Male sexual harassment and female schooling behaviour in the eastern mosquitofish

Among poeciliid fish, male sexual harassment is often intense and is costly for females. In Gambusia holbrooki, sexual harassment can greatly reduce female foraging efficiency when an isolated female is harassed by a single male and these costs are negatively correlated with male length. However, when females are in groups, male harassment is diluted and female foraging efficiency increases. When several males compete for the same female, mating attempts are monopolized by the dominant male and female foraging efficiency also increases. We tested whether females actively vary their schooling behaviour with conspecifics according to the presence of a harassing male. Consistent with the predictions, we found that females swam closer to each other when a male was visible. When chased by a male, females approached a group of males, and when males of different size were available, they preferred to stay close to large males. These results suggest that female schooling behaviour is a flexible strategy and that male sexual harassment may represent an important factor influencing social aggregation in poeciliids.