Declines in Oxytocin Receptor Density and Social Behavior Across a Dispersal-Like Transition in Solitary Hamsters.

Mammals are born into social groups: even species that become solitary begin life seeking social contact with family members. For solitary mammals, dispersal thus marks a major geographic and social transition from their natal group. This transition may be promoted by reduced social tolerance for and reduced interest in family members, and/or by unrelated factors such as increased exploration and activity. Dispersal may also coincide with other developmental events such as weaning or puberty. We investigated developmental changes in oxytocin receptor density in two solitary hamster species (Syrian hamsters: Mesocricetus auratus and Siberian hamsters: Phodopus sungorus) that disperse to individual burrows in the wild. We quantified oxytocin receptor density prior to and after separation from the natal group to determine whether and how neurobiological changes coincide with changes in social behavior. We also quantified transitions in social behavior across development in Syrian hamsters at 2.5, 4, and 8 weeks. Oxytocin receptor densities and distributions reorganized substantially from pre- to post-dispersal ages in both species. Binding decreased across brain regions, with declines in binding in the endopiriform nucleus of both species, and the greatest reduction in hippocampal CA2 of Syrian hamsters. All metrics of social interest and interaction declined across the 2.5-8 week interval-consistent with transition to a solitary lifestyle-except play behavior which peaked in the characteristic juvenile range. Developmental decline in oxytocin receptor density and oxytocin signaling may support transitions in social behavior in solitary mammals.