A non-social and isolate rearing condition induces an irreversible shift toward continued fights in the male fighting fish (Betta splendens).

Effects of rearing conditions were examined in the development of agonistic behaviors in the male fighting fish. In group-I (highly social), fish were communally reared. In group-II (highly social and isolate), fish were individually housed and exposed to the group-I fish through transparent walls until the sexual maturity (from 6 to 12 weeks post-hatch). In group-III (social and isolate), individually housed fish were similarly exposed to other fish within the group. In group-IV (non-social and isolate), individually housed fish were further visually isolated. Agonisitc behaviors were compared among males of the groups-II, -III, and -IV in their fights against the group-I male. The group-IV males showed significantly higher rate of wins than the groups-II and -III males, without differences in the incidence of agonistic behaviors (butt-or-bite, chase, and gill-cover erect) before the termination of the mutual fights. Increased incidence of agonistic behaviors was found after the termination (particularly in the unilateral chase), suggesting that the group-IV males continued to fight even after the opponent male displayed a submission. The aggression was also enhanced in the group-II, when they were thereafter reared in a social isolation after the sexual maturation; a critical period was thus not found. The enhanced aggression was not reversed in the group-IV, when they were thereafter exposed to social stimuli; shift to the continued fights was irreversible. Possible fitness gain of the enhanced aggression was discussed in terms of the adjustability to altered biological resources.