Oxytocin antagonist treatments alter the formation of pair relationships in zebra finches of both sexes.

Oxytocin and vasopressin are known to be important in affiliative behaviors. Although these peptides have been shown to be involved in monogamous pairing behavior in a few mammalian species, their role across monogamous species is not well understood. In particular, monogamy is most common in birds, yet the role of mesotocin and vasotocin (avian homologues of oxytocin and vasopressin) in pair relationships has not been established in any avian species. The goal of the present study was to investigate the effects of an oxytocin antagonist on pairing and pairing-related behaviors in the monogamous zebra finch. To accomplish this, we systemically administered one of three doses of an oxytocin antagonist (1 μg, 5 μg, or 10 μg) or a vehicle to adult male and female zebra finches (in separate experiments) with no prior pairing experience. Subjects were observed over three days and allowed to choose mates. We found that oxytocin antagonists increased the latency to pair and decreased pair formation in both sexes. The effects of these treatments on overall pairing behaviors were more pronounced in females than in males, suggesting sexually differentiated effects on motivation to contact conspecifics. Treatments also reduced courtship, as measured by directed singing, in males. These results suggest that nonapeptides play a key role in pair formation in zebra finches of both sexes, similar to findings in other monogamous species.