Vocalizations produced by humpback whale (Megaptera
novaeangliae) calves recorded in Hawaii
Ann M. Zoidis
Cetos Research Organization, 11 Des Isle Avenue, Bar Harbor, Maine 04609
Mari A. Smultea
Smultea Environmental Sciences LLC, 29333 SE 64th Street, Issaquah, Washington 98027
and Cetos Research Organization, 29333 SE 64th St., Issaquah, Washington 98027
Adam S. Frankel
Hawaii Marine Mammal Consortium, P.O. Box 6107, Kamuela, Hawaii, 96738-6107
Julia L. Hopkins,
and A. Sasha McFarland
Cetos Research Organization, 11 Des Isle Ave, Bar Harbor Maine 04609
Amy D. Whitt
and Dagmar Fertl
Geo-Marine, Inc., 2201 Avenue K, Suite A2, Plano, Texas 75074
共Received 17 July 2007; revised 12 December 2007; accepted 27 December 2007兲
Although humpback whale 共Megaptera novaeangliae兲 calves are reported to vocalize, this has not
been measurably veriﬁed. During March 2006, an underwater video camera and two-element
hydrophone array were used to record nonsong vocalizations from a mother–calf escort off Hawaii.
Acoustic data were analyzed; measured time delays between hydrophones provided bearings to 21
distinct vocalizations produced by the male calf. Signals were pulsed 共71%兲, frequency modulated
共19%兲, or amplitude modulated 共10%兲. They were of simple structure, low frequency 共mean
=220 Hz兲, brief duration 共mean=170 ms兲, and relatively narrow bandwidth 共mean=2 kHz兲. The
calf produced three series of “grunts” when approaching the diver. During winters of the years
2001–2005 in Hawaii, nonsong vocalizations were recorded in 109 共65%兲 of 169 groups with a calf
using an underwater video and single 共omnidirectional兲 hydrophone. Nonsong vocalizations were
most common 共34 of 39兲 in lone mother–calf pairs. A subsample from this dataset of 60 signals
assessed to be vocalizations provided strong evidence that 10 male and 18 female calves vocalized
based on statistical similarity to the 21 veriﬁed calf signals, proximity to an isolated calf 共27 of 28
calves兲, strong signal-to-noise ratio, and/or bubble emissions coincident to sound.
© 2008 Acoustical Society of America. 关DOI: 10.1121/1.2836750兴
PACS number共s兲: 43.80.Ka 关WWA兴 Pages: 1737–1746
Humpback whales 共Megaptera novaeangliae兲 produce
song, nonsong social vocalizations, and nonvocal, surface-
generated percussive sounds 共e.g., caused by breaches, ﬂuke
slaps, pectoral ﬁn slaps, etc.兲 during migration 共e.g., Kibble-
white et al., 1966; Payne and McVay, 1971; Norris et al.,
1999; Dunlop et al., 2005, 2006; Dunlop and Noad, 2007兲,
on low-latitude winter breeding/calving grounds 共e.g., Payne
and McVay, 1971; Winn and Winn, 1978; Tyack, 1981, 1982;
Darling, 1983; Silber, 1986; Payne, 1983; Helweg et al.,
1992; Au et al., 2000兲, and on higher-latitude summer feed-
ing grounds 共e.g., Thompson et al., 1977; Jurasz and Jurasz,
1979; D’Vincent et al., 1985; Sharpe et al., 1998兲. Songs are
well-documented vocalizations produced by males predomi-
nantly on the wintering grounds where they appear to be part
of breeding displays 共e.g., Payne and McVay, 1971; Winn et
al., 1973; Tyack, 1981; Darling, 1983; Helweg et al., 1992;
Au et al.
, 2000; Darling and Bérubé, 2001; Darling et al.,
2006兲. Singers are typically alone but may also accompany
other whales including a mother and calf 共e.g., Tyack, 1981;
Darling, 1983; Glockner and Venus, 1983; Darling and
Bérubé, 2001兲. Songs are characterized by continuous, re-
petitive, highly structured phrases and themes that contain
units with harmonics above 15 kilohertz 共kHz兲 and sounds
above 24 kHz 共Payne and McVay, 1971; Darling, 1983;
Payne, 1983; Helweg et al., 1992; Au et al., 2000, 2001,
2005, 2006; Fristrup et al., 2003; Potter et al., 2003兲.
On the wintering grounds, nonsong social vocalizations
have been documented among adults 共Tyack, 1981; Silber,
1986兲. In contrast to song, adult nonsong vocalizations 共i.e.,
“social sounds”兲 are produced erratically 共variable through
time, often interrupted by silent periods兲, are unpredictable,
do not contain the rhythmic, consistent, and continuous pat-
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terning of song, and are known to range from 50 hertz 共Hz兲
to over 10 kHz, with dominant frequencies below 3 kHz 共Ty-
ack, 1981; Silber, 1986兲. Early publications on nonsong vo-
calizations in surface-active groups of wintering humpbacks
coined the term social sounds to describe that speciﬁc behav-
ioral context 共Payne, 1978; Tyack, 1982, 1983; Tyack and
Whitehead, 1983; Silber, 1986兲. These social sounds have
been reported to occur predominantly in groups of three or
more adults composed of a mature female accompanied by a
male “principal escort” and other males aggressively vying
for a position near her 共Tyack and Whitehead, 1983; Silber,
1986; Clapham et al., 1992兲. Common behaviors include
chasing, charging, inﬂated head lunges, underwater blows,
aggressive contact, and fast surface traveling 共e.g., Darling,
1983; Tyack and Whitehead, 1983; Baker and Herman,
1984兲. Although the full function of social sounds in these
surface-active groups remains uncertain, they appear to in-
voke approach or avoidance responses from other whales,
depending on sex and composition 共Tyack, 1983兲. They also
seem to signal aggression or agitation among males ﬁghting
for dominance or proximity to a female 共Silber, 1986兲.
