Familiarity influences body darkening in territorial disputes between juvenile salmon

Escalated contests between animals are potentially costly because of increa sed energy expenditure and risk of predation or injury. Hence we would expe ct selection to favour any mechanism that avoids unnecessary prolonged figh ting. One such means of avoiding escalated fights could be the use of infor mation gained through individual recognition. Precious work has shown that a darkening of the body colour is closely associated with submission in con tests between juvenile Atlantic salmon, Salmo salar, and it has been hypoth esized that this may act as a visual signal to the opponent. We tested the hypothesis that body darkening is used to reduce the cost of contests betwe en familiar fish such that losers darken more quickly when faced with famil iar than unfamiliar opponents. In contests between unfamiliar fish, submiss ive darkening occurred after more escalated contests in which the loser inc urred more aggression, whereas the opposite occurred when familiar fish wer e in conflict. In addition familiar fish either submitted quickly or engage d in protracted conflicts in which neither fish signalled submission, where as in unfamiliar fish contests were of intermediate duration regardless of whether either fish darkened. We suggest that body darkening is used by fam iliar fish to signal submission to familiar dominants in order to avert a c ostly escalated fight, but familiarity can lead to escalation without submi ssion if perceived competitive asymmetries are finely balanced. (C) 2000 Th e Association for the Study of Animal Behaviour.