Balancing the energy budget in free-ranging male Myotis daubentonii bats.

Mammals use five main, mutually nonexclusive mechanisms to balance energy budgets: torpor, metabolic compensation, change in activity patterns, change in ingested energy, and/or variability in digestive efficiency. Bats, as small and actively flying mammals, have a high mass-specific energy demand; therefore, balancing mechanisms should be pronounced in this group. We found that male Myotis daubentonii exhibited marked variation in the relative importance of these different mechanisms during their period of seasonal activity in response to extrinsic (ambient temperature, insect abundance) and intrinsic (reproduction, body condition) factors. Cold ambient temperatures in spring facilitated long and frequent daily torpor bouts, whereas in early summer, increased energy intake was the dominant factor in energy balancing. Intake was further increased in late summer, when insect abundance was highest, and daily torpor bouts were shorter and less frequent than in early summer. In autumn, males used metabolic compensation to reduce their resting metabolic rate in addition to daily torpor. Metabolic compensation might be one of the mechanisms that allow males to maintain high body temperature during the day while decreasing the need for foraging time at night, thus maximizing their opportunities to mate.