The significance of winter activity by the big brown bat (Eptesicus fuscus): the influence of energy reserves

Abstract

I tested the hypothesis that big brown bats (Eptesicus fuscus) found active during winter months may be forced into activity because of reduced energy reserves. I compared the masses and fat reserves of big brown bats found active with inactive (torpid) bats at a hibernaculum, throughout the winter. Active bats were significantly lighter that those in the hibernaculum during the same period. Active bats had significantly lower fat reserves and lean dry mass than inactive individuals. The majority of bats found active were lighter than the predicted mass of bats leaving hibernation in April. The forearm length of active individuals was significantly shorter than inactive bats. Inactive individuals lost mass throughout the winter while there was no significant correlation between the mass of active bats and the date of capture. The data are consistent with the hypothesis that bats may become active at a critically low energy reserve, possibly to find food.