Despite numerous underwater and/or acoustic studies of
humpback whale groups containing a mother–calf pair in
Hawaiian waters 共e.g., Glockner and Venus, 1983; Tyack,
1983; Silber, 1986; Glockner-Ferrari and Ferrari, 1990; Zoi-
dis and Green, 2001; Cartwright, 2005; Zoidis et al., 2005兲,
only recently have vocalizations been attributed to individual
calves based on observational evidence and underwater
videoacoustic data recorded with a single omnidirectional
hydrophone 共Zoidis and Green, 2001; Pack et al., 2005; Zoi-
dis et al., 2005兲. Reported evidence includes vocalizations
recorded in closest proximity to a calf with no other whales
nearby, strong signal-to-noise ratio, and/or vocalizations pro-
duced coincident to underwater bubble production by the
calf. Although this is strong evidence that humpback calves
vocalize, it cannot be ruled out that the sounds did not come
from another source/whale that was nearby or not visible
共i.e., the omnidirectional hydrophone used in those studies
could not provide measurable evidence, such as directional
bearings, linking acoustic data to a source兲
. Published docu-
mentation that individual calves of other baleen whale spe-
cies vocalize is limited to one calf from each of three differ-
ent species 共the ﬁrst two of which were captive animals兲:
gray 共Eschrichtius robustus兲共Wisdom et al., 2001兲, Bryde’s
共Balaenoptera edeni兲共Edds et al., 1993兲, and North Atlantic
right whales 共Eubalaena glacialis兲共Parks and Tyack, 2005;
Parks and Clark, 2007兲.
Dunlop et al. 共2005, 2006兲 and Dunlop and Noad 共2007兲
remotely recorded nonsong vocalizations among migrating
humpback mother–calf pairs and other groups with a calf off
eastern Australia using an array of ﬁve separately anchored
hydrophone buoys; however, it was not possible to differen-
tiate which individual vocalized. Dunlop and Noad 共2007兲
collectively termed these and other nonsong vocalizations
recorded from any migrating humpback as “nonsong social
vocalizations,” although songs were also recorded. Vocaliza-
tions were further deﬁned as being internally produced by a
whale, including sounds generated from the blowhole. For
the purpose of this paper and terminology consistency, we
use the term “nonsong vocalizations” for sounds attributed to
humpback calves that did not meet the aforementioned deﬁ-
nition of song and were not percussively generated by body
contact with the water surface.
In this paper, directional acoustic data from a two-
element hydrophone array combined with underwater video
empirically conﬁrm that a male humpback whale calf pro-
duced nonsong vocalizations. Additional data indicate that
both male and female calves vocalize. Descriptions of these
vocalizations are presented and compared to earlier reports
and vocalizations from calves of other large whale species.
Contexts of some calf vocalizations are also described.
Underwater humpback vocalizations and behavior were
recorded during the winters of 2001–2006 off southwest
Maui and Kauai, Hawaii. Data were collected by one or two
snorkelers off a 6 – 8 meter 共m兲 inboard or outboard vessel
and by personnel above water. Divers used two systems: a
Panasonic GS200 and/or GS300 digital underwater video
camera equipped with a single omnidirectional hydrophone
共Biomon BM 8263-3c mounted 1 m below the camera, hy-
drophone sensitivity to 80 kHz兲, and a two-element hydro-
phone array used one day in March 2006. The two-element
array consisted of two HTI MIN-96 hydrophones 共frequency
response 2 Hz to 30 kHz兲 mounted 1.5 m apart on a bar
perpendicular to the optical axis of the camera. This conﬁgu-
ration was used to determine left–right bearings to the source
of sounds produced in mother–calf groups 共i.e., the offset of
a sound source from the plane between the two hydrophones
was measured by the time delay of the signal arriving at the
two hydrophones兲. The unit was held parallel to the water’s
surface. A zero offset would indicate that the sound source
was equidistant from the two hydrophones and located on a
vertical plane parallel to the optical axis of the camera. Veri-
ﬁcation that sound共s兲 were produced by a calf for the two-
element array was deﬁned as repeated measurements of sig-
nals within a 10° angle of the plane bisecting the two
hydrophones, with the calf centered in the video frame and
no other whales present in that plane. Frequency response
curves were obtained from the manufacturer speciﬁcations
for the hydrophones and recording equipment. Hydrophones
were calibrated by the manufacturer.
