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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. 

Sociality has evolved on numerous occasions, with important tness bene ts and consequences for individuals in group-living species. Despite the prevalence of group living, it remains challenging to study the proximate factors that promote variation in this behaviour. Meadow voles, Microtus pennsylvanicus, vary in social organization in response to seasonal cues in the wild, providing a remarkable opportunity to study mechanisms that support changing grouping behaviour within a single species. In summer, female meadow voles are territorial and maintain exclusive home ranges, while in winter, they live in mixed-sex groups. Laboratory manipulations of photoperiod reproduce seasonal variation in social behaviour in this species, demonstrated by altered short-term dyadic interactions during behavioural tests. To assess changes in group social dynamics, we implemented an automated tracking system using radiofrequency identi cation technology to monitor free-moving voles housed in long daylengths and short daylengths in multichambered habitats over extended intervals. We developed open-source software to model each animal's position within the habitat and to describe co-occupancy patterns. Photoperiod drove robust and pronounced changes in group social behaviour, recapitulating natural seasonal transitions in the wild. Voles housed in short, winter-like days spent more time in groups, formed larger, more evenly mixed groups and shared a single home-chamber more often than long-day voles. Short-day voles were more active than long-day voles but rested in groups, thus cohabitation was strongly correlated with ultradian rhythms in activity. These metrics of group dynamics will allow more nuanced assessment of the effects of manipulations on social behaviour in voles and other species, as both habitat and software are adaptable for use with other rodents.