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Strength of aggressive display in siamese fighting fish (Betta splendens) toward a conspecific, an alien species (Macropodus opercularis), and a mirror image as affected by prior conspecific visual experience
William M. Miley and Gail Burack

Siamese fighting fish (Betta splendens) were provided with 10 days of visual experience with other conspecifics or were visually isolated from them for 10 days. Bettas were then allowed to display aggressively toward the following stimuli: a conspecific, an alien species (Macropodus opercularis), and a mirror image. Differences between intraspecific and interspecific displays depended on the response measure used. Isolated bettas displayed more frequently to conspecifics than did the visually experienced bettas, and more so than any other group. Visually experienced bettas showed longer latencies to conspecifics than did isolated bettas. However, there were no differences in the other response measures (duration of display and time to absence of responding for 15 min) in any of the stimulus conditions. Whether there are differences between intraspecific aggression, interspecific aggression, and aggression toward a mirror image may depend on the response measures used and the procedures leading up to behavioral testing.
This demonstrated that visually isolated bettas showed more visual signs of aggression, while non-isolated bettas took longer to show those signs and showed them less frequently.

and for the close relative macropodus there are similar findings:
Early behavioral experience and adult social behavior in the paradise fish, Macropodus opercularis L

Jeffrey Kassel and Roger E. Davis

Macropodus opercularis were reared in isolation, with conspecifics, or cross-reared with nonconspecifics. As young adults, cross-reared subjects presented with live conspecific and mirror image stimuli performed social displays less frequently than controls, while isolates did not differ from controls. In a spawning trial, no differences were found between groups. These results suggest that experience with an alien species may reduce the readiness to perform species-specific social displays
Note that while the fish displayed less, and displays are related to ritual aggression this did not hamper their ability to spawn successfully.

So what I would like to draw out from this is that the behavior of the fish towards one another is strongly influenced by the environment they reside in. This behavior makes sense in nature, aggressive displays near other species might lead to the death of the displaying fish, thus the fish can be more cautious around other fish than they are among their own kind.

This is also noteworthy because those interested in fighting fish have reported that fish can get accustomed to each other and be less aggressive, so they desire to prevent this. They prevent line of sight contact for males not to prevent males from staying agitated, but rather to prevent males from becoming more tolerant of each other and to ensure that the sight of another male causes aggression.

The habituation and recovery of aggressive display in Betta splendens

Five Betta splendens were exposed to a mirror for ten days. The mirror was then removed for a recovery period, replaced for 48 hours, removed for a second recovery period, and so on in such a manner that each fish was given recovery periods of 15 minutes, 6 hours, 24 hours, 48 hours and 4 days in an order counterbalanced across subj ects. In most subjects the threat display increased during the first few minutes of mirror exposure Fig. I. It decreased rapidly during the first 24 hours and then more slowly to a level which was low but above zero Fig. 2. The recovery data indicate gradual recovery over the first 1-2 days after removal of the mirror; further recovery was either non-existent or very slow, and no subject showed a full return to the initial level.

Waning of aggressive motivation in Betta splendens

Siamese fighting fish showed a waning tendency to view both their own reflections and a similar fish during trials lasting 32 hr. Although both stimuli elicited aggressive displays, all Ss spent more time viewing their reflections than viewing the live stimulus fish.
One cannot remove the aggression from the fish, and all signs say that even habituated fish that live peacefully in a community tank will kill each other if isolated together alone. However evidence and experience both indicate that one may use another species to reduce the social aggression of the Betta, the extremely tolerant and peaceful snakeskin gourami is an excellent potential choice for a tank mate that can reduce social aggression in smaller anabantoid fish.
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