To analyze acoustic data, the recorded videotapes were
connected to an Apple Mac Mini and raw video data were
transferred to hard disk using the program
IMOVIE HD. Re-
cordings were divided into a series of video clip ﬁles. The
digital video ﬁles were aurally and visually reviewed for po-
tential acoustic signals. Visual inspection of spectrograms
was useful, particularly if signals were low frequency or
pulsed. All digital video clips with audible pulses were ex-
ported as audio interchange ﬁle format ﬁles. Clips down-
loaded from mini digital video tapes were processed using
audio editing software. The sample rate of acoustic record-
ings was 32 or 48 kHz. Single-element recordings were
downsampled by a factor of 3 to allow more detailed visual-
ization of relatively low-frequency sounds, although the
1738 J. Acoust. Soc. Am., Vol. 123, No. 3, March 2008 Zoidis et al.: Humpback calf hydrophone array recordings
original sampling rate data were used to determine the time
Audio data were reviewed in
RAVEN 1.2.1 共Charif et al.,
2004兲 in 30 s segments. Individual analog/digital samples
were plotted and reviewed as a function of time. Detected
pulses were copied and pasted into two different ﬁles to al-
low separate analyses of the same data. In the ﬁrst ﬁle, data
were low-pass ﬁltered and decimated to allow higher reso-
lution of low-frequency components. These data were used
to create spectrograms to provide descriptive measurements
of sounds. Spectrograms were calculated with a 1024 fast
Fourier transform 共FFT兲, 75% overlap, and a Hamming win-
dow. In the second ﬁle, the same original sounds recorded
with the two-element array were band-pass ﬁltered between
100 and 3000 Hz to allow more accurate measurement of
time delays of each sound between the two left and right
channels. Time delays were determined by hand-measuring
the temporal differences between a wave form’s arrival time
at each sensor in milliseconds 共ms兲. Time delays and angles
were converted to bearings relative to the sound source as-
suming far-ﬁeld propagation. Resultant bearings indicated
the angular position of the sound source relative to a plane
parallel to the camera’s optical axis and normal to the line
between the two hydrophones. These bearings are actually
cones 共or hyperbolic surfaces兲 relative to the plane parallel to
the optical axis of the camera. Video from both divers was
then examined to determine the position of the animal共s兲 in
the frame at the time of sound production. The relationship
between the time delay and the angle from the plane parallel
to the optical axis of the camera to the sound source consists
of the simple linear formula:
angle = arcsin关共⌬t ⫻ c兲/A兴, 共1兲
where ⌬t = time difference between elements 共s兲, c = speed of
sound in water 共m/s兲, and A =aperture 共distance兲 between
elements 共m兲. Any single-measured angle could represent a
source in any direction on the hyperbolic surface deﬁned by
the array geometry and the time delay differences 共not nec-
essarily the object in view of the camera, i.e., the sound
source could be directly behind the camera兲. This was ad-
dressed by conducting both underwater and surface constant
scans for animal presence and locations in relation to the
recording diver. We inferred that the source animal was the
one in front of the diver if no animal occurred behind the
diver and no animals were visually detected on the same
plane as the source animal. In addition, during calf vocaliza-
tions, no other groups were visible underwater or at the sur-
face within 300 m.
Representative spectrograms were selected from clips to
illustrate typical acoustic parameters attributable to calf vo-
calizations 共Fig. 1兲. Clips were downsampled by a factor of
8, reducing the sample rate to 6 kHz. Spectrogram illustra-
tions were made with a Hann window, a 512 FFT size, and a
98% overlap. A subsample of vocalizations with loud signal-
to-noise ratios 共10 dB, indicating close proximity兲, and data
supporting the proximal position of the calf relative to other
whales was selected for further analyses. Sounds with low
signal-to-noise ratios, repetitive sounds similar to song,
and/or sounds with more than one animal in the same plane
based on video and ﬁeld notes were discarded. Selected
sounds were divided into pulsed, amplitude-modulated
共AM兲, and frequency-modulated tonal 共FM兲 signals. Pulsed
signals had a distinct amplitude onset as opposed to the sinu-
soidal amplitude envelope of a typical AM signal. Duration
and bandwidth, initial and ending AM and FM frequencies,
number of pulses in a pulsed signal, and the ﬁrst and last
interpulse intervals were measured. It was noted if the inter-
val was constant, increasing, or decreasing with time. It was
not possible to calculate amplitude of sound from the source
animal because we did not have the precise distance mea-
surements necessary to estimate transmission loss with any
degree of certainty.
Statistical analyses were conducted to compare vocaliza-
tions produced by the one calf recorded with the two-element
hydrophone versus the selected subsample of 60 calf-
attributed vocalizations recorded with the single hydrophone.
Three measures of three signal types 共
AM, FM, and pulsed兲
were considered: lowest frequency, bandwidth, and duration.
These measures were chosen as they are independent of each
other. A mean value for each whale, measurement type, and
signal type was calculated. Means were used to avoid un-
equal representation of one animal compared to another, as
the number of vocalizations produced by each animal varied
between focal sessions. Means were compared using a mul-
tivariate analysis of variance 共MANOVA兲 , with the three
dependent measures compared for the two-element hydro-
phone array recordings of one calf in 2006 against the single-
element hydrophone recordings of 21 different calves in
Underwater behavior from the recorded video of all
whale groups was analyzed using a modiﬁed behavioral sam-
pling methodology based on Mann 共1999兲. Behavioral state,
individual behaviors, and vocalizations were noted for 30 s
periods that the calf was in view for at least 15 s. Distance
between mother, calf, and diver and depth of animals below
the water surface were estimated in mother-whale body
lengths similar to Glockner and Venus 共1983兲, assuming an
FIG. 1. Spectrograms of typical signal types attributed to a humpback whale
calf based on sounds recorded with the two-element array. Five pulses are
seen between 0.7 and 1.7 s. These are followed by a long tonal 关or slight
frequency-modulated 共FM兲兴 signal starting at 2.1 s. Above each spectrogram
is a representation of the signal wave form 共i.e., the recorded time series of
voltages from the hydrophone shown on the y axis; time on the x axis兲.
