The serotine bat (Eptesicus serotinus) is common throughout much of continental Europe. In southern Spain up to 30% of the individuals occupaying maternity colonies my be non-breeding females, which is an exceptionally high percentage. This high percentage of non-breeding females in colonies provides a unique resource to attempt to understand the underlying cause of this behaviour. The purpose of the present study is to measure the energy demands of breeding and non-breeding females in colonies; and of non-breeding females when solitary to compare their energy demands and thus assess whether non-breeding do indeed pay an energy penalty for joining maternity colonies, and also to assess th eimpact of colony size on the energy demands of the individuals in the colony. These measurements will be made throughout the summer breeding season using the doubly-labelles water technique, in which my group has a particular expertise. This project will enhance our understanding of the factors underpinning the social structuring of bat populations in Andalucía and more generally