Sex composition modulates the effects of familiarity in new environment.

In many social species, when an individual is associated with familiar conspecifics, it displays an array of behaviours that may confer benefits (e.g., increased boldness and faster habituation to novel environments). In fish, these effects of familiarity have been studied using individuals of only one sex or juveniles. Since shoals often vary regarding sex composition and males and females show different social behaviours, we hypothesised that social familiarity's effects vary with group sex composition. We tested this hypothesis by comparing the exploratory behaviour of groups of two females, two males, and one male and one female Mediterranean killifish, Aphanius fasciatus, which were either familiar or unfamiliar. Pairs of familiar females were bolder than pairs of unfamiliar females, whereas males showed the opposite trend. Pairs of familiar females also showed faster habituation to the novel environment and, at the beginning of the test, were more cohesive compared to pairs of unfamiliar females. Pairs of familiar mixed-sex fish habituated faster to the novel environment than unfamiliar pairs. Pairs of familiar males did not show any beneficial effect of familiarity relative to pairs of unfamiliar males. Hence, the effects of social familiarity on exploratory behaviour, and likely the associated benefits, appear to depend on the sex composition of the pair in the Mediterranean killifish.