Bioacoustics
The International Journal of Animal Sound and its Recording, 2009, Vol. 19, pp. 49–65
© 2009 AB Academic Publishers
CHANGES IN CLICK SOURCE LEVELS
WITH DISTANCE TO TARGETS: STUDIES
OF FREE-RANGING WHITE-BEAKED
DOLPHINS LAGENORHYNCHUS
ALBIROSTRIS AND CAPTIVE HARBOUR
PORPOISES PHOCOENA PHOCOENA
ANA CAROLINA G. ATEM1, MARIANNE H. RASMUSSEN1,
MAGNUS WAHLBERG1,2, HANS C. PETERSEN3 AND
LEE A. MILLER1*
Institute of Biology, University of Southern Denmark, Campusvej 55, 5230 Odense
M, Denmark
2 Fjord & Bælt, Margrethes Plads 1, DK 5300 Kerteminde, Denmark
3 Department of Statistics, University of Southern Denmark, Campusvej 55, 5230
Odense M, Denmark
1
ABSTRACT
Probably all odontocetes use echolocation for spatial orientation and detection
of prey. We used a four hydrophone “Y” array to record the high frequency clicks
from free-ranging White-beaked Dolphins Lagenorhynchus albirostris and captive
Harbour Porpoises Phocoena phocoena. From the recordings we calculated distances
to the animals and source levels of the clicks. The recordings from White-beaked
Dolphins were made in Iceland and those from Harbour Porpoises at Fjord & Bælt,
Kerteminde, Denmark during prey capture. We used stringent criteria to determine
which clicks could be defined as being on the acoustic axis. Two dolphin and nine
porpoise click series could be used to track individual animals, which presumably
focused on the array hydrophones or a fish right in front of the array. The apparent
source levels of clicks in the individual tracks increased with range. One individual
White-beaked Dolphin and three Harbour Porpoises regulate their output signal level
to nearly compensate for one-way transmission loss while approaching a target. The
other dolphin regulated the output differently. For most of the recordings the sound
level at the target remains nearly constant and the echo level at the animal increases
as it closes on the target.
Keywords: Echolocation, biosonar, source level, apparent source level, prey capture,
hydrophone array, White-beaked Dolphin, Lagenorhynchus albirostris, Harbour
Porpoise, Phocoena phocoena
*Corresponding
author. E-mail: lee@biology.sdu.dk
50
INTRODUCTION
All odontocetes studied thus far emit brief clicks at varying repetition
rates presumably as biosonar or echolocation signals to probe their
environment. Some important characteristics of their click signals are
the transmission beam pattern, click intervals, and source levels. The
click is emitted in a directional beam, which has been measured at
vertical and horizontal angles for some species of dolphins. For the
White-beaked Dolphin the estimated 3 and 10 dB beam widths are 8˚
and 10˚ respectively (Rasmussen et al. 2004). This is narrower than
the 10.2˚ and 22.5˚ beam measured for the Bottlenose Dolphin (Au
1993). Echolocation clicks are sent in pulsed modes. When a click is
emitted and the echo is received the next click is transmitted after a
certain lag-time (Au 2000). During aggressive communication (burst
pulses), pulse intervals can be 1 ms or less (Caldwell & Caldwell
1967; Blomqvist & Amundin 2004). Captive dolphins use decreasing
click intervals during prey capture (Morozov et al. 1972) while freeranging dolphins use a wide range of intervals from over 100 ms to
very short intervals depending on behaviour (Lammers et al. 2004;
Rasmussen & Miller 2004).
The sound pressure level of a click at 1 m from the source is
defined as the source level (Urick 1983). Source levels of clicks are often
expressed in dB re 1µPa peak-to-peak (p-p) values. These values, all
in dB re 1µPa (p-p), vary among species and have been described for
many odontocetes such as: the Pygmy Killer Whale Feresa attenuata
ranging from 197-223 dB (Madsen et al. 2004b); False Killer Whale
Pseudorca crassidens between 201-225 dB, Risso’s Dolphin Grampus
griseus 202-222 dB (Madsen et al. 2004a); Killer Whales Orcinus orca
195-224 dB (Au et al. 2004, Simon et al. 2007) and White-beaked
Dolphins Lagenorhynchus albirostris (Rasmussen et al. 2002). Source
levels around 210 dB were found for Dusky Dolphins Lagenorhynchus
obscurus (Au & Würsig 2004) and for the Atlantic Spotted Dolphin
Stenella frontalis (Au & Herzing 2003). Zimmer et al. (2005) reported
source levels of up to 214 dB for Cuvier’s Beaked Whales Ziphius
cavirostris. The highest of all source levels are those reported for
Sperm Whales Physeter macrocephalus, 240 dB re 1 µPa (p-p) (Møhl et
al. 2003). There is also great variation in source levels with distance
to the target (Au & Benoit Bird 2003) as well as amplitude variation
from a single individual depending on the ambient noise level (Au et
al. 1985).
