Maternity colony social structure of myotis in British Columbia, Canada

Female myotis bats may select artificial structures, such as bat boxes and buildings, to form their maternity colonies, and may form preferential associations with other bats based on age or reproductive status. We studied two colonies of little brown myotis (Myotis lucifugus) and Yuma bats (M. yumanesis) in artificial structures near Vancouver, British Columbia, Canada, to assess changes in social organization based on reproductive period. During summer 2019, we used Passive Integrated Technology (PIT-tags and scanners) to track individuals' daily roost selection and social organization, then used social network analysis to compare temporal networks representing critical reproductive periods. Yuma myotis and little brown myotis formed strong, close associations with other bats during pregnancy and lactation but colony organization fragmented during post-lactation as adult and juveniles began roosting independently. Bats segregated by species during post-lactation, whereas they had previously roosted with heterospecifics during pregnancy and lactation. Little brown myotis left the maternity site in August, while Yuma myotis persisted until September and mixed with conspecifics from nearby maternity colonies in a previously unoccupied roost. Our results show that myotis maternity colony organization is not static and reflects myriad temporal, reproductive, and species-specific influences. Significance statement Bats often form associations with other bats, thus driving colony organization and roost use. We studied the social structure of two maternity colonies in the lower mainland of British Columbia, Canada. The colonies contained little brown myotis (Myotis lucifugus) and Yuma myotis (M. yumanensis), which roosted in buildings and bat boxes that enabled larger maternity colonies than may occur in wild habitats. We compared social networks of PIT-tagged adult female bats during critical reproductive periods to monitor how bat behavior changed. Bats formed strong associations with colony members during pregnancy and lactation with very little ingress or egress of individuals from the colony. During post-lactation, however, overall colony organization weakened as bats moved to other roosts and many new bats began roosting with the colony. Little brown myotis left maternity sites first while Yuma myotis mixed with other conspecifics from nearby maternity colonies. Thus, colony organization in little brown myotis and Yuma myotis is not static and bats observed during post-lactation are often not the same animals observed earlier in the summer Additionally, post-lactation mixing between Yuma myotis from different colonies suggests that artificial roosts may provide valuable habitat after the young are volant. Our results show that social organization at maternity colonies is a dynamic reflection of reproductive and species-specific influences.