J. Acoust. Soc. Am., Vol. 123, No. 3, March 2008 Zoidis et al.: Humpback calf hydrophone array recordings 1739
average body length of approximately 13 m for a Hawaiian
mother humpback per Spitz et al. 共2000兲. For example, if a
calf was separated from its mother by two mother-body
lengths this separation distance was estimated to be approxi-
mately 26 m; the margin of error was not measured but was
believed to be approximately ⫾2 m based on the standard
deviation and range of measured body lengths of 26 mother
whales per Spitz et al. 共2000兲. The mother was identiﬁed
based on its closer consistent contact with the calf and geni-
tal morphology, the latter which was also used to determine
sex of calves 共True, 1904; Glockner, 1983兲.
Underwater video recordings of 170 groups containing a
mother-calf humpback were ﬁlmed off Maui and Kauai from
January to March for 1 – 3 months of each of the following
years: 2001, 2002, 2004, 2005, and 2006. A total of
1007 min of videotape was recorded 共mean= 5.0 min per
group, SD= 4.9, range= ⬍ 1–30 min兲. Nonsong vocaliza-
tions were recorded in 110 共65%兲 of the 170 groups with a
calf. Of the total 170 calf pods, nonsong vocalizations were
detected in 87% or 34 of 39 lone mother–calf pairs and 68%
or 76 of 111 mother–calf escorts; no calf nonsong vocaliza-
tions were detected among the 20 mother-calf pairs with two
or more escorts. Many vocalizations were of relatively low
amplitude and were not detected until later aural and spectral
analyses of video and acoustic data.
The two-element array system was used off Maui for
one day on 9 March 2006 for a total of 51 min on three
separate mother–calf groups. Quality acoustic data were ob-
tained for one group, a male calf accompanied by a mother
and escort; they were observed and recorded for 38 continu-
ous min of which 18 min was recorded with the two-element
array set up simultaneous with a second underwater video
camera and single hydrophone. The remaining 109 共95%兲
calf groups with nonsong vocalizations were recorded using
the single hydrophone-video camera set up. Nearly all 共97%兲
of these 109 groups and videotape 共96%兲 were recorded near
Maui; the remaining 3% were near Kauai. Acoustic data and
videotape were analyzed for the one calf recorded with the
two-element array and for a selected subsample of 28 calves
from 109 groups as described here. All 29 calves consisted of
different individuals based on examination of physical at-
tributes of both the calf and mother 关e.g., ﬂuke, pectoral ﬁn,
and body pigment patterns, body creases, sex, and/or scar-
ring 共e.g., Katona et al., 1979; Glockner and Venus, 1983兲兴.
The one vocalizing calf recorded by the two-element array
was male, whereas the 28 calves recorded with the single
hydrophone consisted of 10 males and 18 females. All 29
whales assessed to be mothers based on close association
with the aforementioned calves were sexed via underwater
video data as females.
A. Two-element array recording of one calf
Directional bearings were determined for 21 distinct bio-
logical sound signals from the one male calf based on mea-
sured time delays of recordings between the two hydro-
phones of the two-element array 共Table I兲. We analyzed nine
different representative video clips 共separate time segments
from the 18 min data series兲 of this encounter providing real-
time visual corroboration that the calf was the only animal
present in the area of the recorded sounds. The calf is clearly
visible in the center of all these frames. For example, a signal
identiﬁed as a grunt occurred 160.8 s into clip 15 共Fig. 2兲.
The measured time delay between the left and right hydro-
phones was 0.104 ms, corresponding to an angle of −6.14°,
just slightly offset from the plane between the two hydro-
phones. No other animals were behind the calf being videoed
or behind the ﬁrst diver, verifying that the calf in front of the
hydrophone made the sounds. A different animal behind the
ﬁrst diver is the only other scenario in which this time delay
difference and resulting acoustic signals could possibly have
been recorded. This was ruled out based on analysis of un-
derwater video taken by the second diver, by constant vessel
observer scans conﬁrming that no other animals were nearby,
TABLE I. Distribution of signal types recorded from the one humpback
whale calf with the two-element array and attributed to 28 humpback whale
calves recorded with the single hydrophone. No signiﬁcant differences were
found between sounds recorded with the two-element array vs the single
hydrophone for the three signal types based on lowest frequency, bandwidth,
and duration parameters as indicated here. AM= amplitude modulated, and
Total n 共%兲
AM 2 12 14 共17兲 F共2,1兲 = 0.1285,
FM 4 30 34 共42兲 F共2,4兲 = 0.2483,
Pulsed 15 18 33 共41兲 F共2,3兲 = 1.7988,
n=21 n=60 n=81
Excludes duplicative sounds recorded by a single hydrophone simultaneous
to the two-element array for the one calf recorded with the two-element
array in 2006.
