In several animal species, aggressive experience influences the characteristics and outcomes of subsequent conflicts, such that winners are more likely to win again (the winner effect) and losers more likely to lose again (the loser effect). We tested the olive fruit fly, Bactrocera oleae (Diptera: Tephritidae), as a model system to evaluate the role of the winner and loser effects in male-male territorial contests. Further, we conducted experiments to test if winning and losing probabilities are affected only by the outcome of the previous contests, or whether the fighting experience itself is sufficient to induce an effect. Both winners and losers of two consecutive encounters displayed higher intensity of aggression and fought longer in subsequent contests. In both cases, they achieved higher fighting success than naïve males. The enhanced fighting performance of both winners and losers was stimulated by merely experiencing a contest, not necessarily by the relative outcome of previous fights. Overall, this study highlights the fact that previous victories and defeats both enhance aggressive behaviour in olive fruit flies, allowing them to achieve higher fighting success in subsequent contests against inexperienced males.A ggressive behaviour is widespread throughout the animal kingdom 1,2 . Aggression is a highly flexible behaviour affected by a large number of factors, and is important to ensure survival and reproduction in many species 3 . The evolution of aggression is shaped by a trade-off between the resultant benefits (i.e., securing limited resources) vs. costs (i.e., risk of injury; loss of time and energy), optimising fitness outcomes 4,5 . Aggressive behaviour can be severe between individuals of the same species, as they compete for the same food, territory, and access to mates 6 . Game theory predicts that Evolutionarily Stable Strategies for conflicts between conspecifics may involve stereotyped contests characterized by the ritualised exchange of agonistic signals 7 , which are thought to convey increasingly accurate information for assessing the contenders' chances of winning [8][9][10][11] . Probability of winning can depend on physical disparities (e.g. size, strength, weaponry) 12-15 as well as on aggressive motivation 16 . The latter is a product of several factors, including the presence of resources 17-21 , social upbringing 22 , physical exertion 23 and experience in previous fights [24][25][26] . Learning in the context of aggressive behaviour is widely recognised in animals 4,27,28 , and it has been shown that previous contest experience affects the characteristics and outcomes of contests in many species 4,26 . It is generally acknowledged that behavioural changes during combat that relate to prior experience fall into two general categories. Losing experiences tend to decrease willingness to engage in a contest (i.e. the loser effect), while winning experiences tend to increase willingness to escalate a contest (i.e. the winner effect) 4 . In other words, individuals experiencing a ...