Modulating Aggression Through Experience

Aggressive interactions are a common means of contesting resources for most animals. Considerable variation occurs in whether a specific individual wins a particular aggressive contest. Influences on the behaviour of individuals that might produce this variation include, among many factors, hunger, size, residency and age (Beaugrand et al. 1996; Hsu et al. 2005). Behavioural ecologists have been quite successful in understanding variation in contest outcomes, employing benefit/cost models to predict such things as contest duration and winner (e.g. Riechert 1998). Benefits are immediate or longer-term positive effects on fitness of the individual, such as gaining access to food or mates as a result of the contest. Costs include time and energy spent in the contest as well as the possibility of being injured or the increased chance of being taken by a predator. Among fishes and a few other animals, including some insects, experience in a prior contest has been shown to influence the outcome of a later contest (Hsu et al. 2005). A recent winning experience tends to increase the chances of winning the current contest, while a losing experience tends to decrease the chance of winning. Among species tested so far considerable variation occurs in how much of an effect prior winning or losing experiences produce and how long the effect lasts. For most organisms winning and losing effects may last for different lengths of time and tend to be asymmetrical, with losing tending to have more influence than winning. Recent experiments indicate, as well, that multiple prior contests, rather than just the most recent experience, can affect behaviour during, and outcome of, the current contest (Hsu & Wolf 2001). Most of the evidence from fish contests strongly suggests that the effects of prior contest experience influence the individual’s perception of its own fighting ability and the accumulation of costs in a subsequent contest (Hsu et al. 2005). In contrast to many standard studies of experience effects on learning and memory, the examination of these effects on aggression are complicated by the necessity to consider more than one individual’s experience in understanding contest outcomes. It is most likely the combined experience effects of the contestants that influence outcomes. How these combined effects are integrated into the ongoing behaviour of the contesting individuals is still very uncertain. The effect of prior contest experience wanes over time and has the hallmarks of memory and forgetting. Following the definition of learning offered by Alcock (1993,