FIG. 2. 共Color online兲 Spectrogram of an amplitude-modulated “grunt” sig-
nal produced by the humpback whale calf recorded by the two-element
1740 J. Acoust. Soc. Am., Vol. 123, No. 3, March 2008 Zoidis et al.: Humpback calf hydrophone array recordings
and data indicating that the mother and escort were resting at
deeper water depth and were not within the angle offset
The types and structures of the 21 vocalizations measur-
ably attributed to the same calf via the two-element array
were predominantly pulsed signals 共71%兲, followed by 19%
FM and 10% AM signals 共Tables I–III兲. All 21 signals were
relatively short in duration 共⬍1 s; mean= 208 ms, SD= 147,
range= 89– 664 ms兲 and of predominantly low frequency
共Table II兲. Mean lowest frequency of the 21 signals was
220 Hz 共SD=391.5, range=10– 1464 Hz兲. Mean highest fre-
quency was 2221 Hz 共SD= 1861.4, range 312–7719 Hz兲.
Vocalizations had a relatively narrow bandwidth 共2004 Hz,
SD= 1891, range= 238– 7118 Hz兲 with a mean maximum
frequency of 2.22 kHz 共range=239– 7719 Hz兲. Table II
shows summary statistics for the AM, FM, and pulsed sig-
nals recorded from this calf. “Grunt” vocalizations were the
most commonly heard signal 共90% of 21兲 from this calf
based on aural analyses 共Fig. 2兲.
The calf made a total of four close 共1–5 m兲 approaches
to the diver, once during each of four surfacing bouts. During
three approaches, the calf emitted 4—8 repetitive “grunts”
and looked directly at the diver at its closest point of ap-
proach. The group 共consisting of the mother, calf, and adult
escort兲 remained in the same approximate location, resting
near the shallow 共⬍30 m兲 bottom.
Supplemental information providing a detailed descrip-
tion of this encounter, including behaviors associated with
calf vocalizations and types of vocalizations, is available at
the Cetos Research Organization website 共2007兲.
B. Single hydrophone recordings of calves
A subsample of 28 of the 109 calf groups with non-song
vocalizations recorded only with the single hydrophone was
selected for detailed analyses based on quality of videotape
and acoustic signals. These 28 groups yielded a subsample of
60 sound signals considered of reasonable quality and cir-
cumstance to be attributable to a calf using methods similar
to previous reports 共Zoidis and Green, 2001; Pack et al.,
2005; Zoidis et al., 2005兲 and data from the two-element
array as follows: 共1兲 The same 21 vocalizations made by the
calf veriﬁed with the two-element array were recorded simul-
taneously by the single hydrophone, i.e., the omnidirectional
hydrophone had coincident sounds that were veriﬁed sepa-
rately as coming from the calf by the two-element array data.
共2兲 No signiﬁcant differences were found between the char-
acteristics of three acoustic parameters of selected single-
hydrophone vocalizations and those recorded and linked to
the calf by the two-element array 共Table II兲. 共3兲 27 of the 28
calves that produced vocalizations were alone 共deﬁned as
isolated with no other whales in view within at least 15 m for
at least 15 min and no humpback song sounds recorded兲. 共4兲
Nonsong vocalizations were detected signiﬁcantly more fre-
quently when the calf was at or near its closest point of
approach to the single hydrophone 共⬍5m兲 facing the diver/
recorder whereas the mother and/or escort were resting in
deeper water ⬎10 m from the diver and calf 共n = 49, X
=49.0, p=0.002, df =2兲. 共5兲 The vocalizations had a rela-
tively strong signal-to-noise ratio 共10 dB兲. 共6兲 Two different
calves emitted bubbles concurrent with the time of the vo-
Most 共90%兲 of the above 60 signals were short in dura-
tion 共⬍1.1 s, mean=559 ms, range=75 ms− 2.5 s兲 and of
predominantly low frequency 共overall mean low frequency
of all 60 was signals= 306 Hz, range= 10–1710 Hz兲. FM sig-
nals were most common 共50%兲 followed by pulsed 共30%兲
and AM 共20%兲 signals 共Table I兲. Of the 30 FM signals, eight
were simple upsweeps, six were simple downsweeps, eight
had near-ﬂat frequency contours, and the remaining eight
had inﬂections in their contours 共e.g., U-shaped兲. Nine of the
twelve AM sounds had no strong changes in AM frequency,
two increased, and one had a complex pattern of modulation.
The large single hydrophone AM value of the “highest fre-
quency” 共Table II兲 is the result of measuring all of the side
bands for the 12 AM sounds. The measured bandwidth of
TABLE II. Summary statistics for the amplitude-modulated 共AM兲, frequency-modulated 共FM兲, and pulsed signals for a male humpback whale calf recorded
with the two-element 共T兲, hydrophone array 共n = 21 signals兲, and 60 signals attributed to 28 humpback calves recorded with the single hydrophone 共S兲.The
mean value is provided for each measure with the standard deviation in parentheses. AM= amplitude modulated, FM= frequency modulated, Hz= hertz, and
NA NA 4
NA NA 30
NA NA 164
NA NA 384
Measured bandwidth of the AM signals is larger than that of the FM signals, as only the fundamental was measured and harmonics were excluded.
Large AM value of the category highest frequency is the result of measuring all side bands of the 12 AM sounds.
J. Acoust. Soc. Am., Vol. 123, No. 3, March 2008 Zoidis et al.: Humpback calf hydrophone array recordings 1741
TABLE III. Comparison of vocalizations produced by young of large whale species. Age classiﬁcations are
taken directly from referenced papers and may have been assessed by non-similar standards. Sample size
provided when presented in literature. AM=amplitude modulated, FM=frequency modulated, Hz=hertz, SD
=standard deviation, s=second, and MSB=mean sound bandwidth.
