The ultimate functions of sociality, or the tendency to associate with conspecifics and to live within a social group, are increasingly well understood. However, the proximate mechanisms that mediate this behaviour have received less attention. The oxytocin family of nonapeptide hormones (including isotocin in teleost fish) is thought to play an important role in regulating social behaviour across a wide range of taxa and social contexts. In the current study, we investigated the influence of exogenous administration of isotocin and an oxytocin receptor antagonist on sociality in a cooperatively breeding fish,
Neolamprologus pulcher. In our first experiment, we found that a high (and a low) dose of peripherally administered exogenous isotocin decreased the time spent associating with conspecifics in N. pulcher, while an intermediate dose had no effect relative to control. In our second experiment, we found that a peripheral administration of an oxytocin receptor antagonist increased grouping preference in male N. pulcher. The results of both experiments suggest that IT may inhibit grouping behaviour in this species. These results contribute to a growing body of literature suggesting that the broad generalization that the oxytocin family of nonapeptides facilitate grouping behaviour is overly simplistic, and that specific behavioural effects depend the study species and testing conditions.