ECHOLOCATION SIGNALS OF THE GREATER-HORSESHOE BAT (RHINOLOPHUS-FERRUMEQUINUM) IN TRANSFER FLIGHT AND DURING LANDING

Echolocation signals of horseshoe bats (Rhinolophidae) consist of a re latively long component of constant frequency (CF) which is preceded b y an initial frequency-modulated (iFM) component and followed by a ter minal frequency-modulated (tFM) component. To examine the role of thes e components in echolocation, four bats were trained to fly from a per ch to a landing bar. A dual camera system allowed reconstruction of th e flight paths in three dimensions. Echolocation signals were recorded , analyzed, and correlated with the flight behavior of the bats. It wa s confirmed that during flight the bats compensate the Doppler shifts which are produced by their own flight movement. In free flight they e mit per wing beat one single signal of long duration, with little vari ation in the three signal components. In approach flight the bats redu ce pulse duration and interval with decreasing target range. The iFM i s not varied with respect to target range, suggesting that this compon ent plays little role in the processing of echolocating a target of in terest. The bandwidth of the tFM component is increased while its dura tion is shortened in proportion to decreasing target range, so that th e signal-echo overlap of the FM component is avoided down to a target distance of 15 cm. These concurrent changes suggest that the tFM compo nent is used for ranging. During the last 60 cm of the approach the ba ts compensated for the increase of echo SPL by lowering the emission l evel of the CF component by 6-9 dB and that of the tFM component by 9- 11 dB per halving of range. The specific signal structure of horseshoe bats is discussed as an adaptation for the hunting of fluttering inse cts in highly cluttered environments. (C) 1997 Acoustical Society of A merica.