Understanding the ecological, behavioural and genetic factors influencing animal social systems is crucial to investigating the evolution of sociality. Despite the recent advances in population genetic methods and the analysis of social interactions, long-term studies exploring the causes and consequences of social systems in wild mammals are rare. Here, we provide a synthesis of 15 years of data on the Bechstein’s bat (Myotis bechsteinii), a species that raises its young in closed societies of 10-45 females living together for their entire lives and where immigration is virtually absent. We discuss the potential causes and consequences of living in closed societies, based on the available data on Bechstein’s bat and other species with similar social systems. Using a combination of observational and genetic data on the bats together with genetic data on an ecto-parasite, we suggest that closed societies in Bechstein’s bats are likely caused by a combination of benefits from cooperation with familiar colony members and parasite pressure. Consequences of this peculiar social system include increased sensitivity to demographic fluctuations and limits to dispersal during colony foundation, which have broad implications for conservation. We also hope to illustrate by synthesising the results of this long-term study the diversity of tools that can be applied to hypothesise about the factors influencing a species’ social system. We are convinced that with the expansion of the number of social mammals for which comparably detailed socio-genetic long-term data are available, future comparative studies will provide deeper insights into the evolution of closed societies.