Sex and schooling behaviour in the Trinidadian guppy

We tested the hypothesis that sexual asymmetry in mating costs affects choice of schooling partner in fish. Female guppies, Poecilia reticulata, from the Tacarigua River, Trinidad, associated preferentially with other (familiar) females from their natural wild school, while males did not show such a preference. This implies that wild guppy schools are not random assemblages of conspecifics. Females form the core of natural schools while males seem to trade off the potential advantages of schooling with familiar conspecifics against increased mobility in search of mating opportunities. We discuss the implications of these findings in relation to cooperative behaviour, gene flow and population differentiation.