The aggressivity of adult males and females was evaluated by measuring (1)the frequency of aggressive displays that individuals made toward a mirror image stimulus following varying periods of social isolation, (2) the display frequency in brief, unisexual and bisexual bouts with live opponents, and (3)the reinforcing effects of mirror image stimulation on performance of an approach response. Isolation up to seven days increased aggressivity in both sexes. Males performed lateral display toward a mirror image or a live conspecific more frequently than females. Mirror image stimulation also was a stronger positive reinforcer in males than in females. Investigations in various anabantoid fishes show that aggressive behavior in males can readily be elicited by visual presentation of another male (

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