Arginine vasotocin (AVT) and its mammalian homologoue arginine vasopressin (AVP) 
influence male sexual and aggressive behaviors in many species. We tested the 
effects of AVT and an AVP-V(1a) receptor antagonist on the display of 
alternative male tactics in a tropical coral reef fish, the bluehead wrasse 
Thalassoma bifasciatum. We gave AVT injections to territorial and nonterritorial 
males of the large and colorful phenotype (terminal phase) and an AVP-V(1a) 
receptor antagonist, Manning compound, to territorial males in the field. AVT 
increased courtship independent of status, while its effects on territoriality 
and aggression were dependent upon male status. In territorial males, AVT 
increased courtship and tended to decrease the number of chases toward initial 
phase individuals. In nonterritorial males, AVT increased courtship, chases 
toward initial phase individuals, and territorial behavior while decreasing 
feeding. These are all behaviors rarely seen in nonterritorial males, so AVT 
made these males act like territorial TP males. The AVP-V(1a) receptor 
antagonist had opposite effects. It decreased courtship and territorial defense, 
making these males act more like nonterritorial males. Manipulations of the AVT 
system shifted males within a single phenotype from the nonterritorial social 
status to the territorial social status and vice versa. Since the entire suite 
of behaviors related to territoriality was affected by AVT system manipulations, 
our results suggest that the AVT system may play a key role in motivation of 
behaviors related to mating.