Rapid body colouration change in Oryzias celebensisas a social signal for intraspecific competition

In some species, such as chameleon and cichlid fish, rapid body colouration changes (within seconds or minutes) serve as visual social signals in male-male competition. This study investigated the relationship between aggressive behaviour and body colouration changes in Oryzias celebensis, an Indonesian medaka fish. We analysed aggressive behaviours and corresponding body colouration changes during attack events in a controlled laboratory setting using groups of 3 adult fish in a small tank. In a triadic relationship consisting of 2 males and a female, males with blackened markings attacked more frequently than males without blackened markings and females. Additionally, we observed that the males with blackened markings were seldom attacked by males without blackened markings and females. These tendencies persisted even in groups consisting of 3 males. Our results suggest that the blackened markings in male O. celebensis not only indicate the level of aggression but also serve as a social signal to suppress attacks by other individuals.