POPULATION-STRUCTURE OF TEMPERATE ZONE INSECTIVOROUS BATS IN RELATION TO FORAGING BEHAVIOR AND ENERGY DEMAND

(1) The morphology, echolocation calls, foraging behaviour and prey distribution of two temperate zone species of insectivorous bats was studied to assess the hypothesis that sexual differences in distribution pattern are the result of differences in energy demand. (2) In an area of the eastern slopes of the Rocky Mountains (Canada) characterized by low ambient temperatures and low insect abundance, over 90% of the Myotis lucifugus caught during the summer were adult males while an equal sex ratio of Myotis evotis was caught. (3) M. lucifugus forage low over water on aerial prey, especially chironomids, which are abundant only for a short time after sunset. (4) M. evotis forage along paths and in forest and prey primarily on moths. They take insects from the air but laboratory studies indicate that they are also adept at gleaning prey from the ground and vegetation. This provides a broader, more predictable food resource than is available to M. lucifugus. (5) I suggest that the flexible foraging strategy of M. evotis allows reproductive females, with their high energy demand, to inhabit areas that cannot support reproductive female M. lucifugus. Males of both species can exist in such areas because of lower energy demand and, potentially, the use of torpor under adverse conditions. Selecting such areas and reducing foraging time may benefit males if there are risks incurred while they forage.