Spatial orientation in echolocating harbour porpoises (Phocoena phocoena)
1 Fjord & Bælt, Margretes 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
* Author for correspondence at present address: German Oceanographic Museum, Katharinenberg 14/20, 18439 Stralsund, Germany (e-mail: ursula.verfuss{at}meeresmuseum.de)
Accepted 6 July 2005
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Summary |
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Key words: harbour porpoise, Phocoena phocoena, biosonar, echolocation, echolocation behaviour, signal pattern, spatial orientation, landmark
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Introduction |
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Teilmann et al. (2002) and
Verfuß and Schnitzler
(http://www.uni-tuebingen.de/tierphys/Fledermaeuse/Delfine.htm
or
http://www.uni-tuebingen.de/tierphys/Fledermaeuse/final_report.pdf)
showed that harbour porpoises wait for the echo of an outgoing signal before
sending out the next signal. This echolocation behaviour is also used by
bottlenose dolphins (e.g. Morozov et al.,
1972
) and false killer whales (Pseudorca crassidens;
Thomas and Turl, 1990
). The
time between receiving an echo and emitting the next click is called the lag
time (Au, 1993
) and is
considered to be relatively constant during specific echolocation tasks, as
shown for bottlenose dolphins (e.g. Au et
al., 1981
; Morozov et al.,
1972
) and the false killer whale
(Thomas and Turl, 1990
). A
beluga whale (Delphinapterus leucas), however, used three different
click patterns. In one of these, the beluga produced clicks before receiving
echoes from preceding emissions, making the intervals between clicks shorter
than the two-way-transit time, the time interval between the emission of a
click and the reception of an echo from the target
(Au et at., 1987
;
Turl and Penner, 1989
).
In bats, echolocation is used for spatial orientation and prey capture
(Schnitzler et al., 2003). We
assume that odontocetes use echolocation in a similar way. However, the
biosonar of odontocetes has mainly been investigated in the context of target
detection (reviewed in Au,
1993
; Kastelein et al.,
1999
) and discrimination (reviewed in
Au, 1993
;
Kastelein et al., 1997
) and
not in the context of spatial orientation. Schnitzler et al.
(2003
) postulate that
echolocation in bats evolved primarily for orientation in space or navigation
and that the transition to prey acquisition followed later. The term
navigation is used according to Trullier et al.
(1997
), who defined navigation
as the ability of animals to find, learn and return to specific places.
Schnitzler et al. (2003
)
define three categories of navigation: small-, middle- and large-scale
navigation. Small-scale navigation is the process of moving around in the
immediate environment, with the animal's goals being within its range of
perception. Middle-scale navigation comprises the ability to follow routes to
goals beyond the perceptual range but within the home range of an animal.
Routes are characterized by sequences of places to which animals react with
recognition-triggered responses (Trullier
et al., 1997
). Each place is defined by a certain landmark or
constellation of landmarks, prominent or conspicuous objects that serve as
guides. Large-scale navigation encompasses movements in unfamiliar areas, for
example during migration or homing, which is defined as guided or directed
movements homeward or to a destination.
Nothing is known about how harbour porpoises use their echolocation abilities for spatial orientation. The present paper investigates and compares the echolocation behaviour of two harbour porpoises during orientation tasks in a semi-natural outdoor pool by using the concept of small- and middle-scale navigation. We show that echolocation plays an important role for spatial orientation and that it is used for navigation.
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Materials and methods |
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Animals
Two harbour porpoises (Phocoena phocoena L.), a female named Freja
and a male named Eigil, were involved in this study. The animals were rescued
from a pound net near Kerteminde, Denmark in April 1997 and had an estimated
age of 12 years. During the study period, the animals' ages were
between 2 and 5 years. The body length of the female was 1.49 m and her
mass was
46 kg. The body length of the male was
1.37 m and his mass
was
39 kg.
Experimental procedure
The animals performed three tasks: (1) they were sent from side `a' to side
`b' (`atb'), (2) they were sent from side b to side a (`bta'), and (3) they
were sent from side a to side b, with equipment two underwater cameras
(cam3+4) and a hydrophone array placed in the water near side b (atb +
equipment = `atb+') (Fig.
1).