Species, age classiﬁcation,
Type: Bandwidth 共Hz兲
MSB, SD, range
calf vocalizations linked
with two-element array
共2006: n = 1 calf, 21
Pulsed: MSB 2359,
FM: MSB 1024,
AM: MSB 1305,
Overall range: 238–7118 Overall mean:
0.21, 共SD 0.15兲,
with single hydrophone
共2001–2005: n = 28 calves,
Pulsed: MSB 2808,
FM: MSB 531,
AM: MSB 3519,
Overall range: 140–4000 Overall mean:
with single hydrophone
共1996–2003: n = 8 calves,
Constant rate pulse:
Increasing or decreasing
rate pulse: MSB 775,
Upswept frequency tone:
Overall range: 30–3000
Pack et al., 2005
Adult humpback groups
Social sounds range:
mostly FM upsweeps
0.25– ⬎ 5
Adult male humpbacks
Song range: 20 – ⬎ 24000 Range:
Helweg et al.,
1992; Au et al.,
2000, 2001, 2005,
2006; Fristrup et al.,
Gray whale 共Eschrichtius
robustus兲 female calf, 1.5–7
old 共n=1 calf兲
共240 h of recordings兲
Type 1a “croak” pulses
Type 1b “pop” pulses
Type 3 “moan”
Type 4 “grunt”
Type 1a: 0.039,
Type 1b: 0.072,
Type 3: 0.44,
Type 4: 0.34,
Wisdom et al.,
North Atlantic right whale
female calf 共n =1 calf, 9
Parks and Tyack,
1742 J. Acoust. Soc. Am., Vol. 123, No. 3, March 2008 Zoidis et al.: Humpback calf hydrophone array recordings
these AM signals is larger than that of the FM signals, par-
tially because only the fundamental was measured and the
harmonics were excluded from the measurement.
We recorded unusual behavioral events when calf-
attributed vocalizations were recorded that to our knowledge
have not been previously reported. A selective representation
of these include the following: Repeated grunt series by three
separate calves that increased in amplitude followed by the
mother joining the calf from depth and “herding” the calf
away from the diver, who was less than 5 m from the calf at
the time of sound production; this type of directed movement
behavior by the mother was not seen when no sounds were
recorded. Two of these calves repeatedly produced grunt
sounds and simultaneously created underwater bubble
streams. One of these calves was recorded for a 3 : 04-min
continuous sequence of grunts, the longest duration of any
year and the farthest 共⬎30 m兲 a calf was away from its
mother who was in deeper water and not visible to the diver.
The grunts increased in tempo and coincided with repeated
bubble streams and an underwater “jaw clap.”
Nonsong vocalizations occur in all adult humpback
group compositions on the wintering grounds, including
those with a calf, as documented previously and herein 共Ty-
ack, 1983; Silber, 1986; Zoidis and Green, 2001; Pack et al.,
2005; Zoidis et al., 2005兲. For the ﬁrst time, measured time
delays between array bearings provide measurable evidence
corroborating that a male humpback calf produced 21 non-
song vocalizations as recorded with a nonstatic two-element
array mounted on an underwater video camera. Previously,
vocalizations have been attributed to individual humpback
calves on the wintering grounds based only on observational
evidence and recordings with a single-hydrophone video
camera 共including the 60 vocalizations from 28 other calves
reported herein兲共Zoidis and Green, 2001; Pack et al., 2005;
Zoidis et al., 2005兲. Although convincing, the methodology
used in these studies cannot unequivocally rule out that the
sounds did not come from another source/whale that was
nearby or not visible because one omnidirectional hydro-
phone cannot provide directional bearings spatially linking
acoustic data to a source. However, the two-element array
calf recordings combined with similarities in sound charac-
teristics between this calf and other calf-attributed vocaliza-
tions provide further substantiation that both male and fe-
male humpback calves vocalize.
Nonsong vocalizations appear more common among
wintering humpback calf pods than previously reported.
Herein, nonsong vocalizations were recorded in 65% of 170
calf pods. Silber 共1986兲 recorded nonsong social sounds in
only 7% of 14 mother–calf–escort groups and in none of
seven lone mother–calf pairs. Pack et al. 共2005兲 attributed 49
vocalizations to eight humpback calves off Maui 共the number
of calf pods that did not vocalize was not reported兲. In addi-
tion, we found nonsong vocalizations more common in lone
mother–calf pairs than among mother–calf–escort groups.
Similarly, Zoidis and Green 共2001兲 reported a signiﬁcantly
higher vocalization rate 共10.9 vocalizations per whale per
hour兲 among 12 mother–calf pairs compared to 50 mother–
calf–escort groups 共4.4兲, 25 surface-active groups of three or
more adults with no calves 共4.2兲, and seven adult dyads
共1.7兲. In contrast, both Silber 共1986兲 and Pack et al. 共2005兲
reported nonsong or calf vocalizations primarily among
mother–calf–escorts. Per Pack et al. 共2005兲, ﬁve of eight
calves that vocalized were escorted by a mother and one
escort including one singing escort; the remaining three
groups with such sounds were a mother–calf pair, one
mother–calf with two escorts, and one lone calf 共the deﬁni-
TABLE III. 共Continued.兲
Species, age classiﬁcation,
Type: Bandwidth 共Hz兲
MSB, SD, range
juvenile 共n = 1 calf, 233
Discrete pulse range:
Pulsed moan range:
Edds et al.,1993
Bryde’s whale calf 共n =1
calf, 36 calls兲
Discrete pulse series
Edds et al.,1993
Mean, MSB, median, SD, and/or range are reported if available. Studies did not always report the same units
Maximum recording equipment frequency was 10 kHz.
