Learned recognition of heterospecific alarm signals: The importance of a mixed predator diet

A wide diversity of aquatic organisms release alarm signals upon being attacked by a predator. Alarm signals may 'warn' nearby individuals of danger. Moreover., the signals may be important in facilitating learned recognition of unknown stimuli. It is common for different prey species to respond to each other's chemical alarm signals. In many cases, the responses are learned but no learning mechanisms have been identified to date. In this study we tested whether prey fish can learn the identity of an unknown alarm signal when they detect it in association with conspecific alarm cues in the diet of a predator. Chemical alarm cues are known to be conserved in the diet of predators. We conditioned fathead minnows (Pimephales promelas) with chemical stimuli from predatory yellow perch (Perca flavescens) fed a mixed diet of minnows and brook stickleback (Culaea inconstans), perch fed a mixed diet of swordtails (Xiphophorus helleri) and stickleback or distilled water. Minnows were subsequently exposed to chemical alarm cues of injured stickleback alone. Those minnows previously conditioned with perch fed a mixed diet of minnows and stickleback increased their use of shelter and -froze' significantly more than minnows previously conditioned with perch fed a diet of swordtails and stickleback or those exposed to distilled water. These data demonstrate a mechanism by which minnows can learn the identity of a heterospecific alarm signal.