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First published online November 17, 2005
Journal of Experimental Biology 208, 4399-4410 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01901

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Echolocating bats can use acoustic landmarks for spatial orientation

Marianne Egebjerg Jensen^1,2,*, Cynthia F. Moss¹ and Annemarie Surlykke²

¹ Department of Psychology, University of Maryland, College Park, Maryland, USA
² Institute of Biology, University of Southern Denmark, DK-5230 Odense, Denmark

View larger version (13K): [in a new window]   Fig. 1. A schematic drawing of the experimental setup (not to scale). The net divided a large flight room in two, and the net position could be adjusted to change the location of the opening in the horizontal axis. The center of the hole was approximately 1.5 m above the floor. Two high speed cameras were placed in corners of the room and recorded the bat's flight path. Two microphones placed 30 cm above the floor, one on each side of the net (both 50 cm from the net), recorded the bat's echolocation pulses. A landmark (photo tripod) was placed on the left side of the hole, and a tethered mealworm was hung from the ceiling at various positions on the other side of the net. The bat had to find its way through the net opening in order to get the mealworm reward.

View larger version (36K): [in a new window]   Fig. 2. Flight patterns for three different behaviors (A–C), along with the corresponding spectrograms of the sounds produced by the bats (D–F). (A,D) A bat flew through the hole. (B,E) A bat inspected the hole. (C,F) A bat crashed into the net next to the landmark, at the usual position of the hole relative to the landmark. (A–C) The bats' flight paths are displayed as viewed from above. The net is indicated by a diagonal line and the opening is marked in pink. The pink triangle represents the landmark's position along the net in the specific trial. Blue circles denote the bat's position at the time of vocalization and black arrows indicate flight direction. (D–F) In the spectrograms, some of the pulses are clustered together in sound groups with two or three pulses (bracketed). A vertical red line in the spectrogram shows the time the bat flew through the hole, crashed into the net or started an inspection.

View larger version (23K): [in a new window]   Fig. 3. (A–C) Average number of crashes and flights through the hole per trial, and (D) the average duration of trials. Values are means +1 S.D. Data are shown for each bat (M1, M2, F1) separately. (A) The landmark is placed next to the hole. In the fly through data F1 failed to find the hole in one trial, which explains the error bar for that bat. (B) The landmark is moved to a position away from the hole. (C) Here there was no landmark available. (D) The average duration of a trial, when the landmark was next to the hole, moved away from the hole, or no landmark available.

View larger version (17K): [in a new window]   Fig. 4. Average number of inspections per trial when (A) the landmark was next to the hole, (B) the landmark was moved to another position and (C) there was no landmark available. A and C show all inspections recorded, whereas in B inspections are separated into inspections of landmark and inspections of the rest of the net including the hole. Values are means +1 S.D.

View larger version (24K): [in a new window]   Fig. 5. Average performance of the bats as a function of trial number over a session in the experiment with no landmark at the hole. (A) The average success of flying through the hole. (B) The average time spent on the trials. (C) The average number of crashes into the net far away from the hole and (D) the average number of crashes into the net around the hole. Values are means +1 S.D.

View larger version (26K): [in a new window]   Fig. 6. Typical trials showing signal duration (black line) and pulse interval (PI; blue line) when the bat (A) flew through the hole (F1), (B) crashed into the net (M2), or (C) inspected the net (M2). In C the zero indicates the start of the inspection behavior (see Materials and methods and Fig. 2B). Note in B how the pulse interval shifted up and down as the pulses were produced in little sound groups (in red circle). Further, note the high rate sound group (indicated by arrows) just before the bat crashed, showing very short signal durations and intervals.

View larger version (24K): [in a new window]   Fig. 7. The inner window (i.e. duration of emitted pulse recalculated to distance; blue line), the bat's distances to the net (green line), the mealworm (black line), and the landmark, if present (pink line). Time 0 is when the bat interacts with the net (fly through or crash). The red arrows indicate the point in time where overlap between the inner window and the net echo starts (blue and green lines cross and blue line is above the green line). (A,B) The bat flew through the hole and in (A) experienced an overlap 200 ms before it flew through the hole (see red arrow), but in (B) produced a high rate sound group and overlap between the pulse and the net echo did not occur until 90 ms before the bat flew through the hole (red arrow). (C,D) The bat crashed into the net. In (C) the bat produced a high rate sound group and the overlap occurred around 50 ms before the bat crashed, whereas in (D) the bat did not produce this high rate sound group and experienced an overlap between pulse and net echoes 190 ms before crashing into the net.