Constraints on optimal foraging: a field test of prey discrimination by echolocating insectivorous bats

Abstract. Under controlled laboratory conditions, insectivorous bats use echolocation to make detailed discriminations among targets. However, field evidence suggests that bats do not make such discriminations under natural conditions, although the observations are generally anecdotal. The ability of free-flying aerial insectivorous bats to distinguish among a range of edible and inedible targets was tested in a series of cafeteria-style presentations. Bats responded to and attacked moving prey significantly more often than stationary prey. There was some evidence that prey too large for the bats to consume were distinguished from edible-sized targets, although attacks on large targets were common. Bats attacked small inedible prey (beetles and leaves) as frequently as similarly sized edible prey (moths). Thus, insectivorous bats attack any moving target of an appropriate size and appear not to make the fine-detailed discriminations based on target shape and texture that occur in the laboratory. These results suggest that the lack of discrimination is due to the rapid flight of bats and the short prey detection range inherent in echolocation. Bats have only a fraction of a second between prey detection and capture, possibly not enough time to distinguish among prey even though prey selection should otherwise be beneficial. These results emphasize that laboratory findings regarding sensory capabilities must be verified under natural conditions in the field.