Echolocation clicks have other properties such as peak
frequencies (frequency of maximum amplitude in the spectrum),
centroid frequency (the spectrum divided in two parts of equal energy),
-3 dB bandwidth and rms bandwidth (Au 1993; Madsen et al. 2004a).
White-beaked Dolphin clicks have average peak frequencies of 115 kHz
with a secondary peak of approximately 250 kHz, centre frequency at
51
about 82 ± 4 kHz, and -3 dB and rms bandwidths of 70±12 kHz and
36 ± 2 kHz respectively (Rasmussen & Miller 2002; 2004). The same
values for wild Harbour Porpoises are: peak frequency 129-145 kHz,
centre frequency 130-142 kHz, -3 dB bandwidth 6-26 kHz and rms
bandwidth 5-12 kHz (Villadsgaard et al. 2007).
Localization of a phonating dolphin can be achieved using
hydrophone arrays like a symmetrical “Y” hydrophone array. The “Y”
hydrophone array has one hydrophone in the centre and the other
three hydrophones at the ends of plastic pipes spaced at angles of
120˚ (Schotten et al. 2004). The distance to a sound source can be
determined by measuring the time of arrival differences of the signal
at the centre hydrophone with respect to the arrival times of the signal
at the other three hydrophones (Au & Herzing 2003). This can be done
by using the hydrophone array in a Cartesian coordinate system where
the distance between the dolphin and the centre hydrophone is R and
the horizontal and vertical angles between the dolphin and the centre
hydrophone are φ and θ (Schotten et al. 2004). Since the distances
to the animals are fairly short (a few tens of meters) the absorption
coefficient of sound in water can be neglected. Assuming the animal
directs its sonar beam to the centre hydrophone, the source level (at
1 m) can be calculated from the received level and the calculated
distance to the animal for each emitted click. If the received level is
constant over short distances then the animal is compensating for the
one-way transmission loss by halving the emitted sound level for each
halving of distance to the target (-20 Log R). Near compensation for
one-way transmission loss has been shown for several species of freeranging dolphins (Lagenorhynchus albirostris Rasmussen et al. 2002;
Stenella frontalis Au & Herzing 2003; Orcinus orca Au et al. 2004;
Lagenorhynchus obscurus Au & Würsig 2004; Tursiops sp Jensen et
al. 2009), and for a Harbour Porpoise (Beedholm & Miller 2007).
The first purpose of our study was to determine the change in
click source levels as a function of distance to a target (our hydro­
phone array) from individual, free-ranging White-beaked Dolphins
Lagenorhynchus albirostris. We used several criteria to judge if a
signal was on axis. All previous studies, save one, of source levels
of clicks emitted by free-ranging dolphins deal with populations of
animals. Our second purpose was to determine the change in click
source levels from three captive Harbour Porpoises Phocoena phocoena
as a function of distance to fish prey. The data from one Harbour
Porpoise blindfolded did not differ from those of the same animal
without being blindfolded, underscoring the sole use of biosonar during
prey capture. We found that one individual White-beaked Dolphin,
presumably focused on our array, and the Harbour Porpoises reduced
the source level to nearly adjust for the one-way transmission loss.
Determining on-axis signals from free-ranging dolphins presents
formidable problems even when applying strict criteria. Also the
52
source levels from all dolphins at distances between 5 and 15 m
nearly compensates for one-way transmission loss, similar to what is
reported earlier.
MATERIAL AND METHODS
Data recording
The recordings of White-beaked Dolphin clicks took place in Iceland
during the summer of 2003. The recording equipment consisted of a
hydrophone array (mounted on a three meter long pole) with four
matched hydrophones (TC4034, frequency range 1 Hz-350 kHz ± 3
dB, Reson, Slangerup, Denmark) at the ends of 0.5 m or 1 m plastic
pipes arranged as a symmetrical “Y” connected to a multi channel
amplifier (Etec, Copenhagen, Denmark) and from there to a “lunch
box” computer. The sounds were recorded at 800 ksamples/s on each
of the four channels simultaneously and stored in the hard drive
synchronously with video recordings from an underwater camera
mounted about 10 cm above the centre hydrophone (Rasmussen et al.
2004). The output from a Brüel & Kjær (Nærum, Denmark) piston
phone (B&K 4223) via a special adaptor was used to check the voltage
sensitivity of the hydrophones, which, in each case, did not deviate
from the specified sensitivity. Calibration files each representing
163 dB re 1 µPa rms with noted amplification were stored for later
converting to peak-to-peak (p-p) values by adding 9 dB. The recording
equipment was calibrated out to 13 m at 0˚ (in line with the centre
hydrophone). A calibration curve of the hydrophone array can be
found in Rasmussen et al. (2004). We limited the calculated distances
from 1 m to 15 m in this study.
