Individual vs. social learning of predator information in fish: does group size affect learning efficacy?

Group-living prey have the opportunity to acquire information about predation risk through individual and social experience. Here, we tested the effect of group size on the efficacy of predator recognition learning via injured conspecific cues or social learning, using fathead minnows (Pimephales promelas) as our test species. First, minnows from three group sizes (one, two or four fish) were conditioned to recognize a novel rainbow trout (Oncorhynchus mykiss) as risky through a single pairing of conspecific alarm cues with trout odour. They were then individually tested for the recognition of trout odour alone. No effect of group size was found on the learned response to trout odour, suggesting that group size does not interfere with the individual assessment of risk via alarm cues. During the conditioning, however, singletons displayed a stronger antipredator response compared to fish in groups, suggesting a context-dependent modulation of their antipredator response intensity due to a dilution effect. Fish that had the opportunity to learn to recognize a trout socially from either one or three experienced conspecifics also responded with the same intensity of antipredator response to the trout odour when tested alone. We hypothesize that the dilution of risk due to the increase of group size balanced the increase of tutor-to-observer ratio when observers directly interacted with tutor(s). This study highlights the potentially conflicting information that may be perceived by social individuals during both individual and social predation learning.