Different aggressive behaviours are exaggerated by facing vs. broadside subliminal stimuli shown to socially isolated Siamese fighting fish, Betta splendens

We report and analyse some features of a new phenomenon: socially isolated Betta splendens become extremely hyper-aggressive after seeing brief glimpses of fish models or mirrors. These brief glimpses are below the threshold for releasing aggressive display, so they are considered subliminal aggressive stimuli. The hyper-aggressiveness was observed to last for weeks. To confirm that hyper-aggressiveness was dependent upon the aggressive significance of the subliminal stimuli, we presented socially isolated Betta splendens with subliminal models in either a 'facing' posture (used mainly in aggressive contexts), or a 'broadside' posture (used in many social contexts). The fish shown the aggressive 'facing' subliminal stimuli became more aggressive, while those shown 'broadside' stimuli performed more generalized advertisement behaviours. The display posture of the model, which may incorporate specific features relevant to aggression, therefore determined how the subliminal aggressive stimuli altered subsequent aggressiveness. This difference was also persistent. Subliminal stimuli may thus be implicated in the hyper-aggressiveness so often reported after social isolation. (C) 1997 Elsevier Science B.V.