Data analysis
The sound files were separated into four channels, one for each
hydrophone, using the software SigPro (Simon Boel Pedersen,
Copenhagen, Denmark) to confirm that clicks were recorded on all
hydrophones. The program was mainly used to identify click series
from individual animals. An animal was considered clicking towards
the array when the highest amplitude was recorded on the centre
hydrophone or equally high amplitudes on one or all of the other
three hydrophones (criterion 1 in Table 1). To identify if a dolphin
was clicking alone towards the array, a sequence should contain only
one obvious click train.
After selecting individual click trains and making sure none
of the clicks had overloaded our equipment, clicks were edited
53
TABLE 1
Five criteria used to select on-axis clicks. Criteria 1 to 4 are from
Rasmussen et al. 2004.
1)
2)
3)
4)
5)
A maximum apparent source level (p-p) on the centre hydrophone or equal to one
of the outer hydrophones
Centre frequency on the centre hydrophone should be above 85 kHz;
The vertical and horizontal angles between the dolphin and the centre
hydrophones should be within 35˚;
Dolphin should be at least 1 m from the array;
Distance swum with continuous on-axis clicks should be more than 1 m.
using CoolEdit Pro (version 2, Syntrillium Software, Phoenix, AZ,
USA). The edited click trains were then loaded into MatLab (The
MathWorks, Inc. Cambridge, MA, USA) to calculate the position of
the dolphins, apparent source levels (ASL), angles to the dolphins,
and centre frequencies (using a specially written script). The script
uses the time of click arrivals at the four hydrophones and the speed
of sound in the water to calculate the position of the vocalizing
dolphin according to Schotten et al. (2004). In this paper we use the
term apparent source level (ASL) back calculated to 1 m instead of
source level, which implies that the signal level is reported at 1 m
distance, irrespectively of angle to the source. The clicks were selected
as being on-axis by sorting them with a set of criteria (see below).
All apparent source levels (ASL) given here are expressed in dB re.
1 µPa peak-to-peak (p-p).
After analyzing each sequence carefully, we realized that clicks
in some sequences recorded from free-ranging White-beaked Dolphins
could be off-axis. Therefore, we used two more criteria to select
presumed on axis clicks for source level analysis (criteria 2 and 3 in
Table 1), a criterion to make sure the dolphins were in the far field
(criterion 4 in Table 1) and a criterion to assure a single animal was
on axis and moving toward our array (criterion 5 in Table 1).
The echolocation clicks from three captive Harbour Porpoises
(one male, Eigil, and two females, Freja and Sif) were recorded during
prey capture trials performed periodically between 2004 and 2008 at
Fjord & Bælt, Kerteminde, Denmark. The recording equipment was
the same as that used for the White-beaked Dolphin studies. During
prey capture trials, the animals were sent from one end to another
end of the pool, where a dead fish was thrown into the water 20 to 30
cm directly in front of the array. Fish captures were documented in
video recordings. (an example can be seen here-http://www.acoustics.
org/press/156th/miller.html). The female porpoise, Freja, performed
the task both with and without being “blindfolded” with suction cups
over her eyes.
54
The statistical analyses were performed using the procedure for
mixed linear models (“proc mixed”) in the SAS 9.1.3 system (SAS
Institute Inc. 2004). Individuals were considered as random effects,
with trials nested within individuals, and meter (distance to target)
was considered as a fixed continuous independent variable related
to the time of the experiment. The modelling followed standard
procedures with F and t-tests, as illustrated in West et al. (2007),
chapter 7.
RESULTS
White-beaked dolphins
In total, we analyzed 804 White-beaked Dolphin clicks from 144 click
sequences, in which only 10 sequences (134 clicks) fulfilled the first
criterion in Table 1; long click sequences from dolphins ensonifying
the centre hydrophone with the highest amplitude clicks (or equally
high). Clicks from these 10 sequences are plotted in Figure 1 and show
that apparent source levels increase with Log range (R). Figure 1 (a)
shows all the 134 clicks while Figure 1 (b) shows only those clicks
from dolphins at ranges between 5 and 12 m. There is considerable
scatter in source levels with the maximum value at about 195 dB re.
1 µPa (p-p) and a regression line with a slope of 21 Log R, or nearly
the one-way transmission loss.
Two click sequences fulfilled all five criteria in Table 1, thus
signifying click sequences from two individual dolphins. Table 2
shows the mean values for apparent source levels, maximum distance,
distance swum, centre frequencies and the vertical and horizontal
angles between the dolphin and the hydrophone. The two sequences
are depicted in Figure 2 showing that apparent source levels decrease
while the dolphins approach the array. Apparently, dolphin 1 (black
triangles in Figure 2) follows rather closely the one-way transmission
loss and dolphin 2 does not. The two click sequences were recorded
on different days, so they were emitted most likely from two different
dolphins.
