Inhibition of V1a-like receptors disrupts male-male affiliative relationships formed by triadic cohabitation in large-billed crows

Affiliative relationships, such as parent-offspring bonds, male-female pair-bonds, and within-sex coalitions/alliances, are beneficial for group-living animals to increase fitness and reproductive success. Male-male affiliative relationship, such as coalitions/alliances, have been reported in social mammals and birds in situations where three or more individuals interact, suggesting that male-male affiliative relationships can be formed via interactive situations involving three (or more) individuals. However, no experimental studies have tested this hypothesis on social animals. Many studies on mammals and birds have revealed that an affiliative relationship is underpinned by the oxytocin (OT)/vasopressin (VP) system. However, the role of the OT/VP system has been studied only in parent-offspring bonds and pair-bonds and is lacking in male-male affiliative relationships. In this study, we found the 2-week cohabitation of three male crows in an aviary facilitated affiliative relationship formation between two of the three crows (Experiment 1). In Experiment 2, we examined the effects of peripheral administration of vasopressin 1a receptor (V1aR) antagonist peripheral administration on the social behaviour of affiliated males. We found that the V1aR antagonist caused the disappearance of affiliative behaviour and the resurgence of agonistic behaviour in affiliated males. Our findings provide the first evidence for V1aR involvement in male-male affiliative relationships in vertebrates.