Fish Recognize and Prefer to Shoal with Poor Competitors

Neil B. Metcalfe, Bruce C. Thomson

Abstract

All animals are never equal; even in apparently uniform assemblages such as fish schools, some individuals will be consistently better at acquiring resources or at minimizing their risk of being predated upon, than others. Although many species benefit from foraging in groups, the net pay-off to the individual of joining a group will clearly depend on its composition and foragers should, therefore, be choosy as to which group they join. Here we show for the first time that fish (European minnows) can discriminate between shoals composed of good and of poor competitors. They show a distinct preference for shoaling with fish of low competitive ability, even in the absence of obvious cues such as differences in aggressiveness, size or instantaneous feeding rate. This remarkable ability may explain the paradox of animals rapidly distribution themselves between foraging groups according to the predictions of the Ideal Free Distribution despite universal violation of its key assumption of equal competitive abilities.