Harbour Porpoises
The source levels and distance swam for three Harbour Porpoises
performing prey detection and capture tasks are show in Figures 3 to
5, where one Harbour Porpoise, Freja, performed the task blindfolded
as well (Figure 5b). Similar to the White-beaked Dolphin graphs,
source levels decrease with Log range in all sequences and follow
closely the one-way transmission loss. The slopes given in Figure 3 to
55
Figure 1. Apparent Source Levels (ASL) for on axis clicks from 10 free-ranging
White-beaked Dolphins as a function of distance. (a) shows all dolphin clicks
at ranges from 1 to 15 m. The regression line is given in the figure, n= 134
(b) shows only those clicks from dolphins at distances greater than 5 m. The
regression line is given in the figure, n=120. Note the 25 dB variation in
source levels.
56
TABLE 2
Apparent Source Levels (ASL), maximum distance (dmax) , distance swum (Δd),
centre frequency (Cf), vertical angle (θ) and horizontal angle (φ), for sequences 1
and 2. SD is the standard deviation.
Click
seq.
1
2
Number
of
on-axis
clicks
16
18
ASL
dmax
Δd
(dB)
(m)
(m)
(average±
SD)
177±0.7
186±3.1
8.50
12.56
1.34
2.72
Cf
(kHz)
(average±
SD)
θ
(degrees)
average±
(SD)
φ
(degrees)
average±
SD)
90.6±3.3
90.9±3.6
-27±2 -14.3±2.3
5.9±0.8
1.1±2.7
Figure 2. Apparent source levels (ASL) from two presumably different
individual, free-ranging White-beaked Dolphins as a function of range. The
regression line for dolphin 1 (▼) is given by y=26 Log R +153 dB, n= 16.
The regression line for dolphin 2 (•) is given by y=73 Log (R) +111 dB, n=
18. Apparently dolphin 2 is not focusing on the array even though all of the
signals were classified by us as being on axis. See text for further details.
5 are not significantly different (p > 0.2) with the common slope being
20.4 Log(R), 95% confidence interval 17.0 to 23.9. The common slope
was not significantly different from the 20 Log(R) slope (p > 0.5), but
was significantly different from the zero and 40 Log(R) slopes (p <
0.0001 for each). However, the dB values at the y-intercepts shown in
Figures 3 to 5 are significantly different (p = 0.0008) with an average
y-intercept of 147 dB. Freja emitted the most intense clicks with an
average y-intercept of 149 dB. The average y-intercept for Sif was
147 dB and that for Eigil was 146 dB.
57
Figure 3. Apparent source level (ASL) as a function of range during prey
capture by a male Harbour Porpoise, Eigil. The data cover three different
tracks recorded during three different years (symbols). The regression line is
given in the figure, n= 45.
Figure 4. Apparent source level (ASL) as a function of range during prey
capture by a young female Harbour Porpoise, Sif. The data cover two tracks
made during different years (symbols). The regression line is given in the
figure, n= 15.
58
Figure 5. Apparent source level (ASL) as a function of range during prey
capture by an older female Harbour Porpoise, Freja. The data in (a) cover
three tracks made during different years (symbols) and those in (b), with
Freja blindfolded, cover two tracks made during different years (symbols).
The regression line in (a) is given in the figure, n= 35. The regression line
in (b) is given in the figure, n= 39. (See text for more information.)
59
DISCUSSION
Previous studies on four species of delphinids, including a study of
White-beaked Dolphins using different equipment, showed that source
levels are higher at greater distances and that they decrease with
decreasing range (Rasmussen et al. 2002; Au & Herzing 2003; Au &
Benoit-Bird 2003; Au et al. 2004; Au & Würsig 2004). The same is
reported in this study (Figure 1). In addition, presumed individual
White-beaked Dolphins show the same behaviour (Figure 2).
If the emitted signal is constant in amplitude, the echo
level at the dolphin will increase with decreasing range since the
target strength is constant. Therefore, it is possible that a dolphin
approaching a target will compensate for this increase in echo level
by decreasing the level of its outgoing echolocation signal as suggested
in earlier studies. Similar to the studies cited above, the majority of
click amplitudes in our study decrease in accordance with a one-way
transmission loss or close to a 20 Log R function. With a decrease in
distance, the amplitude of the echoes will double at the dolphin’s ear
if it attenuates the signal by 6 dB per distance halved (-20 Log R),
which maintains a constant ensonification level on the target. Au and
Benoit-Bird (2003) suggested that this reduction in source level with
decrease in range represents a time-varying automatic gain control
(AGC). Another possibility would be for the dolphin to compensate
for the two-way transmission loss (40 Log R) thus keeping the echo
level constant at the ear (Rasmussen et al. 2002; Au & Benoit-Bird
2003). The source levels of White-beaked Dolphin clicks in our study
had a maximum of 26 Log R (95% confidence interval) (except for one
animal described below) and thus were far from compensating for the
two-way transmission loss.
