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Prey-capture success revealed by echolocation signals in pipistrelle bats (Pipistrellus pygmaeus)

Annemarie Surlykke^*, Vibeke Futtrup and Jakob Tougaard

Centre for Sound Communication, Institute of Biology, University of Southern Denmark, Odense Campusvej 55, DK-5230 Odense M, Denmark

View larger version (8K): [in a new window]   Fig. 1. Diagram of the flight room seen from above. (A) Curtain partly dividing the flight cage. (B) Video camera filming across the track. (C) Video camera filming along the track. Brüel & Kjaer microphone and bat detector were also placed at C. (D) Wooden door into the flight room. (E) Concrete pillar. The mealworm catapult and computer operating devices were placed on the floor right under the capture volume indicated by the grey shading.

View larger version (17K): [in a new window]   Fig. 2. Outline of bat and mealworm (red) in a successful capture attempt. (A) Superimposed successive frames (40 ms between frames) from camera B, (see Fig. 1) giving a side view. (B) Successive frames along the flight path (camera C, Fig. 1). The two black dots are reference points on the walls opposite the cameras. Flash 3 did not go off in this trial.

View larger version (32K): [in a new window]   Fig. 3. (A) Overviews of the sonar signals of two trials recorded the same day from bat V. Left: fail, right: capture. (B) High-resolution oscillograms and spectra of the search and two post-buzz signals from each trial, numbered as in the overviews. Spectra of post-buzz signals following successful captures contain notches (right), which are not seen after fails (left). m, the sound of the mealworm catapult; f, some of the negative flash-sync pulses.

View larger version (37K): [in a new window]   Fig. 4. Distribution of NOTCH, interpause interval (IPI) and post-buzz pause (pbP) in post-buzz signals after capture attempts. (A) Wides, (B) touches, (C) captures. Data are pooled for all bats; black triangles indicate group means.

View larger version (16K): [in a new window]   Fig. 5. Post-buzz pause (pbP) plotted as a function of the duration of the head-down stage determined from video recording of the same trial. Points above the solid diagonal line are from trials where pbP was longer than the maximum duration of the head-down stage. The broken line indicates the average duration of the head-down stage (see text for explanation).

View larger version (14K): [in a new window]   Fig. 6. Prey type effects on NOTCH, interpause interval (IPI) and post-buzz pause (pbP) of post-buzz signals follwing successful captures. The x-axis gives the mean mass of each prey type. Horizontal error bars indicate the size range for the prey type. Values are means ± S.E.M. (vertical error bars). N=60 for all points.

View larger version (22K): [in a new window]   Fig. 7. Receiver operating characteristics (ROC) plots for criteria based on the three `raw' parameters, NOTCH, IPI and pbP, and for the combined first canonical discriminant factor (CD 1). For all parameters, 10 different criteria were chosen to give 10 sets of hits and false alarms to define the ROC-curve. Curve are best-fitting ROC curves assuming Gaussian distribution with unequal variance. The area, A, below the ROC curve is indicated in each panel.

View larger version (30K): [in a new window]   Fig. 8. Success in using sound recordings to determine the actual outcome of capture attempts as a function of criterion for all four parameters: NOTCH, interpause interval (IPI) and post-buzz pause (pbP) and first canonical discriminant function (CD1). CD1 gives a high proportion correct over a broad interval of criteria, >80% correct from approx. -1.1 to +0.4. Coloured curves represent individual bats; the thick black curve is the pooled data for all bats.

View larger version (27K): [in a new window]   Fig. 9. Distribution of the first canonical discriminant factor (CD1) in cases of capture, touch or wide. `Fail' shows the distribution for pooled touch and wide data. Data are pooled for all bats; black triangles indicate group means.