For these tasks, both animals were trained to station at one end of the
pool (Fig. 1, side a or side
b). During trials, one animal stayed with a trainer while the other animal was
sent to the opposite side of the pool (side b or side a, respectively), where
a second trainer splashed at the water surface to attract the animal's
attention. When the porpoise headed towards the `destination point', which is
approximately 0.5 m in front of the second trainer, the trainer lifted her
hand 30 cm above the water surface. The porpoise had to touch the
trainer's hand to end the behavioural trial. No target was submerged into the
water during a trial. The holding pool was positioned in a corner of the
enclosure at the starting end to minimize disturbing the experimental
procedure.
Experimental set-up and trials
Synchronised video and high-frequency sound recordings were made from the
porpoises during all orientation tasks. Experimental sessions were done on
days with good water clarity and calm weather with no or little rainfall to
assure good visibility and recording conditions. Two surveillance cameras were
used for the video recordings. One camera
(Fig. 1, cam1) was fixed on
wires approximately 5.3 m above the water surface, giving a top view of part
of the west end of the pool. The second camera
(Fig. 1, cam2) was fixed
9.4 m above the water surface on the Fjord & Bælt exhibition
centre wall and was used to analyse the porpoises' behaviour at the east end
of the pool. No recordings were made for the atb+ trials with cam2. On days
with atb+ trials, two video cameras in underwater housings (Evamarine,
Geretsried, Germany) were mounted 2 m apart on a horizontal steel rod and
fixed to a vertical steel pole in the harbour-side corner of side b
(Fig. 1). Both cameras were
placed 0.25 m under the water surface. This equipment was not operational in
these studies and served as temporarily introduced objects during atb+
trials.
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Signals from the hydrophones were amplified by 52 dB and high-pass filtered
at 100 Hz using Etec amplifiers (Etec, Copenhagen, Denmark;
Fig. 2). The sound was recorded
on three channels of a RACAL Store 4D high-speed magnetic tape recorder (Racal
Instruments GmbH, Bergisch Gladbach, Germany) at a speed of 60 inches
s1, giving a bandwidth of 300 kHz.
The synchronization of all video and sound recordings was done with a custom-built VITC/LTC time code generator (Universität Tübingen, Tierphysiologie, Tübingen, Germany; Fig. 2).
The atb+ trials were recorded on seven days in October and November 1998 with 25 experimental sessions, totalling 117 trials (56 trials with Freja and 61 trials with Eigil). The atb and bta trials were recorded on six days in May and June 2000 with 12 experimental sessions, totalling 65 trials (16 atb and 15 bta trials with Freja, and 18 atb and 16 bta trials with Eigil).
All video and sound recordings were visually scanned for quality, defined as reasonably good sighting of the involved porpoise on the recordings of both video cameras (cam1 and cam2) and a reasonable signal-to-noise ratio for the emitted echolocation click series on the sound recordings. A total of 43 out of 182 trials were chosen for detailed analysis of the echolocation behaviour, including 16 trials for atb (eight for Freja, eight for Eigil), 17 trials for bta (nine for Freja, eight for Eigil) and 10 trials for atb+ (five for Freja, five for Eigil).
Video analysis
Selected video sequences were digitized with a frame grabber card (HASOTEK
frame grabber FG42; Rostock, Germany). The video sampling rate was 25 images
s1, giving a 40 ms time interval between frames.
Motion analysis was done frame by frame. The relative position of the tip of the animal's rostrum within each successive frame was determined from the video recordings of cam1 and cam2. For frames in which the porpoise was not visible, its position was interpolated.
For analysing the distance from the porpoise to its destination, we defined
an arbitrary `reference point'. We did this by examining the porpoises'
echolocation behaviour, which showed a clear decrease in click interval during
the approach (see the Results). The decreasing click interval indicated that
the animal had locked its sonar on to a landmark somewhere near the end of the
pool. This landmark might have been the front edge of the pontoon where the
trainer sat (0.5 m behind the destination point) or the net at the end of
the pool (
3.5 m behind the destination point). We therefore chose the
`reference point' to be midway between the front edge of the pontoon and the
net, a point 1.5 m between these (Fig.
3). The distance between the calculated position of the porpoise
and the reference point was defined as `distance to reference'.
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Reconstruction of the swimming path was considered successful when the track from each camera overlapped at the middle of the pool, which was common to both camera views. Tidal differences that changed the distance between cameras and water surface were taken into account for each session. With this method, distances could be calculated with a maximum error of 5%.
In atb+ tasks, only one surveillance camera (cam1) was used (Fig. 1). Distances to reference beyond the view of cam1 were interpolated by using a polynomial fitting formula of the swim speed obtained for each porpoise in atb trials. With the assumption of a similar swim speed in atb trials and atb+ trials, the missing distances in atb+ trials could be calculated.
