ECHOLOCATION AND OBSTACLE AVOIDANCE IN THE HIPPOSIDERID BAT ASELLIA-TRIDENS

The echolocation sounds of the hipposiderid bat Asellia tridens consist of a constant frequency (cf) component followed by a frequency modulated (fm) terminal downward sweep of 19-21 kHz. The cf-part constitutes about 7/10 of the entire signal. In individual roosting animals the frequencies of the cf-part of consecutive sounds (resting frequency) is kept very constant but varies from bat to bat. In 18 Asellia tridens resting frequencies between 111-124 kHz have been measured. The sound duration in roosting and free flying bats is between 7-10 ms. In the approach and terminal phase of bats landing on a perch or flying through obstacles, the sound duration is reduced and the repetition rate increased the nearer the bat approaches the target. At the end of the terminal phase sound durations of a minimum of 3 ms have been measured. Flying bats lower their emission frequency in order to compensate for Doppler shifts caused by the flight movement. The echofrequency is therefore kept constant about 150-200 Hz above the resting frequency. In flights through obstacles consisting of vertically stretched wires with different diameters, the bats were able to avoid wires down to a diameter of 0.065 mm whereas at 0.05 mm the percentage of flights without collisions is far below the chance level. The results demonstrate that the echolocation behavior of the hipposiderid bat Asellia tridens does not differ fundamentally from that of rhinolophid bats. As a result, a new suggestion for categorization of bats producing cf-fm orientation sounds is put forward. © 1979 Springer-Verlag.