Siamese fighting fish, Betta splendens, have been extensively studied due to their aggression and stereotypical displays. Many studies have focused on their characteristic opercular flaring, while the less aggressive and less energetically costly lateral display have been comparatively understudied. Many factors have been shown to influence aggression in Bettas, notably body length and the personality trait of boldness, however, the role that colour plays in determining an individuals aggressiveness is much less clear. The role of colour has only been briefly studied, and based on human interpretations of colour, i.e. limited to what the receivers eyes and sensory systems actually can process and discriminate, with results suggesting blue males are more aggressive than red males. Using male-male interactions, measuring opercular flaring and lateral display we found that colour and personality do play a role in determining the degree of aggressiveness in Betta splendens. Blue-finned males were more aggressive, performing longer lateral displays more frequently. Blue fins are a phenotype observed in wild type males and is likely selected for to allow visual cues to travel through the murky water they inhabit. Body mass was positively correlated with lateral display frequency, and opercular flare frequency and duration. Finally, neophobic individuals, individuals that were less willing to approach a novel object, were more aggressive, performing significantly more lateral displays. This indicates that personality may impact fighting strategy, with males either choosing to end conflicts quickly with more aggressive displays or to outlast their opponent with less energetically costly displays.### Competing Interest StatementThe authors have declared no competing interest.

The determinants of aggression in male Siamese fighting fish, Betta splendens