Two competing activities of temperate insectivorous bats during the fall swarming period have direct fitness consequences: fat storage for hibernation and mating. This study investigated whether interindividual variation in body condition (as a metric of stored fat; body mass/forearm length) correlated with reproductive status and influenced swarming behaviour of adult male little brown myotis (Myotis lucifugus (Le Conte, 1831)) in Nova Scotia, Canada. We predicted that bats in good body condition would more likely be reproductive and would be more likely to remain at, and closer to, a swarming site than males in poor body condition. As predicted, males in good body condition were more likely to be in advanced reproductive states than those in poor body condition. However, contrary to the prediction, males in good body condition spent significantly less time at the swarming site than males with poor body condition. There was no difference between bats of contrasting body conditions in the probability of relocating them or how far from the swarming site they roosted. Because variation in swarming behaviours of male M. lucifugus at a swarming site was not explained by body condition, one or more other factors (e.g., social, energetic) must be important.