Dolphin echolocation signals are emitted in a directional beam,
and signals recorded distant from the axis of the beam (off-axis clicks)
are distorted, with distorted signal waveforms, attenuation of higher
frequencies components, and decreasing peak frequencies (Au 1993;
Au & Nachtigall 1997). Simon et al. (2007) suggested that when using
only the criteria of highest (or equally high) amplitude at the centre
hydrophone, some of the clicks may be off-axis and were recorded
from a direction where the apparent source level was not changing
with off axis angle. And, as pointed out earlier (Madsen et al. 2004b,
Madsen & Wahlberg 2007), there is still no analytical method that
can discriminate exactly on and off axis clicks. The highest amplitude
at the centre hydrophone is definitely the major indication of on-axis
clicks. However, when more conservative criteria were considered,
such as centre frequency, angles between dolphin and the equipment
and others (Table 1), many clicks that were previously considered on–
axis were discarded. On the other hand, these conservative criteria
will exclude weak on-axis clicks (Madsen & Wahlberg 2007). Spectral
60
parameters are influenced by signal level (Au 1993) and choosing a
centre frequency above 85 kHz could have excluded on-axis clicks
with lower intensities from our data set.
After using all five criteria in Table 1, only two click sequences
were found from presumed single individuals. The first sequence of
clicks was short and emitted from a dolphin that swam only 1.3 m.
But even so, a slight decrease in source level with decrease in range
could be calculated, confirming the tendency observed in our results
using a liberal criterion. The clicks from this individual fell close to
the one-way transmission loss curve in a similar manner to what was
already described for other dolphin species, including White-beaked
Dolphin (Rasmussen et al. 2002; Au & Herzing 2003; Au & BenoitBird 2003; Au et al. 2004; Au & Würsig 2004).
Just because a dolphin’s clicks pass all our criteria for being onaxis does not mean the animal is interested in our recording array.
The second sequence from an individual dolphin seen in Figure 2
shows that apparent source levels decreased drastically (72 Log R)
with decreasing change in range, even though the range was not long
(≈ 3 m). In this case the clicks did not fall anywhere close to the
one-way or two-way transmission loss curves, and this pattern does
not seem to be from a dolphin interested in our array. This dolphin
did not regulate its output level either by 6 or 12 dB per distance
halved, but by 22 dB per distance halved while approaching the
array. Nevertheless, the dolphin still reduces its output signal while
approaching a “target”, just like in the first sequence. Either the
dolphin was not interested in the array, or there is more plasticity in
the source level-to-range regulation than that indicated from analyses
of pooled results and from prey capture sequences by individual
Harbour Porpoises.
The source levels given in Rasmussen et al. (2002) are about 20
dB greater than those given here, which is significant. An increase in
environmental noise caused an 18 dB increase in source levels of a
Beluga Delphinapterus leucas (Au et al. 1985). We feel that differences
in environmental and recording conditions may have influenced source
levels. Data for the Rasmussen et al. (2002) paper were collected in
1998 using a different vessel and different recording system from
that used by us in 2003 for this study. However, background noise
measurements were not made during recordings made in 1998 or
2003.
In addition, our criteria are more rigorous for selecting click
sequences that could be attributed to a single individual. Rasmussen
et al. (2002) used the first criterion from Table 1 to select on-axis
clicks and found one sequence of one dolphin looking straight to the
underwater video camera. Here we used five criteria to select on-axis
clicks and found two sequences from individual dolphins emitting
clicks towards the hydrophone array. Therefore, it is more likely that
61
the clicks presented here are on-axis than those shown in Rasmussen
et al. (2002).
The Harbour Porpoises at Fjord & Bælt were trained to capture
dead fish in front of the hydrophone array and, therefore, swam
directly towards the array. Even so signals must qualify criterion 1
in Table 1 to be considered on axis. The Harbour Porpoises in our
study reduced the level of their outgoing signals while approaching
the target (Figures 3-5). The levels of their signals decreased by about
6 dB per distance halved, closely following the one-way transmission
loss curve (20 Log R), in a similar fashion as described for the Whitebeaked Dolphins mentioned above. The regression lines of the three
porpoises were not significantly different and had a common slope of
20.4 Log R.
It was clear in our study and documented in another study
(Verfuss et al. 2009) that the inter click interval (ICI) decreases
(click rate increases) as the amplitude decreases when the Harbour
Porpoises approach the fish prey. However, we could not quantify
this relationship since not all emitted clicks during a prey capture
sequences were on the acoustic axis according to our criteria. However,
when one of the porpoises, Eigil, spontaneously changed the rate of
his echolocation clicks while stationary at a small plastic square;
the amplitude of his signals followed rather closely a 20 Log (ICI)
function (Beedholm & Miller 2007). Thus the decrease in amplitude
with increasing click rate as the animal closes on the target may
reflect limitations imposed by the sound production mechanism at
high clicks rates (Beedholm et al. 2006).