Sound analysis and correlation with video recordings
The sound sequences of the chosen trials were played back at 16-fold
reduced speed and digitised with a sampling rate of 51.2 kHz, resulting in an
effective sampling rate of 819.2 kHz. The `click interval', which is the time
between two successive clicks, was analysed by saving the onset time of each
click into a text file. This was done with custom-made software (Sona-PC; B.
Waldmann©, Tübingen, Germany) with an accuracy of 156
µs. The software also showed the onset of each video frame and its specific
frame number, which were used to correlate sound and video recordings. It was
thus possible to correlate a particular click or click interval with a
distance of the porpoise to our arbitrary reference point. Analysis began from
the first click recorded in a trial and stopped with the clicks emitted when
reaching the destination point, which was 0.5 m in front of the pontoon.
This is called a `navigational trial' for atb and bta. For atb+, the
navigational trial ended when the porpoise reached the hydrophone array,
2 m in front of the pontoon. The first click and last click of a
navigational trial were used to determine the time and distance navigated.
Also, the total number of clicks recorded during the navigational trial was
determined (see Table 1).
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Click exclusion criteria
Not all clicks of a click train were captured by the hydrophones. Harbour
porpoises possess a very directional transmission beam pattern
(Au et al., 1999), and pauses
occur in the echolocation train as the animals move the beam away from the
recording hydrophone. Recordings from the three hydrophones used in the bta
trials confirmed beam scanning by our porpoises. Therefore, all click
intervals longer than 120 ms, indicating that the animal directed its sonar
beam away from the hydrophone, were excluded from the analyses.
During the experimental trials, the porpoises locked their sonar onto spots near the end of the pool, indicated by a decrease in the click interval during the traverses. In the beginning and near the end of each trial, the porpoises did not swim straight towards the destination point (see Results, Swimming behaviour). To assure inclusion of those parts of the traverses during which the porpoises swam directly towards the end of the pool, and therefore most likely focus on the same spot, only a middle range from 26 m to 12 m was chosen for analysing click intervals and lag times for all tasks (see Fig. 3).
Relationship between click interval and distance to reference
For assessing the relationship between click interval and the distance to
the reference point, the click interval and corresponding distance data pairs
of all analysed click trains (echolocation click sequences) were pooled
separately by animal and task. The data pairs were then grouped into
distance-to-reference classes using a bin of 1 m. Median click intervals and
the 25% and 75% quartiles of each class were determined because the data were
not normally distributed. The same procedure was done with the corresponding
lag time and distance-to-reference data pairs. The `lag time' is the time
difference between the click interval and the corresponding two-way-transit
time to the reference point. We calculated the two-way-transit time of the
porpoises' clickecho pairs to the reference point assuming the speed of
sound in water to be 1.5 m ms1, giving a slope of 1.3 ms
m1.
A regression analysis was performed on the 1 m bin median click interval values from 26 m to 12 m (Fig. 4) for each porpoise and task. Median values comprised at least three click intervals in each bin and results from at least three trials. The slope of regression with 95% confidence interval was determined (Table 1; Fig. 5).
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Results |
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Echolocation behaviour
The porpoises continuously emitted echolocation signals in all trials. The
mean number of clicks recorded during navigational trials and the
corresponding mean time and distance covered are given in
Table 1. The click interval
decreased with decreasing distance [26 m to 12 m of the total distance
navigated (32 m)] to our arbitrary reference point approximately 1.5 m behind
the front edge of the pontoon (Fig.
3). The decrease in median click interval
(Fig. 4) issignificant for both
porpoises and for all tasks (Freja atb, r2=0.863; Eigil
atb, r2=0.893; Freja bta, r2=0.938;
Eigil bta, r2=0.973; Freja atb+,
r2=0.486; Eigil atb+, r2=0.913;
P0.006). However, the slopes of the regressions for click
intervals are not significantly different from that of the two-way-transit
time (1.3 ms m1), except for Freja during atb (slope
0.908±0.228, 95% confidence interval) and for Eigil during atb+ (slope
1.861±0.361, 95% confidence interval)
(Fig. 5).
