Generalization of predators and nonpredators by juvenile rainbow trout: learning what is and is not a threat

Learned recognition of novel predators allows prey to respond to ecologically relevant threats. Prey could minimize the costs associated with learning the identity of both predators and nonpredators by making educated guesses on the identity of a novel species based on their similarities with known predators and nonpredators, a process known as generalization. Here, we tested whether juvenile rainbow trout, Oncorhynchus mykiss, have the ability to generalize information from a known predator (experiment 1) or a known harmless species (experiment 2) to closely related but novel species. In experiment 1, we taught juvenile trout to recognize a predatory pumpkinseed sunfish, Lepomis gibbosus, by pairing pumpkinseed odour with conspecific alarm cues or a distilled water control. We then tested the trout for a response to pumpkinseeds and to novel longear sunfish, Lepomis megalotis (same genus as pumpkinseed), rock bass, Ambloplites rupestris (same family as pumpkinseed) or yellow perch, Perca flavescens (different family). Trout showed strong learned recognition of pumpkinseed and longear sunfish odour and a weak learned response to rock bass odour but no recognition of yellow perch. In experiment 2, we used latent inhibition to teach juvenile trout that pumpkinseeds were harmless. During subsequent predator learning trials, trout did not learn to recognize pumpkinseed or longear sunfish odour as potential threats, but they did learn that rock bass and yellow perch were threatening. Taken together, these results demonstrate that juvenile rainbow trout can generalize learned recognition of both predator and nonpredator odours based on the phylogenetic relatedness of predators.