Bioacoustics
The International Journal of Animal Sound and its Recording, 1997, Vol. 8, pp. 47-60
0952-4622/97 $10
© 1997 AB Academic Publishers
ACOUSTIC COMMUNICATION SIGNALS OF
MYSTICETE WHALES
PEGGY L. EDDS-WALTON
Mari.ne Biological Laboratory, Woods Hole, MA 02543 and Department of Biology,
University of California, Riverside, CA 92521*
ABSTRACT
"Vtysticete (baleen) whales produce a variety of vocalizatiom; and sounds, but relatively few
;[these have been well described with accompanying behavior. This review concentrates
on the vocalizations consistently associated with behavioral interactions or acoustic
exchanges between or among conspecif1cs. These communication "signals" have been
categorized for this review as contact calls of single animals out-side of the breeding season
(including cow-calf pairs), vocalizations reported during the breeding season (often
designated as "songs"), and calls produced by active groups of whales that may or may
not have a reproductive function. While much remains unknown, the data obtained thus
far indicate that the social vocalizations of baleen whales have structural/functional
similarities with those of other mammals and birds.
Key words: baleen whales, communication, voealization, contact calls, song
INTRODUCTION
The Mysticeti (baleen whales) differ from the Odontoceti (toothed
whales) in both morphology and life history due to their filter-feeding
habits. Most baleen whales undertake annual migrations from high
latitude, summer feeding areas, to lower latitude, winter breeding areas
with warmer waters but little, if any, of their preferred food. On tbe
feeding grounds, the activities of both juvenile and adult individuals
are influenced greatly by prey distribution and availability. On
breeding grounds, the activities of sexually mature animals are more
likely to be influenced by the presence of potential mates and potential
competitors for mates. Vocalizations are known to be an important
component of behavior in both areas, and during migration, for many
species of baleen whales. For animals that range so widely across
'ceans, the advantages of communication across many kilometers has
*Send correspondence to: Dr. P. L. Edds~\-Yalton, Department of Biology, Spieth Hall,
University of California, Riverside, CA, 92521.
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no doubt been a strong selective force in the vocalizations of these
species.
Studies of vocal communication in a variety of vertebrates have
shown that _frequency . CO!llP2.I1<l.!lts and temporal charact~J:i§hf<§,
including duration and intercall intervals (the silent periods between
vocalizations), are important sources of "information" (e.g. Busnel 1963,
1968, Morton 1977). Amplitude variations provide limited potential for
information content because reverberation and echoes can distort
amplitude modulations and only the louder portions of the call will
travel well. Long distance communication is facilitated by frequency
sweeps within the call and numerous. repetitions of the call, both of
which can make a vocalization stand out from the background noise
(Wiley and Richards 1978),.Jrequency sweeps or broadband calls also
provide more clues for binaural sound localization than do constant
frequency, narrowband tones. Lastly, vocalizations with sharp onsets
are more easily located than those with gradual rise times (Marler
1967). Among the vocalizations of baleen whales frequency sweeps,
sharp onsets, and variations in temporal patterns are likely to be
important for communication as in birds and terrestrial mammals.
In most studies of communication, the behavioral significance of
a call is determined initially by observing natural interactions between
conspecifics and quantifying the changes in behavior following a
vocalization (Busnel 1968). In addition, a call "type" can be reproduced
and the "artificial" call altered in various ways to determine what
components are necessary to get the predicted response from a conspecific. This kind of study has not been attempted with baleen whales
as yet.
Even more difficult to determine are the components of a
vocalization that carry "information" from the sender to the receiver,
The species of the caller as well as behavioral state (as simple as "I am
available for interaction" versus "do not approach'' or "go away") and
individual identification ("' am a particular individual of this species")
are potentially encoded in the vocalization (see Emlen 1972, Schleidt
1976). These kinds of studies are very difficult to undertake and
interpret when working with large marine mammals. Excluding any
other potential source of stimulation beyond the experimental stimulus
is extremely difficult in the field, and repeated experimentation under
controlled laboratory conditions with filter feeding animals 6-30 m long
has not been possible.
Acoustic recordings have been made of all baleen whale species;
however, few of these species have been well studied both behaviorally
and acoustically. Many recordings are from remote hydrophones, either
floating (e.g. Navy sonobuoys) or bottom-mounted, with no concurrent
behavioral observations. Even with behavioral observations,
determining whether communication is occurring between baleen
whales is obviously a daunting task.