Some odontocete species do not regulate the source level in
the manner observed in the species studied here. Beaked whales
Mesoplodon densirostris and Ziphius cavirostris and the Sperm Whale
Physeter macrocephalus maintain high output levels until abruptly
changing to low levels and high click rates during the “buzz” indicating
prey capture (Madsen et al. 2002, 2005).
Besides regulating the output from the sound generator, animals
may also control their hearing abilities. Such an “auditory controlled”
AGC system has been described for the False Killer Whale Pseudorca
crassidens by recording auditory evoked brain potentials (Supin et al.
2004; 2005; 2007). These authors show that Pseudorca crassidens was
capable of regulating its hearing sensitivity, presumably by a forward
masking mechanism, thus compensating for the changes in echo level.
Pseudorca crassidens did not change the level of transmitted sonar
signals with changing target distances (Supin et al. 2007).
Two questions arise at this point. Why do some dolphins and
the Harbour Porpoise show an apparent “motor controlled” AGC while
approaching a target? Why does the false killer whale apparently
have an auditory controlled AGC and keep the transmitted signal
level constant?
62
As the White-beaked Dolphin and the Harbour Porpoise inhabit
highly reverberant environments such as near shore and shallow
waters, they are forced to find their prey in a cluttered environment. As
described by Au and Turl (1983) and Turl et al. (1991), reverberation
is the sum of echoes scattered from objects and in-homogeneities
in the medium. Reducing the level of the outgoing signal while
approaching a target would reduce echoes from “uninteresting” objects
(clutter) smaller than the target and improve the signal-to-clutter
ratio. The level of some clutter echoes may actually fall below the
auditory threshold and “disappear” from the animal’s auditory image.
Why does the level of reduction follow more closely the one-way
transmission loss? In this case the ensonification of the target (prey)
will be nearly constant and independent of distance. Provided that
the echolocation clicks of odontocete predators are sufficiently intense
(source level about 200 dB re. 1 µPa (p-p) for the Harbour Porpoise,
see Villadsgaard et al. 2007), prey that are able to hear these signals,
like several fish species in the herring family Alosinae (Wilson et al.
2008), will lack information on proximity of the predator, giving it an
advantage over the prey (Verfuss et al. 2009).
If clutter is not a problem then keeping the outgoing signal level
independent of distance for long ranges, as reported for some beaked
whales and the sperm whale (Madsen et al. 2005), will maintain a
high signal-to-noise ratio and a better image of the target. Should
these species have a central auditory AGC like that reported for the
false killer whale (Supin et al. 2007), this, in addition, might improve
target identification by providing finer sensory control and reducing
specialization for sound production.
In conclusion, the present study supports the general tendency
shown by some dolphins and the Harbour Porpoise to reduce the
apparent source level while closing in on a target. This source level
regulation may improve signal-to-clutter ratios, since it would be
advantageous to reduce clutter echoes while approaching a target.
Since the signal attenuation closely follows the one-way transmission
loss, a constant ensonification level is maintained on a prey target
and a prey that can hear odontocete signal frequencies cannot sense
the change in distance to an approaching porpoise or dolphin.
We are only beginning to uncover the biosonar world of freeranging odontocetes. Especially important are comparative studies
of species in different habitats to determine the flexibility of their
biosonar. For example, are there really differences in biosonar
systems of coastal species contra pelagic species and if so how are
the biosonar systems adapted to specific environments? Are there
odontocete/prey interactions or do odontocetes enjoy unhindered
access to prey species? The increasing use of archival tags
(Jones et al. 2008) attached to individual odontocete species will
profoundly increase our understanding of how these echolocators
63
use their biosonar during orientation and prey capture in different
environments.
ACKNOWLEDGMENTS
We thank the field assistants, and especially Helga Ingimundardóttir
for her help in Iceland. Thanks to Dr. Peter T. Madsen, Aarhus
University, and anonymous referees for valuable suggestions to
improve the manuscript. Data for Harbour Porpoise source levels
were obtained during courses in Marine Mammal Biology held at
the Marine Biology Research Centre and Fjord & Bælt, Kerteminde,
Denmark. The first author would like to thank the Biological Institute
for financial support and her family and friends. The Harbour
Porpoises are maintained by Fjord & Bælt, Kerteminde, Denmark,
under Permit No. J.nr. SN 343/FY-0014 and 1996-3446-0021 from the
Danish Forest and Nature Agency, Danish Ministry of Environment.
We acknowledge the staff at the Fjord & Bælt for their cooperation.
REFERENCES
Au, W. W. L. & Turl, C. W. (1983). Target detection in reverberation by an echolocating
Atlantic bottlenose dolphin (Tursiops truncatus). J. Acoust. Soc. Am., 73, 16761681.
