Oxytocin and vasopressin are neuropeptides involved in a wide variety of affiliative behaviors across vertebrates. Their involvement in faciliting monogamous pair bonds in mammals has been well established and recent work on zebra finches has shown an important role for these hormones in avian pair bonds as well. However, studies on the role of hormones in maintaining pair bonds after initial bonds have formed are sorely lacking. Indeed, the role of mesotocin and vasotocin (avian homologues of oxytocin and vasopressin) may play in maintaining 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 pair maintenance behaviors in the monogamous zebra finch.
We measured behavior associated with pair bond maintenance in zebra finches, including allopreening, clumping, male singing, and copulation. We also measured aggression, as mesotocin has been linked to certain types of aggression in social animals. Zebra finch pairs that had successfully raised offspring were considered experienced partners and used for this study. Eight pairs of birds (8 males, 8 females) were injected with 5ug of a mesotocin blocker while eight pairs of control birds (8 males, 8 females) were injected with saline. The birds were separated for 1 hour and then placed in a reunion cage for 30 minutes, where their behavior was video taped. Birds were observed on the first day after no injection to establish baseline levels of behavior and then injected for three days in a row (Day 1, 2, 3). We tested for significant differences in the time and incidence of behaviors for each day vs. baseline values using Kruskal tests with treatment (hormone blocker vs. saline injection) as a predictor variable.
We found no difference between control and hormone-blocker treatments in the amount of time spent allopreening or clumping for either sex (copulation occured to infrequently to compare). However, males in the hormone blocker treatment sang less to females, while females in the hormone blocker treatment had a reduction in aggressive bouts against males. These results suggest that mesotocin/vasotocin may play a role in regulating some pair maintenance behaviors but play no role in others. These results demonstrate that hormones continue to regulate some, but not all, pair-bonding behaviors in experienced zebra finches.