[Origins and consequences of aggression in Etroplus maculatus (Cichlidae, Pisces). The problem of drive]

Increased aggressive activity between male and female in isolated pairs of Etroplus maculatus has hitherto been explained either by the drive hypothesis or by the external stimulus hypothesis. In an attempt to investigate these two hypotheses the partners of 48 pairs were separated and their aggressiveness towards their mirrored images and their non-aggressive activities were recorded the following day and again 4 days later after 180 brief presentations of an adjacent compartment. For both sexes aggressiveness was lowered if this compartement was empty (isolation) and heightened if it held the original mate. Females also reacted more aggressively on seeing conspecific strangers of both sexes. There was no significant alteration of non-aggressive activities in any of the series. The increase of aggressiveness caused by the presence of the mate was normally offset by various investigated mechanisms preventing its eruption against the partner. Attacks only occurred if the equilibrium between aggressiveness and inhibition was disturbed. All results and those of other authors are discussed; it is concluded that the behaviour of E. maculatus can be better explained in terms of external stimulation than in terms of an endogenous production of aggressive drive energy.