First published online February 27, 2009
Journal of Experimental Biology 212, 823-834 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.022137
Echolocation by two foraging harbour porpoises (Phocoena phocoena)
Ursula K. Verfuß1,2,*,
Lee A. Miller3,
Peter K. D. Pilz2 and
Hans-Ulrich Schnitzler2
1 Fjord & Bælt, Margrethes Plads 1, DK-5300 Kerteminde, Denmark
2 Tierphysiologie, Zoologisches Institut, Universität Tübingen, Auf
der Morgenstelle 28, D-72076 Tübingen, Germany
3 Institute of Biology, University of Southern Denmark, DK-5230 Odense M,
Denmark
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Fig. 1. The sections of a click train in a fish capture trial. The before-splash
section ends with the splash, which is produced when a fish hits the water
(fish trials) or by a boat hook (no fish trials). In fish trials, the
after-splash section is divided into a far stage and a near stage, which
begins with the transition to progressively shorter click intervals.
Horizontal brackets in the after-splash section indicate the mean click
interval over five consecutive clicks. The transition point is defined as
being the longest click interval in the first mean of the series of
continuously decreasing mean click intervals to a value below 20 ms in the
near stage.
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Fig. 2. Swimming paths of Freja in no fish (A) and fish trials (B) with no eyecups
and when blindfolded (with eyecups). The hatched circle is the area of fish
release. The three black dots show the hydrophone array (H), and the two black
squares represent the two underwater video cameras (U). The arbitrary
reference point (X) is 1.5 m behind the front edge of the pontoon. White dots
on the swimming paths indicate 5 m intervals and the 30 m, 20 m and 10 m
points show distance to the reference. The content of the grey frame in B is
shown enlarged in C with the swimming paths of Freja (thin lines) and the fish
prey (thick lines). Swimming paths of Freja and prey from the same trial are
marked with the same grey shade. Black circles indicate the position of the
fish catch or catch attempt.
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Fig. 3. (A) Mean swim speed (±s.d.) of the two porpoises, Eigil and Freja,
and of Freja blindfolded (Freja bf) in no fish trials (grey bars) and fish
trials (white bars). *Significant difference in swim speed between
fish and no fish trials for Eigil (P<0.05).
***Significant difference in swim speed between Freja with and
without eyecups (P<0.001). The speed of the same individuals in
the orientation task described previously
(Verfuß et al., 2005 ) is
shown for comparison (hatched bars). (B) Swim speed of the porpoises Eigil and
Freja in fish trials (black line) and no fish trials (grey line). The swim
speed is given as the running mean averaged over 10 video frames (0.4 s). The
circles and horizontal bars give the mean distance to the reference
(±s.d.) of the porpoise at the time the splash occurred in fish (black)
as well as in no fish trials (grey). Number of trials for Eigil/Freja/Freja bf
are: no fish=5/5/4 and fish=5/5/5.
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Fig. 4. Click interval of typical click trains emitted by Freja (A) and Freja
blindfolded (B) during no fish trials and fish trials. The click interval is
plotted against time relative to the arrival of the porpoise in front of the
hydrophone array in no fish trials (distance to reference, 5 m), and relative
to the time of fish catch in fish trials (distance to fish, 0 m). As
additional information, the corresponding distance is given (second
y-axis in grey) to the reference (dark grey line) and to the fish
(light grey line). The time of splash (S), the presumed time of prey detection
(*), the time at the transition from far to near stage (T) and fish
capture (C) are also shown. The black bars on the x-axes mark the
time stretch DR=26–12 m, from which data were
included in further analyses. Clicks with intervals >120 ms were not
plotted (marked by X in B). The sudden increase in click interval shown by the
arrow at about –6 s (19 m) for Freja (blindfolded) in the no fish trial
(B left) indicates a shift to another landmark (see Results).
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Fig. 5. Click interval plotted against distance to reference
(DR) for Freja with no eyecups and when blindfolded during
no fish trials (A) and fish trials (B). White circles indicate click intervals
before and black dots click intervals after the splash. The grey line marks
the two-way transit time to the reference point (A,B) or to the fish (C). Data
after the splash in B are displayed in C against distance to fish
(DF). Number of trials for Freja/Freja bf are: no fish=5/4
and fish=5/5.
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Fig. 6. Mean click interval after the splash for Eigil (black circles), Freja (grey
triangles) and Freja blindfolded (Freja bf; white diamonds). The grey line
indicates the two-way transit time to the reference point (A) or to the fish
(B). A shows mean click interval with no fish present. B gives mean click
interval of far stage and near stage over the distance to the fish. Note that
the click intervals are rather constant around about 50 ms in the far stage
for fish trials. See text for further explanations. Number of trials for
Eigil/Freja/Freja bf are: no fish=5/5/4 and fish=5/5/5.
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Fig. 7. The phases of the echolocation behaviour of odontocetes during foraging
according to the change of click interval over time or distance to prey. When
harbour porpoises search for prey in a confined space or at shallow depths
(A), the click interval may indicate range locking to obstacles. This is also
seen in the echolocation sequence of a finless porpoise [adapted with
permission from Akamatsu et al. (Akamatsu
et al., 2005); B] where a sudden rise in click interval suggests a
switch, S, from one landmark to another positioned further away. (Black dots
show the click intervals, the grey dashed lines indicate the estimated
regression line of the click interval within different parts of the
echolocation sequence.) Beaked whales in a pelagic open environment (C) may
use click intervals adapted to maximum perceptual ranges. After detecting a
prey, D, the approach phase begins and the whale closes in on the prey. In the
initial part of the approach phase, different signal patterns have been
observed. In porpoises the mean click interval remains rather constant, maybe
as an adaptation to prey with ultrasound hearing (A,B). Beaked whales either
continue to emit clicks with long intervals when they have detected a single
prey item, presumably for keeping a large perceptual range (Ca), or shorten
the click interval perhaps for adapting their perceptual range to schooling
prey patches (Cb). At a certain distance to the prey the click interval
progressively decreases indicating a transition, T, to the terminal part of
the approach phase with rapidly decreasing click intervals. At close distances
to the prey the terminal part ends with the buzz that is characterized by
constant and very short click intervals (absent in B). The buzz ends with the
capture of the prey, C, which is followed by an increase in click interval.
For further explanation see text. Please note the difference of ordinate scale
in A to C.
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