Communally breeding bats use physiological and behavioural adjustments to optimise daily energy expenditure

Causes and consequences of living in closed societies: lessons from a long-term socio-genetic study on Bechstein's bats

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 synthesizing the results of this long-term study the diversity of tools that can be applied to hypothesize 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.

Bats are able to maintain long-term social relationships despite the high fission-fusion dynamics of their groups

Elephants, dolphins, as well as some carnivores and primates maintain social links despite their frequent splitting and merging in groups of variable composition, a phenomenon known as fission-fusion. Information on the dynamics of social links and interactions among individuals is of high importance to the understanding of the evolution of animal sociality, including that of humans. However, detailed long-term data on such dynamics in wild mammals with fully known demography and kin structures are scarce. Applying a weighted network analysis on 20,500 individual roosting observations over 5 years, we show that in two wild Bechstein's bat colonies with high fission-fusion dynamics, individuals of different age, size, reproductive status and relatedness maintain long-term social relationships. In the larger colony, we detected two stable subunits, each comprising bats from several family lineages. Links between these subunits were mainly maintained by older bats and persisted over all years. Moreover, we show that the full details of the social structure become apparent only when large datasets are used. The stable multi-level social structures in Bechstein's bat colonies resemble that of elephants, dolphins and some primates. Our findings thus may shed new light on the link between social complexity and social cognition in mammals.

Group decision-making in fission-fusion societies

The prevalent view of group splitting during group decisions is that a beneficial consensus has not been reached because time constraints, different individual information, or inter-individual conflicts lead to fission instead of a compromise. However, societies with high fission-fusion dynamics may allow their members to avoid consensus decisions that are not in their favour without foregoing grouping benefits that arise from collective behaviour. Moreover, by forming temporary subgroups that represent individual preferences better than the group as a whole fission-fusion societies could avoid a permanent break up even in situations where conflicts among their members are to strong to reach a consensus.