Short and long-lasting behavioral consequences of agonistic encounters between male Drosophila melanogaster

In many animal species, learning and memory have been found to play important roles in regulating intra- and interspecific behavioral interactions in varying environments. In such contexts, aggression is commonly used to obtain desired resources. Previous defeats or victories during aggressive interactions have been shown to influence the outcome of later contests, revealing loser and winner effects. In this study, we asked whether short- and/or long-term behavioral consequences accompany victories and defeats in dyadic pairings between male Drosophila melanogaster and how long those effects remain. The results demonstrated that single fights induced important behavioral changes in both combatants and resulted in the formation of short-term loser and winner effects. These decayed over several hours, with the duration depending on the level of familiarity of the opponents. Repeated defeats induced a long-lasting loser effect that was dependent on de novo protein synthesis, whereas repeated victories had no long-term behavioral consequences. This suggests that separate mechanisms govern the formation of loser and winner effects. These studies aim to lay a foundation for future investigations exploring the molecular mechanisms and circuitry underlying the nervous system changes induced by winning and losing bouts during agonistic encounters.Across the animal kingdom, aggression between conspecifics often accompanies the competition for food, mates, and territory. Although an innate behavior, aggression is a highly adaptive trait as well, with animals learning from previous experience and changing their behavior in response to new challenges. In competition for rank, for example, previous fighting experience influences the outcome of subsequent contests: prior defeat decreases whereas prior victory increases the probability of winning later contests. These have been called “loser” and “winner” effects (1). Such effects have been observed in many species, including fish (2), birds (3), and mammals (4). In general, loser effects persist longer than winner effects (5). The durational asymmetry observed between loser and winner effects has been hypothesized to participate in stabilizing social hierarchies among conspecifics (6).Fruit flies (Drosophila melanogaster) exhibit a variety of simple and complex social behaviors, including aggregation (7), courtship (8), and aggression (9) in which learning and memory have been demonstrated or postulated to serve important roles (10–12). Thus, characterizing the molecular basis of memory formation, retention, and retrieval is crucial to ultimately understanding the adaptability of these social behaviors. In Drosophila, a variety of operant and classical training paradigms have been used to subdivide memory into distinct categories. Short-term memory (STM) lasting minutes to hours is induced by a single training session, whereas long-term memory (LTM) lasting days usually requires repeated training sessions and involves de novo protein synthesis (13). A large number of studies have been carried out using olfactory, visual, social, and place memory paradigms. These have allowed the functional and molecular characterization of neuronal circuits and the identification of numerous genes underlying learning and memory (14–16). Included are mutations in rutabaga (rut, type 1 adenylyl cyclase) that interfere with learning and STM formation (17); amnesiac (amn, peptide regulator of adenylyl cyclase) that affect STM retention (18); and crammer (cer, inhibitor of a cathepsin subfamily) that prevent LTM formation (19). Whether rut, amn, and cer serve roles in the learning and memory that accompanies aggression remains unknown.Male–male aggression in fruit flies was first described almost 100 y ago (20). Since then, considerable progress has been made in understanding its expression and regulation (21–26). In competition for food resources and territory, male–male agonistic encounters, composed of stereotyped behavioral patterns, usually result in the formation of dominance relationships (9). During the progression of fights, both flies modify their fighting strategies: The ultimate winners chase and lunge at their opponents to gain sole access to the resources, whereas the losers retreat from the resources after receiving such attacks (9, 10).In second fights (30 min after first fights), losing flies display greater submissive behavior and never win against naïve or experienced opponents, revealing short-term loser effects (10). Evidence for winner effects, however, was not found (11). Recently, in olive fruit flies (Bactrocera olea) it was found that previous losing and winning experiences both increased the aggressiveness of the flies. This suggests that the consequences of losing or winning may vary across species (27).We previously suggested that fights between male flies function as operant learning situations in which males learn to use their most advantageous fighting strategy during fights and then continue to do so in subsequent contests (28). In an attempt to optimize the learning and memory associated with aggression, we designed new “handling-free” behavioral chambers (29). These proved to be more desirable for studying the formation of loser effects (12). By using these experimental arenas and pairing familiar opponents in second fights we previously showed that changes in fighting strategies could be developed by both winning and losing flies. This allowed us to suggest the existence of short-term winner effects along with the previously demonstrated loser effects (12). A more detailed examination of these short-term effects is presented here along with experiments attempting to measure the intrinsic changes in fighting abilities of losing and winning flies.In the present study, we ask (i) whether a single fight can lead to the formation of loser and winner effects and how long these effects persist, (ii) whether flies adopt different fighting strategies in second fights depending on their opponents, (iii) whether longer-lasting behavioral effects result from sequential repeated defeats or victories, (iv) whether protein synthesis is required for the short- or long-term effects observed, and (v) whether mutations in genes involved in learning and memory affect aggressive behavior.