Directional sensitivity of echolocation in the horseshoe bat,Rhinolophus ferrumequinum
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The directionality of hearing ofRhinolophus ferrumequinum (RF) at its reference frequency has been determined by behavioral means, taking advantage of their tendency to automatically alter the constant frequency component of their emitted frequency to compensate for an upward shifted echo frequency.
Directionality was measured by presenting a sinusoidally frequency-shifted echo from straight ahead of the bat, and determining the relative effectiveness of narrow band noise from different angles around the bat in masking the signal and stopping compensation (Fig. 1).
The directionality determined in this way (Fig. 2) shows a maximal difference in sensitivity of approximately 45 dB between the directions of greatest and least sensitivity in the hemisphere in front of the bat. On the horizontal axis, sensitivity falls off by approximately 35–40 dB from the midline to the side. The drop in sensitivity is also sharp at angles below the horizon, less abrupt above the horizon.
Several features of the directionality pattern are shown to be altered by altering pinna and antitragus position (Fig. 3).
Monaural ear plugging causes a 15–20 dB rise in signal SPL necessary to evoke compensation. It also results in a large increase in noise effectiveness on the side of the intact ear, little or no change on the side of the plugged ear (Figs. 4, 5).
These results, coupled with the directionality of pulse emission inRF (Schnitzler and Grinnell, 1977), indicate a very sharp and nearly symmetrical drop in echo effectiveness at angles more than about 30‡ away from the axis of the bat's head (Fig. 6). Probably all useful echoes, except from very large objects, come from angles no greater than 30‡–40‡ off this axis.
Difficulties in interpreting some features of the behavioral data are discussed.
KeywordsMaximal Difference Frequency Component Narrow Band Behavioral Data Relative Effectiveness
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