Au, W. W. L., Carder, D. A., Penner, R. H. & Scronce, B. L. (1985). Demonstration of
adaptation in beluga whale echolocation signals. J. Acoust. Soc. Am. 77, 726-730.
Au, W. W. L. (1993). The Sonar of Dolphins. New York: Springer-Verlag.
Au, W. W. L. & Nachtigall, P. E. (1997). Acoustics of echolocating dolphins and small
whales. Mar. Fresh. Behav. Physiol., 29, 127-162.
Au, W. W. L. (2000). Hearing by whales and dolphins: An overview. In Hearing by
Whales and Dolphins (Ed. by W. W. L. Au, A. N. Popper & R. R. Fay), pp. 1-42.
New York: Springer-Verlag.
Au, W. W. L. & Herzing, D. (2003). Echolocation signals of wild Atlantic spotted
dolphin (Stenella frontalis). J. Acoust. Soc. Am., 113, 598-604.
Au, W. W. L. & Benoit-Bird, K. J. (2003). Automatic gain control in the echolocation
system of dolphins. Nature, 423, 861-863.
Au, W. W. L., Ford, J. K. B., Horne, J. K. & Allman, K. A. N. (2004). Echolocation
signals of free-ranging killer whales (Orcinus orca) and modeling of foraging
Chinook salmon (Oncorhynchus tshawytscha). J. Acoust. Soc. Am., 115, 901-909.
Au, W. W. L. & Würsig, B. (2004). Echolocation signals of dusky dolphins
(Lagenorhynchus obscurus) in Kaikoura, New Zealand. J. Acoust. Soc. Am., 115,
2307-2313.
Beedholm, K., Miller, L. A. & Blanchet, M. A. (2006). Auditory brainstem response
in a harbor porpoise shows lack of automatic gain control for simulated echoes. J.
Acoust. Soc. Am., 119, 41-46.
Beedholm, K. & Miller, L. A. (2007). Automatic gain control in harbor porpoises?
Central versus peripheral mechanisms. Aquat. Mamm., 33, 67-73.
Blomqvist, C. & Amundin, M. (2004). An acoustic tag for recording directional pulsed
ultrasound aimed at free-swimming bottlenose dolphin (Tursiops truncatus) by
conspecifics. Aquat. Mamm., 30, 345-356.
64
Caldwell, M. C. & Caldwell, D. K. (1967). Intraspecific transfer of information via the
pulsed sound in captive odontocetes cetaceans. In Les Systems Sonars Animaux
Biologie et Biunique (Ed. by R. G. Busnel), pp 879-936. Jouy-en-Josas: Laboratoire
de Physiologie Acoustique.
Jones, B. A. Stanton, T. K. Andone, C. L. Johnson, M. P. Madsen, P. T. &
Tyack, P. L. (2008). Classification of broadband echoes from prey of a foraging
Blainville’s beaked whale. J. Acoust. Soc. Am. 123. 1753-1762.
Jensen, F. H., Beider, L., Wahlberg, M., and Madsen, P. T. (2009) Biosonar adjustments
to target range of echolocating bottlenose dolphins (Tursiops sp.) in the wild.
J. Exp. Biol. (in press)
Lammers, M. O., Au, W. W. L., Aubauer, R. & Nachtigall P. (2004). A comparative
analysis of echolocation and burst-pulse click trains in Stenella longirostris. In
Echolocation in Bats and Dolphins (Ed. by J. A. Thomas, C. F. Moss & M. Vater),
pp. 414-419. Chicago: Univ. of Chicago press.
Madsen, P. T., Payne, R., Kristiansen, N. U., Wahlberg, M., Kerr, I. & Møhl, B. (2002).
Sperm whale sound production studied with ultrasound time/depth- recording tags.
J. Exp. Biol. 205, 1899-1906.
Madsen, P. T., Kerr, I. & Payne, R. (2004a). Echolocation clicks of two freeranging, oceanic delphinids with different food preferences: false killer whales
Pseudorca crassidens and Risso’s dolphins Grampus griseus. J. Exp .Biol., 207,
1811-1823.
Madsen, P. T., Kerr, I. & Payne, R. (2004b). Source parameter estimates of echolocation
clicks from wild pygmy killer whales (Feresa attenuata). J. Acoust. Soc. Am., 116,
1909-1912.
Madsen, P. T., Johnson M., Aguilar DeSoto N., Zimmer W. M. X & Tyack P. (2005).
Biosonar performance of foraging beaked whales, Mesoplodon densirostris. J. Exp.
Biol., 208, 181-194.
Madsen, P. T. & Wahlberg, M. (2007). Recording and quantification of ultrasonic
echolocation clicks from free-ranging toothed whales. Deep-Sea Research I, 54,
1421-1444.
