Body tactile stimulation intensifies appeasement behavior in a cichlid fish.

Social touch, provided by body tactile stimulation (TS), is known to trigger appeasement behavior and social stability in mammals. Recent studies have shown that TS also plays a role in regulating social interactions in fish by reducing aggression between individuals. While this effect may stem from decreased aggression in dominants, the potential contribution of increased appeasement behavior in subordinates has been overlooked. Here, we investigated whether TS facilitates the onset of appeasement behavior in the cichlid fish Geophagus iporangensis, a species that exhibits clear dominance-subordination relationships. Isolated individuals were assigned to one of two treatments, either with TS (provided by plastic sticks lined with silicone bristles) or without it for 21 days. Then, each fish was paired with a socially experienced dominant individual to enable rapid hierarchy resolution. Initial appeasement scores did not differ between treatments. However, individuals who experienced TS exhibited significantly higher appeasement scores during the final 5 min of the contest than the control group. Additionally, fish under TS showed an increase in the appeasement score after 10 min of agonistic interaction, while it did not change in the treatment without TS. Overall, this study provides the first evidence that TS can mitigate aggressive interactions by intensifying subordinates' appeasement behavior, thereby affecting social dynamics similarly to mammals.