Communication by means of visual signals occurs during the competitive, aggressive and sexual interactions of many animals. Some animals such as fish are able to change their body coloration rapidly, and there is evidence that this is used as a means of signalling. However, the precise meaning of such signals is rarely understood. We examined whether (1) darkness in juvenile Atlantic salmon is associated with submission, and (2) changing to a darker colour could act as a signal to the opponent and hence induce a change in its behaviour. We found that both the sclera of the eye and the overall body coloration tended to darken in fish that were losing territorial encounters, while victors retained their original coloration. The darkening was rapid and usually occurred during a period of sustained attacks by the opponent; however, the aggression level decreased as soon as the losing fish had become darker. We suggest that the darkening of the losing fish was associated with submission and may result in a change in the behaviour of its opponent, so minimizing the potential risk of injury during unnecessarily prolonged fights. (C) 1999 The Association for the study of Animal Behaviour.
