FM echolocating bats avoid pulse-echo ambiguity by shifting broadcast frequencies.

Big brown bats pursue flying insects in open areas, but they also hunt for prey near vegetation. The proximity of obstacles requires emission of sounds at short interpulse intervals (IPIs) to guide collision-avoidance maneuvers, yet the bat has to locate obstacles distributed in depth for path planning, which requires longer IPIs. If IPIs are too short, the stream of echoes for one sound will still be arriving when the second sound is emitted, creating ambiguity about matching echoes to broadcasts. We investigated how bats cope with pulse-echo ambiguity by recording sounds with a miniature radio microphone (Telemike) carried during flights through a dense array of obstacles (vertically hanging chains) that were distributed in depth. Array density forced bats to emit FM sounds at short intervals (strobe groups) with consequent pulse-echo ambiguity. We found that when ambiguity occurred, bats shifted the FM sounds in strobe groups upward and downward in frequency by several kiloHertz to distinguish each broadcast’s echoes. Overlap of echo streams from the bat’s own FM broadcasts causes extreme clutter interference, and bats react by slightly shifting FM frequencies to distinguish one sound’s echo stream from another sound’s. [Research supported by an ONR grant.]