Increased aggressiveness after brief social isolation of adult fish: a connectionist model which organizes this literature.

We examine the literature on fishes' aggressive behaviour after social isolation, in the light of a connectionist adaptive control system model for robot motivation and learning. If animals used the model's motivation modules, then social isolation would cause two simultaneous processes to occur-one progressively increasing motivation, the other decreasing display readiness. The readiness decrement would have the temporal flexibility typical of motivation changes, and would disappear soon after social stimuli reappeared. The incremental effect would be synaptic, and would be temporally more stable. Due to this stability difference, brief post-isolation aggression tests would tend to show that social isolates have depressed attack readiness, but longer tests would uncover the underlying increase in aggressiveness. In reviewing the literature, we find that this has been overwhelmingly the case. A new hypothesis about the adaptive value of the increase of aggression during isolation is outlined, which may help make the phenomenon more understandable.