The influence of sex, line, and fight experience on aggressiveness of the Siamese fighting fish in intrasexual competition.

We examined the influence of sex, line, i.e., broods from different parents, and previous fight experience on the aggressiveness of the Siamese fighting fish Betta splendens in intrasexual competition. The innate aggressiveness of the fish against their mirror images was measured on the day prior to the direct fight with other individuals, and it was found to be influenced by the line type but not by the sex. In the direct fight with other individuals, the males invested more effort in the fight than the females. In addition, the individuals of a particular line that exhibited a lower innate aggressiveness spent less time in the direct fight and were often losers when compared with those of other lines. After the direct fight with other individuals, the aggressiveness of the fish against their mirror images was remarkably influenced by the outcome of the direct fight, i.e., the winners exhibited more aggressive behavior, whereas the losers exhibited a lesser degree of aggressive behavior. This influence of the previous fight experience on subsequent aggressiveness was the greatest in the individuals of the line that have exhibited the lowest innate aggressiveness. However, the positive effect of the winning experience or the negative effect of the losing experience on subsequent aggressiveness decreased following several days after the previous fight increased.