The mean lag time, as indicated by the difference between the two-way-transit time to the assumed reference point and the corresponding click intervals (Fig. 4), is depicted in Fig. 6 and the values are given in Table 1. There are no significant differences between the lag time calculated for Eigil in atb and bta trials (Pcorr=0.952, d.f.=14) and between Eigil and Freja in atb trials (Pcorr=0.738, d.f.=15) and in atb+ trials (Pcorr=0.070, d.f.=8). There are significant differences for the other combinations shown in Fig. 6, ranging from Pcorr=0.024, d.f.=14 (Eigil and Freja, bta) to Pcorr=0.000197, d.f.=12 (Freja atb+ and atb). Both animals show significantly longer lag times for atb+ trials compared with atb trials.
There are two reasons for the lower mean number of clicks recorded from the animals in atb+ trials. First, the lag times are longer in atb+ trials. Second, the atb+ navigational trials are shorter, since these ended when the porpoises reached the hydrophone array approximately 2 m in front of the destination (the pontoon) where the navigational trials for atb ended.
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Discussion |
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Our conclusion supports the findings of Kastelein et al.
(1995) that harbour porpoises
in a pool used the same amount of echolocation in light as well as darkness.
Nevertheless, echolocation is complemented by visual information. The female
porpoise, Freja, was trained to catch live fish with and without opaque
eyecups (U. K. Verfuß, L. A. Miller, P. Pilz and H.-U. Schnitzler,
unpublished). Click intervals were hardly affected, but a significantly slower
swim speed when wearing eyecups gave an increased number of clickecho
pairs per metre travelled. This behaviour increased the received information
flow per distance travelled, which was assumed to compensate for the lack of
vision. Also `resident' killer whales (Orcinus orca) in Canadian
waters showed no change in echolocation activity with water clarity
(Barrett-Lennard et al.,
1996
).
Range-locking behaviour and lag time
Thus far, range-locking behaviour has been tested either with swimming
odontocetes approaching a target (Morozov
et al., 1972) or with stationary odontocetes locating single
targets offered at different distances (reviewed in
Au, 1993
). Range locking means
that the next click is emitted after reception of an echo and a specific lag
time. In bottlenose dolphins, lag times between
15 ms and 45 ms have been
reported (Morozov et al.,
1972
; Au,
1993
).
In our investigations, the porpoises were not asked to detect any target.
Nevertheless, both animals demonstrated an obvious range-locking behaviour
when moving from the starting to the destination point. Lag times were
approximately 14 ms to19 ms if they approached the destination point without
equipment in the water (atb and bta) and were distinctly longer, approximately
2636 ms, with equipment in the water (atb+)
(Table 1). The continuous
decrease of click interval combined with a nearly constant lag time at
distances from 26 m to 12 m suggest that the porpoises acoustically locked on
to a distant reference point somewhere near the destination. We cannot
determine whether the reference point was the front edge of the pontoon on
which the trainer was positioned (0.5 m behind the destination point) or
the net at the end of the pool (
3.5 m behind the destination point). We
therefore calculated the lag times to an arbitrary reference point that was
about midway between the front edge of the pontoon and the net.
The demonstrated range-locking behaviour indicates that the porpoises use information from a specific object in the background, a landmark, when approaching the destination point. This clearly shows that the porpoises use echolocation for spatial orientation even when swimming along stereotyped routes. If the landmark is picked up immediately after leaving the starting point, the echolocation task constitutes small-scale navigation, since the goal is within the perceptual range of the animal. In this case, the lag time should be about constant throughout the whole route from the start to the destination point, but this only occurred in the central portion of the traverses (Fig. 3).
In middle-scale navigation, where the goal is beyond the perceptual range
(see Introduction), an animal follows a route along several landmarks. In so
doing, the porpoises should switch with their lock-in behaviour from one
landmark to the next. We think that such behaviour explains the differences in
the means of the lag times shown by Freja in experiments atb and bta and the
individual differences between Freja and Eigil in bta
(Fig. 6). We assume that while
the animals used the same landmark in atb, in bta Freja locked onto a pile of
stones (see Fig. 1)
representing a landmark positioned 3 m in front of the reference point at
side a whereas Eigil mostly used a landmark near our reference point.
