Opponent familiarity influences the audience effect in male–male interactions in Siamese fighting fish

It is well established that communication in social organisms often occurs within a network of individuals rather than within signaller–receiver dyads. Most of these investigations, however, have not assessed how other social factors might influence communication. In the present study, we investigated whether previous exposure to an opponent influences audience effects on male–male interactions in Siamese fighting fish, Betta splendens. Males interacted with an unfamiliar opponent or a familiar opponent in the presence of a male, a female or no audience. Opponent-directed gill flaring was greatest when opponents were unfamiliar with one another and a male audience was present. In contrast, opponent-directed tail beats were most frequent when a female audience was present, especially when opponents were unfamiliar. Opponent familiarity appears to have a greater effect on opponent-directed gill flaring than on tail beats. This may reflect the difference in the function of these behaviours as tail beats are commonly used in courtship as well as during aggressive encounters. Our findings suggest that audience type and the social history of the opponents work together to influence aggressive interactions in this species.