Overriding the oddity effect in mixed-species aggregations: group choice by armored and nonarmored prey

Because "odd" individuals often suffer disproportionately high rates of predation, solitary individuals should join groups whose members are most similar to themselves in appearance. We examined group-choice decisions by individuals in armored and nonarmored species and predicted that either (1) the oddity effect would result in preference for conspecific groups for solitary individuals of both species, or (2) individuals in the armored species would prefer to associate with groups containing individuals of the more vulnerable species. Armored brook sticklebacks (Culaea inconstans) and nonarmored fathead minnows (Pimephales promelas) have the same predators and often occur together in streams. In mixed-species shoals, yellow perch (Perca flavescens) attacked minnows earlier and more often than sticklebacks. We tested whether solitary minnows and sticklebacks preferred to associate with conspecific or heterospecific shoals under conditions of both low and high predation risk. When predation risk was high, minnows preferred to associate with conspecifics over heterospecifics, as predicted by the oddity effect. In contrast, sticklebacks preferentially associated with groups of minnows over groups of conspecifics when predation risk was high. When predation risk was low, solitary individuals of both species preferentially associated with conspecific over heterospecific shoals. Stickleback shoal choices under low-risk conditions may have been influenced by interspecific competition for food. In feeding experiments, minnows were more efficient foragers than sticklebacks, so it should benefit sticklebacks to avoid minnows unless predation risk is high. Therefore, for armored prey, the benefits of associating with more vulnerable prey appear to override the costs of both the oddity effect and food competition when predation risk is high. Copyright 2003.