Therefore, our calculated two-way-transit time for Freja bta was too long by
approximately 4 ms. If we add this value to the calculated lag time of 14.4 ms
shown in Table 1, we get a new
lag time of 18.4 ms. This is very close to the value of 19.2 ms determined for
Freja in the atb trials and is also closer to the value of 16.8 ms for Eigil
in bta. Another example for the switching from one landmark to another is the
change from very short lag times at the beginning of a trial to the more
constant values later in the trial then to long lag times at the end of the
trial in the atb experiments for both porpoises (see
Fig. 3D, atb). These changes
are explained if the porpoises first used the nearby holding pool as a
landmark, then switched to the landmark at the opposite side of the pool and
finally used the corner of the enclosure as a landmark when turning just
before reaching the destination point, as they often did. An earlier release
of the landmark at the opposite end of the pool by Freja in atb+ trials, where
obstacles are a few metres in front of the destination point, could explain
why click interval increases at shorter distances to reference
(Fig. 4); thus, the lag time is
overestimated in this case. Eigil seems not to be disturbed by introduced
obstacles since the click intervals decrease monotonically in the atb+
experiment (Fig. 4).
Inconclusion, these harbour porpoises lock their biosonar on to landmarks
during spatial orientation tasks.
Our experiments atb and bta are rather similar to those of Morozov et al.
(1972), where five dolphins
(Tursiops truncatus) had to swim in a netted aquarium over a distance
of 2530 m to get a dead fish. In the 24 m to 4 m section of the
traverse, the lag times were
20 ms, or similar to what we found with
harbour porpoises. They assumed the dolphins were range locking on to the prey
but it seems more likely that the animals locked on to some object in the
background in the range of 24 m to 4 m and on to the target thereafter. Range
locking has also been demonstrated in stationary Tursiops truncatus
that had to detect or discriminate targets offered at different distances (for
a review, see Au, 2000
). In
such experiments, lag time values between 19 ms and 45 ms have been measured.
Au (2000
) points out that in
these experiments targets have been used that are totally alien to the
animals, and the environments in which the experiments were made may also have
been rather unnatural. Additionally, the animals were tested after long
training periods. We therefore suggest caution when stating that echolocating
odontocetes will lock onto every natural target, including prey. If distant
large targets that can serve as landmarks and small targets such as swimming
fish are present at the same time, we assume that range locking is mainly
connected to spatial orientation and not to prey detection. Madsen et al.
(2005
) recorded the biosonar
performance of deep-diving beaked whales (Mesoplodon densirostris)
while foraging in open water and found no range locking during prey capture.
The lack of landmarks on which to lock in deep waters might explain this
behaviour. On the other hand, finless porpoises (Neophocoena
phocaenoides) in an isolated waterway showed a clear range-lock behaviour
during foraging, starting up to 42 m from the presumed prey capture
(Akamatsu et al., 2005
). The
authors interpreted this behaviour as detection and approach of a potential
prey target. Accepting the use of landmarks for orientation in odontocetes, it
might be more likely that at such distances the finless porpoises lock on to
larger objects in potential foraging areas and that prey detection occurs
after arrival at the foraging site.
Read and Westgate (1997)
describe orientation behaviour by wild harbour porpoises. They concluded from
their satellite tracking studies that porpoises moving out of the Bay of Fundy
into the Gulf of Maine did so by following the 92 m isobath, which probably
represents an important movement corridor. To keep acoustic contact with a
bottom contour is a typical small-scale navigational task, whereas the
migration to the Gulf of Maine can be attributed to middle- or large-scale
navigation, depending on the familiarity of the destination.
The described spatial orientation behaviour of our harbour porpoises and
that reported by Read and Westgate
(1997) is similar to the
spatial orienting behaviour of bats, which also use echolocation to follow
routes along a sequence of landmarks and contours such as tree lines, forest
lanes and edges (Verboom et al.,
1999
; Schnitzler et al.,
2003
). Both animal groups show a kind of guidance behaviour for
maintaining a certain spatial relationship to landmarks
(O'Keefe and Nadel, 1978
;
Trullier et al., 1997
;
Mallot, 1999
).
Lag time versus processing time
It is assumed that lag time is necessary to process the information of the
preceding pulseecho pair (Morozov
et al., 1972). We found that lag time is task dependent. In the
experiments atb and bta (no equipment near the destination point), we measured
lag times between 14 ms and 19 ms. In the experiment atb+, with a more complex
spatial situation (equipment in the water), the lag times were longer, with
means around 36 ms for Freja and 26 ms for Eigil. This may indicate that the
animals now needed more time to process the more complex information in the
pulseecho pairs. This result corroborates the findings of Au et al.
(1981
). They measured lag
times of only 7.09.4 ms in a detection experiment with Tursiops
truncatus. These values are considerably shorter than the lag times of
1822 ms measured in a discrimination task in an earlier experiment with
the same animals (Au, 1980
).
