contents of the fluids transmitted, the contexts in which trophallaxis occurs and the roles these behaviors play in colony life. We identify six contexts where trophallaxis occurs: nourishment, short- and long-term decision making, immune defense, social maintenance, aggression, and inoculation and maintenance of the gut microbiota. Though many ideas have been put forth on the evolution of trophallaxis, our analyses support the idea that stomodeal trophallaxis has become a fixed aspect of colony life primarily in species that drink liquid food and, further, that the adoption of this behavior was key for some lineages in establishing ecological dominance. Trophallaxis plays a vital role in colony physiology, nutrient distribution, and communication, and doing so underscores the necessity of understanding this complicated suite of behaviors in all of its manifestations.