The nonapeptides arginine vasopressin (AVP; including its non-mammalian homolog 
arginine vasotocin, AVT) and oxytocin (OT; including its non-mammalian homologs 
mesotocin, MT, and isotocin, IT) regulate social behavior, including aggression 
and reproduction, via receptors conserved across vertebrates. In monogamous 
prairie voles, the vasopressin and oxytocin pathways are crucially important for 
pair-bond formation, specifically by influencing affiliative behavior toward the 
mate and aggression toward non-mates. Monogamous social systems are found in 
diverse taxa. We hypothesized that the AVT/IT pathways are associated with 
mating behavior in monogamous teleost fishes. We used the monogamous convict 
cichlid, Amatitlania nigrofasciata, to test this idea. In the first experiment, 
we treated males with a general nonapeptide receptor antagonist during pair-bond 
formation. Control males were treated with vehicle. On the first day of 
treatment we observed a significant reduction in both affiliative behavior 
toward the potential mate and aggression toward neighbors. However, the 
antagonist did not prevent the pair-bond from forming and the behavioral effects 
disappeared on subsequent treatment days. In the second experiment, we 
administered on three consecutive days the AVP/OT receptor antagonist to males 
that were in an established pair-bond. In established pairs, male affiliation 
towards the mate and aggressive behavior towards territorial neighbors were not 
affected by the antagonist. Our results indicate that the basic social behaviors 
typically mediated by the AVP/OT pathways may provide the building blocks 
necessary for monogamous mating behavior.