Refueling while flying : foraging bats combust food rapidly and directly to power flight

Abstract

Two quickly accessible energy sources are available for exercising animals: carbohydrates, such as glycogen, and lipids. Mammals combust mostly glycogen during increased aerobic muscle work. When glycogen stores have been depleted, only the fraction of metabolism which is fuelled by lipid combustion can be maintained, i.e. the animal “hits the wall”.2. Flying bats perform endurance aerobic work several levels higher than locomotion on the ground. However, bats, such as the insectivorous Noctilio albiventris, have few glycogen stores when they start to hunt at sunset. We hypothesized that fasting bats fuel most of their metabolism with lipids, even during energy demanding flight.3. Since fat stores are depleted in 13C in relation to the diet we expected exhaled breath of fasting bats emerging from the roost to be depleted in 13C in relation to the diet and also in relation to conspecifics that had fed.4. Mean δ13Cbreath of emerging fasting bats was indeed significantly depleted in 13C in relation to δ13Cdiet. In contrast, 13Cbreath of foraging bats was similar to δ13Cdiet.5. Our data suggests that fasting bats combusted mostly lipids, whereas satiated animals combusted insects nutrients when flying. In contrast to mammals with terrestrial locomotion, bats may fuel endurance aerobic work mostly through lipid and not through glycogen combustion when no food is available and thus may not hit the wall.