Noseleaf motions impart dynamic signatures on bat biosonar pulses

Abstract

Old-World leaf-nosed bats (Hipposideridae) are echolocating bats with nasal biosonar pulse emission. The nostrils in these animals are surrounded by baffle shapes (“noseleaves”) that have been shown to function as beamforming devices. In addition to their elaborate static geometries, the noseleaves can also undergo non-rigid shape changes that coincide with biosonar pulse emission. Prior work by the authors based on biomimetic baffle shapes has demonstrated that baffle motions similar to the ones seen in bat noseleaves can result in time-variant device properties, i.e., distributions of signal power over direction and frequency that also change with time. However, it remained to be established whether similar effects occur in bats. To answer this question, biosonar pulse emission in hipposiderid bats (Hipposideros armiger and Hipposideros pratti) has been studied with synchronized arrays of high-speed video cameras and ultrasonic microphones. Noseleaf motion was characterized by tracking the position of nine landmark points on the noseleaf. Temporal variability in the distribution of signal energy across spatial directions and ultrasonic frequencies was quantified using pair-wise measures of similarity applied to time-windowed segments of the pulse waveforms. Pulses that were accompanied by unequivocal noseleaf motions showed a significantly larger variability across time than those that were not.