Social plasticity and individuality shape variation in contest behaviour

Physical contests are critical in most animals in determining access to limited resources such as territories, food and sexual partners. Individuals should base their decision to engage and escalate a contest on the potential returns and the probability of winning against a specific opponent. Although variation in competitive ability should maintain variation in contest behaviour within populations, empirical evidence linking contest behaviour to competitive ability at the among- and within-individual levels remains limited. Here, we used an inbred line of the highly territorial and aggressive fly Drosophila prolongata to test how pre-existing variation in competitive ability drives phenotypic variation in contest behaviour. Specifically, we quantified the degree to which individual differences (i.e. individuality) in two key traits determining competitive ability, body size and weapon size, contribute to variation in territoriality and aggressiveness. Although territoriality and aggressiveness were repeatable, we found that behavioural plasticity in response to both focal and opponent morphological traits largely explains variation in both behaviours. Thus, even in the absence of genetic differences and under identical laboratory conditions, individuals consistently differ in contest behaviour while still adjusting their responses to the social context. We suggest that variation in the micro-environment, by shaping competitive ability through body size and weapon size, plays a crucial role in driving both between- and within-individual variation in social behaviour.