Abstract
All animals interact with conspecifics during their life, and nearly all also display some form of aggression. An enduring challenge, however, is to understand how the experiences of an individual animal influence its later behaviours. Several studies have shown that prior winning experience increases the probability of initiating fights in later encounters. Using behavioural assays in the laboratory, we provide evidence that, in Argentine ants (Linepithema humile), the mere exposure to an opponent, without the encounter escalating to a fight, also increases the probability that it will display aggression in later encounters. Argentine ant workers differ in their propensity to attack non-colonymates, with some ants repeatedly aggressive and others consistently more docile. Although 78 per cent of the workers were consistent in their behaviour from one encounter to the next, workers that did change their behaviour after an encounter with a non-colonymate more often changed from non-aggressive to aggressive, rather than the reverse. Surprisingly, a single encounter with a non-colonymate increased a worker's propensity to fight in encounters up to a week later. An encounter with a non-colonymate also increased the probability that a worker would attack ants from a colony that it had not previously encountered. Thus, these interactions lowered the overall aggression threshold, rather than stimulating a specific aggressive response to a particular foreign colony. Finally, our data suggest that aggression towards non-colonymates increases with age.