Effects of aggression and wing removal on brain serotonin levels in male crickets, Gryllus bimaculatus

When pairs of adult male crickets (Gryllus bimaculatus) that had been housed individually for 7 days were placed together, they fought, and dominant–subordinate relationships were formed within 1 min. Aggressive behavior by the dominant male was repeated during the period in which the two males were kept together. Immediately after 10 min of aggressive interaction, brain serotonin (5-hydroxytryptamine, 5-HT) levels were unchanged in dominant males and significantly reduced in subordinate males. The emission of aggressive song by dominant males is known to be abolished by removal of the wings. All wings were thus removed from male crickets. After 7 days of isolation, pairs of wingless males were placed together. The wingless males fought and formed dominant–subordinate relationships within 1 min. The wingless, dominant males displayed aggressive behavior. Brain 5-HT levels in the wingless males were reduced immediately after 10 min of aggressive interaction, and no significant differences in brain 5-HT levels were detected between the dominant and subordinate males, unlike the case for intact males. These data indicate a difference in brain serotonergic activity between dominant and subordinate male crickets during aggressive interaction, and suggest that aggressive behavior by dominant male crickets rapidly reduce brain 5-HT levels in subordinate ones. Furthermore, the data suggest that aggressive song is responsible for the change in brain 5-HT levels.