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This review is limited to vocalizations that are associated with
social behavior in baleen whales. Descriptive terms used bere (e.g.
growl) are those of the original authors and tend to represent the aural
impression of the human listener rather than the spectral
characteristics. When possible, the spectral characteristics will be
included. For each of the vocalizations considered here, observers
reported either consistent behavioral interactions associated with these
vocalizations or an acoustical response from a conspecific. Given this
distinction, the vocalizations may be called communication "signals" as
defined by Busnel (1963). Vocalizations will be described within three
contexts: contact calls produced by siJlgle animals outside of the
breeding season; calls produced during the winter breeding season;
calls produced by active groups that may or may not be reproductive in
nature.
In this paper, a contact call will be defined as a call produced by
a single whale, an adult of either sex or a calf, physically separated
from a conspecific, that results in approach or acoustic interaction
between the caller and a conspecific. Since we cannot determine the
intent of the caller or the respondent, the contact calls of adult animals
in areas where breeding is not known to occur may or may not have
reproductive significance. We can only observe tbat these calls result in
the interaction of previously separated animals. When calves call and
interact with a single adult, the adult is assumed to be female, but in
general the sex of the adult animals interacting acoustically in feeding
areas is not known.
In breeding areas, calls produced by an individual whale tend to
be more stereotyped and may occur in series that have longer durations
than call sequences heard in a feeding area. These call series have
been compared to bird song. In some species, the call series commonly
heard in breeding areas are also heard intermittently during migration
to the breeding areas. The primary distinction for the breeding calls is
their preponderance in breeding areas, and their rarity in feeding
areas.
Calls produced by active groups may occur in feeding areas or
breeding areas. The identity of the caller in a group is not always
obvious, but the context is consistently one in which animals are in
close proximity with much accompanying surface activity. As in contact
calls, the intent of the caller is not known, and the calls produced by
active groups are associated with social interactions that may or may
not have reproductive implications.
Vocalizations without concurrent behavioral observations will not
be covered in this review. Sounds produced by flippers or flukes are not
included since consistent behavioral responses from conspecifics have
not been documented. Readers interested in reviews of all types of
baleen whale sound production are referred to Watkins and Wartzok
(1985), Ridgway and Harrison (1985) and Clark (1990).
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CONTACf CALLS
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A landmark playback e>;periment conducted by Clark and Clark (1980)
showed definitively that southern right whales Eubalaena australis
responded to conspecific vocalizations. Southern right whales
approached an underwater speaker emitting conspecific sounds, but did
not approach the same speaker if a humpback whale Megaptera
novaeangliae song or noise were being broadcast. Clark (1982) also
provided evidence that their most common vocalization, a frequency
modulated upsweep (between 50 and 200 Hz, 0.5--1.5 sec duration, is
the contact call. Upcalls are produced primarily by single individuals
(but also by swimming groups) that join other upcalling individuals or
groups. Upcalling ceases when previously separated animals or groups
are in close physical proximity. This call type is also produced by
newborn calves, and vocal exchanges occur with their mothers when
they are visually separated (Clark 1983). Another call type described as
·"growl-like", is produced by mothers when their calves wander "several
hundred meters away"; the calf then returns to her side or the mother
goes to the calf (Clark 1983). Adult-calf pairs are usually silent when
they are in close proximity.
There is also evidence for contact calls between mother and calf
bowhead whales Balaena mysticetus. Calves are often left at the
surface while the mother dives (Wursig et a!. 1985a). During one
observation of a cow-calf reunion, series of calls were recorded as the
cow and calf approached each other. The vocalizations ceased upon
their reunion (Wursig and Clark 1990). Calls in two different
frequency ranges were heard, and Wursig et a!. (1985a) speculated
that the higher frequencies were produced by the calf. Ljungblad et al.
(1980) recorded vocalizations from a cow-calf pair for an hour. They
also described two call types with different frequency ranges. One was
of lower frequency than the other, with nearly constant frequency
(100-195 Hz, durations 0.65-2.56 sec) and evenly spaced harmonics,
which may be indicative of a pulsed call (Watkins 1967). The second
call type had a slight upsweep and higher frequencies (50-580 Hz)
with shorter durations (0.3-0.85 sec) and no harmonics. Unfortunately,
they could not distinguish one as the adult call and the other as the calf
call.
Two recordings from adult and calf Bryde's whales B. edeni
indicated that vocal exchanges are used to maintain acoustic contact
between them while the adult is feeding (Edds et a!. 1993). The adult
calls were tonal, with varying frequency modulations (90--180 Hz; 0.10.6 sec). The calls attributed to the calf were individual broadband
pulses (700--900 Hz; 25-40 msec) that were produced in series (4-11).