Produced by 1.5 months of age.
Produced at 7 months of age.
1– 2 year old animal
10 was indicated simply as “duration” in Edds et al. 共1993兲 and appears to be a range
compared to other numbers presented in the original document table.
Recorded with an omnidirectional hydrophone on two occasions when only ﬁrst-year free-ranging calves were
present at the surface, whereas an adult companion was diving. The discrete pulses were loudest when a calf
was close to the hydrophone and thus may have been produced by a calf 共Edds et al., 1993兲. Discrete pulses
were recorded only near these individual calves during opportunistic studies of free-ranging Bryde’s whale
J. Acoust. Soc. Am., Vol. 123, No. 3, March 2008 Zoidis et al.: Humpback calf hydrophone array recordings 1743
tion, i.e., duration of how long a calf was “alone” was not
indicated兲. Five of these calves were males and three were
females 共Pack et al., 2005兲. In comparison, we attributed
vocalizations to 11 male and 18 female calves.
Calf vocalizations recorded with the two-element array
were simple in structure and limited in repertoire with short
duration and interpulse interval, and predominantly low fre-
quency and relatively narrow bandwidth. These characteris-
tics were not signiﬁcantly different from other vocalizations
attributed to humpback calves in this study and are similar to
acoustic characteristics reported in earlier reports of calf vo-
calizations 共Zoidis and Green, 2001; Pack et al., 2005; Zoi-
dis et al., 2005兲共Table III兲. Calves appear to produce se-
quences of the same sound in some instances. Repeated
grunt bouts were made by the two-element-array recorded
calf and were also recorded near calves by the single hydro-
phone. Similarly, Pack et al. 共2005兲 reported that four calves
produced sound sequences at a mean interval of 4.4 s, the
spectral characteristics of which were similar to the grunt
sounds we recorded 共Fig. 2兲. In addition, vocalizations were
documented simultaneous to bubble emissions from the
blowhole for both the two-element array recorded calf as
well as calves recorded with the single hydrophone herein
and in Pack et al. 共2005兲 共Figs. 1 and 2兲.
Pack et al. 共2005兲 found that calf-attributed vocaliza-
tions were statistically similar to “noncalf social sounds”
共terminology from Pack et al. 2005兲 but were signiﬁcantly
different from adult song units; the former were noted to
have a narrower frequency range and shorter duration 共Table
III兲. Our results for calf vocalizations based on both the two-
element array and single hydrophone recordings generally
corroborate these ﬁndings, as do earlier reports that song is
more complex than nonsong adult and calf vocalizations
共Silber, 1986; Zoidis and Green, 2001; Zoidis et al., 2005兲.
In addition, pulsed signals were frequent among calves com-
pared to the known repertoire of pulsed sounds produced by
adult humpbacks in Hawaii 共e.g., Payne and McVay, 1971;
Darling, 1983; Payne, 1983; Silber, 1986; Helweg et al.,
Au et al., 2000兲. However, the most common type of
signal recorded by the single hydrophone was FM signals, as
found by Silber 共1986兲 for groups of three or more adults off
Maui 共Table III兲. Most AM signals we recorded near calves
had no strong evidence of changes in AM frequency. In con-
trast, social sounds of surface-active adult humpback groups
typically exhibit considerable modulation in the AM fre-
quency, producing the sensation of a frequency sweep 共Sil-
Underwater video documenting behaviors and surround-
ings concurrent with two-element array recordings were im-
portant in providing empirical real-time evidence that a calf
produced vocalizations. This approach allows an animal to
be monitored visually and acoustically 共i.e., it remains in
view兲 for extended periods, so that underwater behaviors can
be continuously recorded concurrently with vocalizations.
Previous studies 共e.g., Tyack, 1981, 1983; Darling, 1983; Sil-
ber, 1986兲 recorded nonsong sounds among adult humpback
whales by deploying a single hydrophone from a small ves-
sel. More recently, Dunlop et al. 共2005, 2006兲, and Dunlop
and Noad 共2007兲 recorded nonsong vocalizations in Australia
remotely from shore using a stationary array. No directional
data linking vocalizations to individuals were presented in
these previous studies.
The function and biological signiﬁcance of calf vocal-
izations are unknown. Several potential hypotheses are pro-
posed. Some types of calf vocalizations may elicit the moth-
er’s approach. In most cases, a calf closely 共⬍5to10m兲
approached the diver either silently or with nonsong vocal-
izations during a surfacing bout, with no reaction from the
mother. However, when on three occasions a calf emitted
repeated grunt vocalizations, with increasing incidence and
amplitude, twice with bubbles and once with an accompany-
ing jaw clap, the mother surfaced quickly, approached, and
seemingly intentionally “herded” the calf directly away. Cer-
tain sounds 共e.g., FM signals, repetitive grunts, particularly
those that increase in amplitude兲 and/or bubble streams may
function as either isolation or “alarm” calls by the calf to
alert and/or call the mother. Alarm or isolation calls occur
among mother-young groups of other mammalian species in-
cluding dolphins 共reviewed in Tyack, 2000兲, bats 共Balcombe,
1990兲, and primates 共Robinson, 1982兲. The calf may also
produce vocalizations when encountering a novel stimulus
共i.e., a close encounter with a diver or boat兲, or as unidirec-
tional contact communication from the calf to the mother
关i.e., where only one individual recognizes the call of the
other and not visa versa 共Torianni et al., 2006兲兴. Documen-
tation is lacking as to whether humpback mothers or other
adults use sound to communicate with calves. To date, the
published evidence has not shown this.
