Fiddler crabs are generally considered polygamous as they tend to live in dense mixed sex colonies with numerous neighbours and individually defended territories. We show that the Australian fiddler crab, Uca capricornis, is socially monogamous based on behavioural experiments and observations of neighbouring males and females. The unusual relationship between neighbouring males and females in U. capricornis is selected for and maintained by intrasexual aggression and the ability to recognise and defend their partner.