Who swarms with whom? Group dynamics of Myotis bats during autumn swarming

For many animal taxa, group-living is a strategy where the cohesion provided by groups confers fitness benefits to individuals. Bats are highly gregarious with many species living in groups with complex social structures. During the summer, many temperate species are sexually segregated among roosts where females have been found to exhibit dynamic social structures and males remain understudied. We studied the group dynamics of little brown and northern Myotis bats (Myotis lucifugus and Myotis septentrionalis) during autumn swarming, a period for which social interactions are largely unknown. Using capture–mark–recapture surveys, we characterized the occurrence and frequency of age and sex groups occurring at swarms. Within a night, young-of-the-year associated more often with other bats than did adult males and females. Further, they associated more often with other young-of-the-year than adults. We found no evidence to support the maternal guidance hypothesis predicting that there would be associations between mother–offspring pairs. Adult male and female bats associated less frequently with each other and were captured alone most often. When males were captured in groups, these groups were more likely to be composed of multiple males and in M. lucifugus, males had preferred male associates they grouped with over multiple nights. Groups formed during the autumn swarming season may represent cohort groups of young bats learning of the location of sites and groups of males that are potentially cooperating to secure more mating opportunities.