First published online March 12, 2009
Journal of Experimental Biology 212, 1011-1020 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.024620
Acoustic scanning of natural scenes by echolocation in the big brown bat, Eptesicus fuscus
Annemarie Surlykke1,
Kaushik Ghose2 and
Cynthia F. Moss*2
1 Institute of Biology, University of Southern Denmark, Odense, DK-5230,
Denmark
2 Department of Psychology, Institute for Systems Research, University of
Maryland, College Park, MD 20742, USA
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Fig. 1. Schematic 3-D (A) and top view (B) of the flight room (7 m x 8 m
x 2 m) and experimental set-up for the dual task. A net is stretched
across the flight room and one of two openings (hole a or b) allows the bat
access to a food reward (tethered insect; red oval) on the other side of the
net. The bat's task is to find the tethered insect behind one of these
openings and fly through it to access the food. A partition net divides the
back end of the room to allow the bat access to the food reward only if it
flies through the correct opening. Two high-speed video cameras (Cam 1 and Cam
2) were used to reconstruct the 3-D flight path of the bat and positions of
other objects in the room. Full bandwidth recordings of the bat's echolocation
calls were taken with two microphones (1 and 2) positioned on the floor. A 16
microphone array (green circles in A, black circles along the inside perimeter
of the inner box in B) positioned along three walls was used to reconstruct
the beam pattern of each sonar emission.
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Fig. 2. (A) Schematic illustration of the method used to calculate the direction of
the big brown bat's sonar beam axis and the tracking angle from microphone
array recordings. (B) Temporal relationship of pulse and echo(es) from the
bat's perspective. The pulse duration (Dur) defines the distance in front of
the bat where the echo from an object will overlap with the outgoing pulse.
Decreasing the pulse duration can prevent overlap between the bat's sonar
vocalization and the echo. If there are several objects at different ranges,
the bat may experience overlap with echoes from closer objects and avoid
overlap with more distant objects.
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Fig. 3. The two panels show 3-D plots (axes, m) aligned with plots of pulse
interval (PI) and pulse duration. Time=0 s is when the bat flies through the
hole. In A the worm was 58 cm behind the hole, on-axis, and the bat flew
straight from the hole to seize the prey. In B the worm was 56 cm behind the
hole, off-axis, and the bat made a `U-turn' before capture. When the bat
approached the net, the pulse interval and duration were reduced, as shown in
the lower panels.
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Fig. 4. Data from two selected trials, when the big brown bat flew towards and
through the net hole and captured the insect on the other side. The four
panels (A–D) for the two trials show beam directing data up to selected
video frames (number displayed in each panel). The dashed red line extends the
beam axis of the last call displayed in each panel, illustrating how the beam
was directed at either the right edge (RE) or the left edge (LE) of the hole,
until the bat shifted its beam axis to the more distant worm (W). The plots on
the right summarize schematically the bat's beam-directing behavior in the two
selected trials. The edges of the holes are shown as horizontal lines, with LE
as the upper line and RE as the lower line. Each call is represented by a
filled black circle on either of these lines, according to beam axis. The red
letters (A–D) refer to the corresponding panels. The plots show how the
bat scanned sequentially back and forth between the left and right edges of
the hole until 200–300 ms before crossing the net, when it shifted its
beam direction towards the worm on the other side of the net. In trial
7.17.1.14 the bat took a `quick look' at the left edge with one call (in frame
157 of C).
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Fig. 5. Summary of the beam aim of the calls produced by the big brown bat, subject
HP4, as it approached the net and flew through the hole to get the insect in
18 successful trials with the insect far behind the net. Beam aim for each
sonar call is displayed as a function of time, with all trials aligned with
respect to the time when the bat flew through the net hole (zero on the
abscissa). Blue line segments above the time axis for each trial show
vocalizations directed at the left edge of the net hole, and black line
segments below the time axis show vocalizations directed at the right edge of
the net hole. Vocalizations directed at the mealworm are shown with red
triangles pointing to the right. Beam-directing behavior was taken from the
beam axis measurement, ±5 deg. The plots illustrate how the bat scanned
back and forth between the edges of the hole. Well before crossing the net
hole (time zero) the bat shifted its gaze to the more distant prey (red
triangles).
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Fig. 6. The plots show tracking angle distributions for sonar calls directed at the
left and right edges of the net, as well as the worm before and after passing
the net. Sonar beam tracking angles  15 deg. were assigned to the left or
right edges of the net hole or the worm. The few vocalizations that did not
fall within this range were not assigned to any object. Data in this figure
include vocalizations that were produced by the bat in both successful and
unsuccessful trials.
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Fig. 7. (A) Top, schematic illustration of the beam-directing behavior in a
selected trial (see also Fig.
4). The big brown bat scanned back and forth between the left and
right edges of the hole until ca. 250 ms before flying through the hole, when
the beam axis shifted to the worm. Bottom, duration of sonar calls from the
same trial, converted to distance (blue curve), plotted as a function of time
relative to net crossing (time zero). The magenta curve shows the distance
between the bat and net, with a minimum at time zero, when the bat flew
through the hole. The black curve shows the distance between the bat and worm.
Sonar signal duration (blue) decreased as the bat approached the net, but
started increasing approximately 300 ms before it crossed the net, creating
overlap between call and net echoes around –250 ms, corresponding with
the shift of beam aim to the worm (red dashed double-headed arrow). (B) Time
of worm fixation (Y-axis) plotted against time of pulse–echo
overlap (X-axis) over 16 trials.
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© The Company of Biologists Ltd 2009
