Changes in body mass and fat reserves in pre-hibernating little brown bats (Myotis lucifugus)

Changes in body mass, fat mass, lean dry mass, and energy content of little brown bats, Myotis lucifugus (LeConte), captured at a cave in southern Vermont, were quantified during the pre-hibernation period in late summer and autumn. Adults of both sexes showed maximum rates of increase in body mass from mid-August to mid-September, during which time the average gain was 2.3 g for males and 2.1 g for females. These gains represent 32.9% and 29.6% of the pre-hibernation body masses for adult males and females, respectively. Young-of-the-year of both sexes weighed about 1 to 2 g less than adults during most of the pre-hibernation period. In mid-September, adult females weighed significantly less than a cohort of adult females captured at a maternity roost on the same date. Adult bats reached their maximum pre-hibernating body mass in mid-September, whereas young bats reached their maximum pre-hibernating body mass one month later. From mid-July to mid-September, we found no significant differences in mean body mass between young males and females, but in mid-October when they entered hibernation, young females weighed significantly more than young males and almost as much as adult females. Young males and females arrived at the swarming-hibernation site in late summer with an average lean body mass and fat index approximately 20% lower than adults. By early October, young females achieved minimum adult levels of lean dry mass, but by the time they entered hibernation the lean mass of young males was still about 10% lower than adults. During the pre-hibernation period, lean dry mass and fat mass of all bats were significantly correlated with body mass. Regression equations derived from these data were used to estimate total energy content of bats. The acquisition of maximum fat reserves by M. lucifugus in the pre-hibernation period may be as important for successful reproduction as it is for sustaining hibernation. The fattest adult males may gain a reproductive advantage if they acquire enough energy reserves to sustain autumn mating and hibernation, and engage in multiple matings during the winter. The fattest females should gain a reproductive advantage by maximizing fat reserves before entering hibernation, and retaining sufficient energy reserves at the end of hibernation to facilitate ovulation. Relatively low survival and reduced fecundity in females at northern latitudes may reflect the relatively low fat reserves deposited by young females in their first autumn.