Not always positive: tactile body stimulation does not lower aggressiveness and increases stress in the Siamese fighting fish

Artificial body tactile stimulation (TS) has been shown to improve fish welfare by reducing aggressive behavior and stress, thereby serving as a sensory enrichment. Here, we tested the effect of TS on aggressive behavior and stress response in the Siamese fighting fish (Betta splendens), an ornamental species reared worldwide and renowned for its high aggressiveness. Twenty isolated male fish were exposed either to TS (provided by a frame with vertical plastic sticks lined with silicone bristles, inserted in the middle of the tank) or to a control treatment (sticks without bristles) for 22 days. Individual aggressive behavior was evaluated before and after the treatment period using mirror tests and staged pair fights. We measured cortisol, brain serotonin (5-HT), and dopamine (DA) to infer stress and possible associations with aggressiveness and the reward system. Although fish actively interacted with the bristles, TS did not alter overt physical fighting but significantly increased frontal and lateral displays. Furthermore, TS elevated cortisol levels, decreased 5-HT, and increased the 5-HIAA/5-HT turnover ratio. DA levels remained unchanged, indicating no activation of neural reward pathways. Contrary to our expectations, TS did not mitigate aggressiveness in this species but rather seemed to prompt an increase in both behavioral and physiological reactivity (cortisol levels and 5-HT activity) to fighting. Our findings indicate that artificial selection for high aggression in B. splendens may alter sensory processing to touch, causing physical contact to act as a stressor rather than an enrichment.