Thus, different echolocation tasks affect the mean lag time used by these
dolphins. The dependence of lag time on the difficulty of an echolocation task
reflects the neuronal process that is necessary to extract the task-relevant
information from the pulseecho pairs without being disturbed by the
succeeding pulse. It is plausible that a simple task such as detection (the
presence or absence of a target) needs less processing time than a more
complex task such as discrimination or categorization of targets.
Spatial orientation
In the present study, the click interval depended on the spatial
orientation tasks. The porpoises were locked onto specific places in the
background that we call landmarks. From this fact, we derive the general
hypothesis that odontocetes use echolocation not only for foraging but also
for spatial orientation. This hypothesis is supported by data from other
studies also indicating the use of landmarks by odontocetes.
Teilmann et al. (2002)
conducted an experiment where a harbour porpoise had to detect a target at
distances ranging from 12 m to 20 m. In all trials of this experiment, a
constant mean click interval of 59 ms was measured, resulting in mean lag
times changing between 32 ms and 43 ms. The porpoise apparently did not lock
onto the target. The authors describe the experimental pool as a 34 mx20
m outdoor floating net-enclosure with a small (3.6 mx2.9 m) research pen
at one side, where the porpoise stationed for each trial. The net of the
enclosure was covered with marine fouling. Assuming that the animal used the
end of the enclosure as a landmark and locked onto it, we can calculate the
lag time. This landmark was positioned at a distance of 30.4 m from the
porpoise (34 m length of pool minus 3.6 m length of research pen), which
results in a two-way-transit time of 40.5 ms and a lag time of 18.5 ms. This
lag time is comparable with the lag times found in Freja and Eigil during atb
and bta tasks.
The findings of Akamatsu et al.
(1998) that click intervals of
odontocetes in a natural surrounding are longer than those of animals kept in
pools reflect, in our opinion, the different distances to the background,
which encloses guiding landmarks in both environments. The shorter click
intervals used by animals in pools indicate that the landmarks used for
spatial orientation were closer, resulting in a shorter two-way-transit time
for the distance porpoise to landmark.
An observation of Goodson et al.
(1994) also supports our
hypothesis that odontocetes use echolocation for spatial orientation. He found
shorter click intervals when Tursiops truncatus was foraging between
pier heads. The nearby piers were potential landmarks, giving a short
two-way-transit time for the clickecho pairs, and were therefore
allowing the dolphin to shorten the click interval to keep the lag time
constant.
Our view that the click interval is chosen in relation to distance to the
background is also supported by Goold and Jones
(1995), who recorded click
intervals from sperm whales (Physeter macrocephalus) at the beginning
of a dive, which were long enough to receive echoes from the seabed before the
production of the next click. They therefore suggested that seabed proximity
may have some influence on the click rate in sperm whales.
Also, the observation of Hooker and Whitehead
(2002) that the click interval
in northern bottlenose whales (Hyperoodon ampullatus) is somehow
connected either to the distance to the research vessel or to the diving depth
points in the same direction.
Our data, but also the findings of other studies, support the postulate by
Murchison (1980) that:
"...the adoption of the word `target' into biosonar terminology may
have brought a subtle bit of inappropriate conceptual simplicity with it
because it implies that the experimenter knows the one stimulus, among all the
stimuli available, that the echolocating animal is attending to. This implies
that the stimulus defined as target by the experimenter is, at all times
during the experiment, defined as target by the echolocating animal. Analysis
of repetition rate/target-to-animal distance relationship might be affected by
these assumptions and implications".
When interpreting experimental and field data, one must remember that the
echolocation systems of odontocetes have evolved not only for the detection,
localization and classification of single targets (such as prey) but also for
spatial orientation. We even assume similar to the evolution of
echolocation in bats (Schnitzler et al.,
2003) that it is more likely that the evolution of
echolocation in odontocetes occurred in two steps; first, the evolution of
echolocation for spatial orientation and, second, a later transition for prey
acquisition. This conceptual framework calls for a new view on field data of
odontocetes orienting and foraging in different types of habitats. For
instance, animals living near the coast or in rivers (e.g. harbour porpoises
or finless porpoises) can use echolocation for spatial orientation in relation
to the background and for prey acquisition whereas pelagic animals (such as
beaked whales) can use it only for prey acquisition if they have no acoustic
contact to the bottom. This may explain differences in the use of echolocation
by pelagic and coastal odontocetes, similar to differences found in bats in
the two guilds: `open space aerial foragers' and `edge space aerial foragers'
(Schnitzler et al., 2003
).
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Acknowledgments |
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References |
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