CONSTRAINTS ON OPTIMAL FORAGING - A FIELD-TEST OF PREY DISCRIMINATIONBY ECHOLOCATING INSECTIVOROUS BATS

Under controlled laboratory conditions, insectivorous bats use echoloc ation to make detailed discriminations among targets. However, field e vidence suggests that bats do not make such discriminations under natu ral 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 cafe teria-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 e dible-sized targets, although attacks on large targets were common. Ba ts attacked small inedible prey (beetles and leaves) as frequently as similarly sized edible prey (moths). Thus, insectivorous bats attack a ny moving target of an appropriate size and appear not to make the fin e-detailed discriminations based on target shape and texture that occu r in the laboratory. These results suggest that the lack of discrimina tion is due to the rapid flight of bats and the short prey detection r ange inherent in echolocation. Bats have only a fraction of a second b etween prey detection and capture, possibly not enough time to disting uish among prey even though prey selection should otherwise be benefic ial. These results emphasize that laboratory findings regarding sensor y capabilities must be verified under natural conditions in the field.