Lateralization of displays during aggressive and courtship behaviour in the Siamese fighting fish (Betta splendens).

Evidence for right-left asymmetries in eye use at the individual level in the Siamese fighting fish, Betta splendens, is reported. When faced with their mirror image (in two daily trials of 10 min each), adult male Betta splendens showed consistency in their right or left eye use during threat lateral displays. Moreover, if one side was preferred by an individual to exhibit the lateral displays, then the duration of the displays on that side was longer than the duration of the displays on the other side. Similar findings were obtained when a sample of animals was tested for eye use during courtship displays in the presence of a female. Furthermore, consistency in eye use was observed in fish tested first with the mirror and then, 2 months later, with the female. Results are discussed with respect to the issue of the evolution of brain lateralization.