A few studies have been able to individually link vocal-
izations to young of other baleen whale species including
gray 共Wisdom et al., 2001兲, northern right 共Parks and Tyack,
2005; Parks and Clark, 2007兲, and Bryde’s whales 共Edds et
al., 1993兲; however, some of these studies have involved
captive animals and may not be representative of free-
ranging animals 共Table III兲. Although the vocalizations them-
selves are quite different between species and are limited to a
few individuals, their overall characteristics are generally
similar to humpback calf-attributed vocalizations in terms of
simplicity, limited repertoire, short duration, and predomi-
nantly low and narrowband frequency, when compared to
adults of the same species 共Edds et al., 1993; Wisdom et al.,
2001; Parks and Tyack, 2005; Parks and Clark, 2007兲共Table
III兲. A captive juvenile Bryde’s whale produced primarily
pulsed moans with amplitude and frequency modulation, and
on two occasions, free-ranging isolated Bryde’s calves were
linked with series of 4–11 discrete pulse calls 共Edds et al.,
1993兲. Series of pulsed vocalizations 共i.e., pulse trains兲 were
also commonly produced by a captive gray whale calf 共Wis-
dom et al., 2001兲 as well as an entrapped juvenile humpback
共Beamish, 1979兲 and humpback calves reported herein and
by Pack et al. 共2005兲 共Table III兲. Published individual sound
conﬁrmation of Northern right whale calves is limited to one
free-ranging female based on bearing data collected with an
acoustic array on the feeding grounds 共Parks and Tyack,
2005; Parks and Clark, 2007兲. This calf produced stuttered
“screamlike” calls interrupted by many pauses when alone at
the surface as the other adults in the group dove. The calls
1744 J. Acoust. Soc. Am., Vol. 123, No. 3, March 2008 Zoidis et al.: Humpback calf hydrophone array recordings
were generally of higher pitch and often longer in duration
than adult female “screams” used to initiate surface-active
adult groups 共Parks and Tyack, 2005兲.
The biological signiﬁcance of vocalizations produced by
humpback calves and how it may change depending on so-
cial structure, environmental cues, ontogeny, and behavior,
as well as whether mothers vocalize, remains to be further
investigated. Circumstances of calf-attributed vocalizations
recorded with a single hydrophone provide strong indication
that the calf is most likely the sound source rather than the
mother, including in lone mother–calf pairs. Compelling ob-
servational data include vocalizations recorded coincident
with bubble emissions by humpback calves during both two-
element and single hydrophone recordings, similar to hump-
back whale calves and a juvenile reported elsewhere
共Beamish, 1979; Zoidis and Green, 2001; Pack et al., 2005;
Zoidis et al., 2005兲 A captive juvenile Bryde’s whale emitted
bubbles only when vocalizing, throughout the vocalizations,
and only from the right blowhole 共Edds et al., 1993兲. Dol-
phins sometimes emit bubbles simultaneous to sound pro-
duction 共Dahlheim and Awbrey, 1982; McCowan, 1995; Mc-
Cowan and Reiss, 1995; Dudzinski, 1996; Herzing, 1996;
Killebrew et al., 1996, 2001兲, although this relationship is
not consistent 共Caldwell et al., 1990; Fripp, 2005兲. However,
it cannot be ruled out that some sounds recorded by the
single hydrophone were produced by the mother without cor-
roboration from directional acoustic data. Northern right
whale mothers vocalize to their calves when they become
separated, presumably to reunite the two 共Parks and Clark,
2007兲. Similar contact calls may occur between humpback
mothers and calves but have not yet been recorded or iden-
tiﬁed, possibly due to low-amplitude source levels that are
difﬁcult to detect except at close range. The relatively low
amplitude of calf vocalizations we recorded may be why
calves have only recently been reported to vocalize 共Zoidis
and Green, 2001兲.
Further analyses of the acoustic characteristics and be-
havioral context of calf vocalizations are underway, includ-
ing their role as a potential indicator of stress, with implica-
tions for management concerns in areas with elevated
anthropogenic activity and underwater noise such as Hawaii.
The authors thank Randy Bates, Charles Bishop, Dan
DenDanto, M. Green, Carol Hart, Thea Jenson, Terry Mc-
Cabe, Don Moses, Dan Shapiro, Ethan Silva, Jessica Shar-
man, J. Pantukhoff, and Norbert Wu for their assistance and
support with ﬁeld portions of this project. They thank Pete
Gehring, Thomas F. Norris, Shannon Rankin, Carol Spencer,
P. Stevick, and Jenelle Black for reviewing this manuscript
and/or for their editorial or technical input. They thank all
our donors and patrons that have funded this work so that
this publication was possible. They are also grateful to E. M.
Blair, J. Dennis, S. Katona, D. Kearney, P. Tadd, A. Waldron,
and A. Wright for their support and encouragement over the
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