Rats Benefit from Winner and Loser Effects

Prior fighting experience of opponents can influence the outcome of conflicts. After a victory, animals are more likely to win subsequent contests, whereas after a defeat animals are more likely to lose, regardless of the identity of opponents. The underlying mechanisms and the adaptive significance of these winner and loser effects are as yet unknown. Here, we tested experimentally whether agonistic behavior of male wild-type Norway rats is influenced by social experience, and we investigated whether this might reduce fighting costs (duration of contest, risk of injury) in subsequent encounters. Rats were randomly assigned to receive either a losing or a winning experience and subsequently tested with unfamiliar, naive opponents. We found that most rats with a winning experience won the subsequent encounter, and all rats with a losing experience lost the next contest. Previous winners attacked more rapidly in the subsequent encounter and reduced their aggressive behavior sooner; the contests were decided more quickly, which saved time and behavioral effort to the winner. Previous losers received less aggression in the next encounter, despite emitting fewer submissive ultrasonic calls than in the preceding contest, thereby reducing the risk of being injured by the opponent. Thus, anonymous social experience influenced rats' subsequent behavior toward size-matched, naive, unknown social partners. Furthermore, apparently, they benefit from showing winner and loser effects in intraspecific contests by saving time, energy, and risk.