Abstract
Pairs of adult male crickets, Gryllus bimaculatus, fight and immediately determine winner and loser statuses. The winner male repeatedly produces an aggressive (rival) song by rubbing his forewings together. In this study, I removed the plectrum, a sound-producing structure in the forewing, from male crickets and measured their brain serotonin (5-hydroxytryptamine: 5-HT) levels immediately after a 10-min aggressive interaction. Pairs of plectrum-removed males fought and established clear winner-loser relationships, like the case of intact males. The plectrum-removed winner males frequently rubbed their forewings together, but were unable to produce song. Aggressive interaction reduced significantly brain 5-HT levels in the plectrum-removed males, regardless of their winner and loser statuses. Furthermore, the reduction of brain 5-HT was detected primarily in the central body, a group of neuropils spanning the midline of the brain. In contrast, in pairs of intact males, aggressive interaction reduced brain 5-HT levels in the loser males, but not in the winner males. Plectrum removal alone did not affect the brain's 5-HT levels. These results suggest that aggressive song emitted by the winner male cricket prevents the reduction of 5-HT levels in his own brain, especially in the central body.