Alternating calls were produced by the cow and the calf when the calf
was alone at the surface and while the female approached the calf. The
calls ceased when the female and calf were reunited (Edds eta!. 1993).
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Finbacks Balaenoptera physalus vocalize in feeding areas, such
as the St. Lawrence Estuary, where they occur most often as single
animals, but they also occur as pairs whose members change over time,
indicating social interactions (Edds and Macfarlane 1987). A low
frequency, downsweeping call, generally with initial frequencies below
35 Hz and final frequencies around 20-18 Hz, is believed to be a
contact call. This call type is often referred to as the "20 Hz pulse" (see
Watkins 1981 for an historical review) because the frequency sweep is
usually centered around 20 Hz, but there is considerable variety in the
frequencies produced (Edds 1988). The 20 Hz call is commonly recorded
from single individuals, and vocal exchanges have been reported by
Watkins (1981), Edds (1988) and McDonald (1995). Although no
playback studies have been conducted with finbacks, Watkins (1981)
described the approach of a distant finback to a finback producing
downsweeps from as far as 10 km away. McDonald et al. (1995)
reported three finbacks producing downsweeps alternately while
separated by several kilometers. In that case, the downsweeping calls
appear to have been used to maintain contact among separated
individuals rather than to initiate interactions. Similar alternating calls
by three physically separated bowhead whales Balaena mysticetus
were described by Clark (1991). Similar exchanges probably occur
among individuals of other species as well but are difficult to document
since the activities of all the animals must be monitored simultaneously
to show they are communicating with each other.
Vocalizations of the gray whale Eschrichtius robustus have been
recorded in their breeding lagoons, during migration, and on feeding
grounds (Cummings et al. 1968, Dahlbeim et al. 1984, Moore and
Ljungblad 1984). Broadband sounds or "pulses" are the most common
vocalization recorded, with frequencies of 100-3,000 Hz. Pulses may be
produced singly or in series. Moans or grunts with lower frequencies
(20-200 Hz) have been reported variously as common (Cummings et a!.
1968) and. relatively uncommon (Dahlheim et a!. 1984).
To date, no gray whale vocalization has been described explicitly
as a contact call; however, there are recordings in which the occurrence
of a contact call is suggested by the behavior of the animals. Gigi, a
juvenile gray whale held in an aquarium for a year, produced "pulses"
(100-10,000 Hz, in series of 2 sec duration) and "clicks" (2-6 kHz, 1. 2 msec duration) when released in the vicinity of a pod of migrating
gray whales (Fish et a!. 197 4), but the behavioral significance of the
sounds was not apparent. Norris et al. (1977) reported hearing Hpulses"
after releasing a calf that had been captured for radio-tagging. The
mother was about 300 m away. She swam directly toward the calf, and
the two were reunited. In addition, Norris et a!. (1977) heard "sharp
clicks" from two male calves both while they were stranded and during
the approach of the mother gray whale to her calf. The clicks ended
when the mother and calf were reunited. Therefore, two hroadband call
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types, a lower "pulse" and a higher "click", occurred in contexts in which
a contact call (or separation call) would be expected.
WINTER (BREEDING) VOCAL!7ATIONS
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Tembrock (1963) described different classes of sounds produced by both
terrestrial and marine mammals. Vocalizations associated with
"pairing" were described as having a different "phonetic character" from
other sounds in the repertoire. In addition, pairing vocalizations were
described as sequences of sounds that are often rhythmic and may be
produced in stanzas. Patterned repetitions of a single sound, short
sequences of several different sounds, or long sequences of a wide
variety of sound types, have been reported for different baleen whale
species during the winter breei:ling season. Although actual copulation
has been observed very rarely, the reproductive cycles of most baleen
whales are known from whaling records of the occurrence of pregnant
females and females with calves. In those species, there are peaks in
reproduction that correspond to breeding in the winter or early spring,
with approximately a one year gestational period.
For fin backs, temporal patterning of the 20 Hz call is associated
with migratory behavior and acoustic activity in suspected breeding
areas (Watkins et al. 1987, Mellinger and Clark 1995), Watkins et al.
(1987) suggested that the patterning is a simple "song". The initial
frequency, the frequency sweep and the intercall interval are produced
consistently by an individual, and there is some evidence for individual
variation in those parameters (Mellinger and Clark 1995). It is
important to note, however, that Mellinger and Clark (1995 and
personal communication) have discovered patterned 20 Hz calls from
finbacks in northern waters during the winter breeding season.
