Echolocation range and wingbeat period match in aerial-hawking bats.

Aerial-hawking bats searching the sky for prey face the problem that flight and echolocation exert independent and possibly conflicting influences on call intervals. These bats can only exploit their full echolocation range unambiguously if they emit their next call when all echoes from the preceding call would have arrived. However, not every call interval is equally available. The need to reduce the high energetic costs of echolocation forces aerial-hawking bats to couple call emission to their wingbeat. We compared the wingbeat periods of 11 aerial-hawking bat species with the delays of the last-expected echoes. Acoustic flight-path tracking was employed to measure the source levels (SLs) of echolocation calls in the field. SLs were very high, extending the known range to 133 dB peak equivalent sound pressure level. We calculated the maximum detection distances for insects, larger flying objects and background targets. Wingbeat periods were derived from call intervals. Small and medium-sized bats in fact matched their maximum detection range for insects and larger flying targets to their wingbeat period. The tendency to skip calls correlated with the species' detection range for background targets. We argue that a species' call frequency is at such a pitch that the resulting detection range matches their wingbeat period.

Directional hearing in grasshoppers: neurophysiological testing of a bioacoustic model

A recently proposed biophysical model for directional hearing in grasshoppers was tested using complex stimulus situations, with two loudspeakers, one on either side of the animal, synchronously emitting sinusoids with defined phase and amplitude relationships. Hearing responses were determined from whole nerve recordings and compared with the predictions of the model. In Schistocerca gregaria, there were only minor differences between the predictions of the model and measurements and, by reducing the value of the gain of the internal sound path measured previously, a close agreement was achieved between model and measured hearing responses. In Chorthippus biguttulus, larger discrepancies between model calculations using the values measured previously and neuronal response functions were found in both shape and amplitude. A better fit between measurements and model predictions was achieved by increasing the values of the internal delay over those measured previously. The measurements presented here indicate high inter-individual variability of the parameters of the internal pathway, with a range of 60 degrees for the internal phase delay. Calculating the directional characteristics using this range of values for the internal delay indicated that sufficient directional information was available down to 5 kHz. Increasing the value of the internal delay over that measured in an earlier study therefore provides an explanation for the discrepancy between the poor directional information attributed to C. biguttulus in that study and the excellent lateralization ability of males of this species at 5 kHz.