Hormone levels increase in response to video playback in the Siamese fighting fish

The mechanisms of action of hormones regulating aggression in fish remain poorly understood. One possibly confounding variable is the lack of standardization in the type of stimuli used to elicit aggression. The presentation of controlled stimuli in videos, a.k.a. video playback, can provide better control of the fight components. However, this technique has produced conflicting results in animal behaviour studies and needs to be carefully validated. Here, the response of male Betta splendens to a matched for size conspecific fighting behind a one-way mirror presented live or through video playback was compared. Despite their non-interactive nature, both stimuli elicited a strong aggressive response by focal fish, which started with stereotypical threat displays and progressed to overt attacks. Overall, the frequency and duration of aggressive behaviours and swimming activity were similar towards live and video stimuli. Post-fight plasma levels of the androgen 11-ketotestosterone were elevated as compared to controls, regardless of the type of stimuli. Cortisol also increased in response to the video images, as previously described for interactive live fights in this species. The study shows for the first time in a fish a robust endocrine response to video stimuli and supports the use of this technique for researching aggressive behaviour in B. splendens .