Just as humans circle around a fight to watch the outcome, animals too gather at the sidelines of a brawl. Researchers have observed this behaviorknown as eavesdroppingin a range species, from Siamese fighting fish to nightingales. And now Rufus Johnstone from the University of Cambridge has modeled for the first time just how eavesdropping affects levels of aggression within a group. His surprising conclusion, reported today in the Proceedings of the National Academy of Sciences, is that eavesdropping can stir up violence.

To develop his model, Johnstone built upon Maynard Smith's classic simplification of animal aggression, the Hawk-Dove game. The basic idea is that animals at odds adopt one of two strategies, that of a combative hawk or a peace-seeking dove. Hawks automatically beat doves, and two hawks or two doves in conflict face an equal chance of winning. Played out over many rounds, the game shows that both hawks and doves do well if they are in the minority. If too many animals follow the same strategy, though, the approach no longer confers an advantage.

Johnstone set up his version of the game such that challenges between hawks and doves proceeded as in Maynard Smith's game, but eavesdroppers followed their own set of rules: if they faced an animal that had won its previous fight, the eavesdroppers would play the dove. If, however, their opponent had recently lost, they would act like hawks (see chart). Again it turned out that eavesdroppers, like hawks and doves, were most successful if they were rare. (They prosper when they can best predict other players' behaviors. But because they themselves act unpredictably, their strategy weakens as their numbers increase.)

In other ways, though, the results were unexpected. Many researchers thought that eavesdroppers used the information they gleaned to avoid fights with dangerous challengers, and so the behavior should lessen the amount of violence in a group. Instead Johnstone found that individual aggression was more common in his game than in the standard Hawk-Dove scenario. "In the presence of eavesdroppers, there is an extra benefit to be gained by winning a contest," he writes. "An individual that is victorious in one round is more likely to win in the next, because its opponent is less likely to mount an escalated challenge. This advantage of a positive "image" or "reputation" effectively adds to the value of the contested resource, and thus favors increased aggression."