Morozov, V. P., Akopian, A. I., Burdin, V. I., Zaitseva, K. A. & Sokovykh, Y. A. (1972).
Tracking frequency of the location signals of dolphins as a function of distance to
the target. Biofizika, 1, 139-145.
Møhl, B., Wahlberg, M., Madsen, P. T., Heerfordt, A. & Lund, A. (2003). The
monopulsed nature of sperm whale clicks. J. Acoust. Soc. Am., 114, 1143-1154.
Rasmussen, M. H. & Miller, L. A. (2002). Whistles and clicks from white-beaked
dolphins, Lagenorhynchus albirostris, recorded in Faxafloi Bay, Iceland. Aquat.
Mamm., 28, 78-89.
Rasmussen, M. H., Miller, L. A. & Au, W. W. L. (2002). Source levels of clicks
from free-ranging white-beaked dolphins (Lagenorhynchus albirostris Gray 1846)
recorded in Icelandic waters. J. Acoust. Soc. Am., 111, 1122-1125.
Rasmussen, M. H., Wahlberg, M. & Miller, L. A. (2004). Estimated transmission beam
pattern of clicks recorded from free-ranging white-beaked dolphins (Lagenorhynchus
albirostris). J. Acoust. Soc. Am., 116, 1826-1831.
Rasmussen, M. H. & Miller, L. A. (2004). Echolocation and social signals from whitebeaked dolphins, Lagenorhynchus albirostris, recorded in Icelandic waters. In
Echolocation in Bats and Dolphins. (Ed. by J. A. Thomas, C. F. Moss & M. Vater),
pp. 50-53. Chicago: University of Chicago press.
SAS Institute Inc. (2004). SAS/STAT® 9.1 User’s Guide. Cary, NC, USA: SAS
Institute Inc.
Schotten, M., Au, W. W. L., Lammers, M. O. & Aubauer, R. (2004). Echolocation
recordings and localization of wild spinner dolphins (Stenella longirostris) and
pan tropical spotted dolphins (S. attenuata) using a four hydrophone array. In
Echolocation in bats and dolphins. (Ed. by J. A. Thomas, C. F. Moss & M. Vater),
pp. 393-400. Chicago: University of Chicago press.
65
Simon, M., Wahlberg, M. & Miller, L. A. (2007). Echolocation clicks from killer whales
(Orcinus orca) feeding on herring (Clupea harengus) in Norwegian waters. J.
Acoust. Soc. Am., 121, 749-752.
Supin, A. Ya., Nachtigall, P. E., Au, W. W. L & Breese, M. (2004). The interaction of
outgoing echolocation pulses and echoes in the false killer whale’s auditory system:
Evoked potential study. J. Acoust. Soc. Am., 115, 3218-3225.
Supin, A. Ya., Nachtigall, P. E., Au, W. W. L. & Breese, M. (2005). Invariance of
evoked-potential echo-responses to target strength and distance in an echolocating
false killer whale. J. Acoust. Soc. Am., 117, 3928-3935.
Supin, A. Y., Nachtigall, P. E. & Breese, M. (2007). Evoked-potential recovery during
double click stimulation in a whale: A possibility of biosonar automatic gain control.
J. Acoust. Soc. Am., 121, 618-625.
Turl, C. W., Skaar, D. J. & Au, W. W. L. (1991). The echolocation ability of the beluga
(Delphinapterus leucas) to detect targets in clutter. J. Acoust. Soc. Am., 89, 896901.
Urick, R. J. (1983). Priciples of Underwater Sound. New York: McGraw-Hill.
Verfuss, U. K., Miller, L. A., Pilz, P. K. D. & Schnitzler, H.-U. (2009) Echolocation by
two foraging harbour porpoises (Phocoena phocoena). J. Exp. Biol. 212 (In Press).
Villadsgaard, A., Wahlberg, M. & Tougaard, J. (2007). Echolocation signals of wild
harbour porpoises, Phocoena phocoena. J. Exp. Biol., 210, 56-64.
West, B.T.; Welch, K.B. & Gałecki, A.T. (2007). Linear Mixed Models. A Practical
Guide Using Statistical Software. Boca Raton: Chapman & Hall/CRC.
Wilson, M., Acolas, M.-L., Bégout, M.-L., Madsen, P. T. & Wahlberg, M. (2008). Allis
shad (Alosa alosa) exhibit an intensity-graded behavioral response when exposed
to ultrasound. J. Acoust. Soc. Am., 243EL, 1-5.
Zimmer, W. M. X, Johnson, M. P., Madsen, P. T. & Tyack, P. L. (2005). Echolocation
clicks of free-ranging Cuvier’s beaked whales (Ziphius cavirostris). J. Acoust. Soc.
Am., 117, 3919- 3927.
Received 23 December 2008, revised 18 February 2009 and accepted 27 February
2009