Group-size preferences in a shoaling cichlid

Abstract

Abstract Group size is an important criterion in social decisions. Accordingly, assessing quantities is common in many animals. In fishes, studies on numerical abilities focus on a limited range of species. Arguably, cichlids show the greatest variability of social patterns among vertebrates. Nevertheless, knowledge about their quantitative abilities is scarce. Here we use the Lake Tanganyika cichlid Lamprologus callipterus to scrutinize the quantitative abilities of fish in the context of shoaling. Females chose between different numbers of conspecifics, varying in absolute and relative number differences. In half of the trials both shoals were composed of familiar sisters, while all fish were unfamiliar non-kin in the other half. Test fish consistently preferred the larger of two shoals, irrespective of the ratios. Their activity differed significantly between familiarity/relatedness treatments, indicating recognition of this parameter. L. callipterus therefore has fine-tuned discrimination skills, adding to the evidence that quantitative abilities are widespread in fishes.