Mellinger (pers. comm.) hypothesized that these could have been produced by nonreproductive juveniles or receptive females that did not
migrate, or that the patterned calls may have additional functions
beyond advertisement of reproductive readiness.
Watkins et al. (1987) observed finbacks while they produced
their long trains of patterned 20 Hz calls: the whales appeared to stay
at. a depth of approximately 50 m, moving little. These observations
suggest that the whale is advertising his/her location. Vocalizing
finbacks stopped vocalizing when approached by conspecifics (Watkins
et a!. 1987). This behavior may be interpreted in two ways: the caller
ceased calling because a conspecific had responded and a desired
interaction resulted; or the caller ceased calling because the respondent
was a dominant animal that discouraged calling. As is true for much of
the behavior of baleen whales, the interpretation of surface activity is
difficult.
Minke whales B. acu.torostrata may produce a patterned call in
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their winter range that is not a simple patterning of the calls recorded
in summer feeding areas. The first acoustical recording of minke
vocalizations was from an individual at a breathing hole in the
Antarctic (Schevill and Watkins 1972). The calls were downsweeping
like those of the finback, but with shorter durations (0.2-0.3 sec) and
higher frequencies (130-60 Hz). Edds (1980) recorded very similar
downsweeping vocalizations from Northwest Atlantic minkes in the St.
Lawrence Estuary. Based on nearly daily observations of single
animals, and the rarity of pairs of minke whales, Edds (1980)
suggested that the minke downsweeps may serve a spacing function in
that feeding area. Differe,nt sounds. have been reported by Winn and
Perkins (1976) (also see Thompson ef a!. 1979) from the Caribbean
where minke whales may be breeding. The Caribbean vocalizations
have shorter durations (50-,-70 msec), higher frequencies (100-200 Hz)
and more rapid repetition rates (2-7/sec). These "thump trains" are
common in the late winter and early spring (Clark and Mellinger,
personal communication) and are believed to be a reproductive call of
the minke whale, although concurrent observations of sociaVsexual
behavior and sound production have not been made to date. Individual
thump trains often can be distinguished by consistent differences in the
temporal patterning of the thumps in the series, and there are slight
variations in the frequency bands of these very rapid frequency sweeps
(too rapid to be distinguished easily by human hearing) that may also
vary with the individual caller (personal observations). If these thump
trains are associated with reproduction, interesting comparisons can be
made with the patterned pulses of the closely related finbacks.
Essentially, the minkes are repeating a series of syllables as opposed to
the finback repetition of a single syllable.
Unlike other members of the Balaenidae, there is good evidence
that bowheads "sing" (repeatedly produce 1-2 themes for up to 10 h;
Ljungblad et al. 1982; illustrated in Wursig and Clark 1990). Social
and sexual activity have been observed during the spring migration
when these recordings were made (Wursig and Clark 1990).
Individual variation is apparent in these songs as well as differences
in the song from year to year (Clark 1990). The significance of these
annual changes is not known, but the variations are of interest when
comparing the songs of different species of baleen whales. Although
the bowhead is in a different taxonomic family, the bowhead song may
be considered an intermediate on a continuum of song complexity
among baleen whales, between the simple repetition of a single call
type found in the finback and the much greater variety of sounds
repeated in the song of the humpback whale. It is important to note
here that although the structures of the songs are interesting to
compare across baleen whale species, the functions of the songs may
differ among these species.
The most complex song known among baleen whales is the song
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of humpback whales. This long (15-20 min) complex series of various
call types is produced by slow moving or stationary, solitary males
(discussed in Tyack 1981), and an occasional female (Tyack, personal
communication). A nonsinging whale may approach a conspecific
producing song, or a singer may approach another individual or group,
and the singing stops (Tyack 1981). In general, singing whales are
separated by 5 km or more (Tyack 1981), indicating that singers may
avoid other singers. Unlike the reproductive calls of most mammals, the
humpback song changes gradually during the breeding season.
Components of the song may be modified or may disappear, and new
components are added (Payne et al. 1983, Guinee et al. 1983). Given
the variety of sound types in the song, the consistent ways in which the
song changes, and the similarity of songs sung by different individuals
in a given year, the actual structure of the song is likely to have some
importance. Since singers are spaced on the breeding grounds, and no
two singers are likely to be at exactly the same place in the song at the
same time, a listener that knows the song can "follow" a particular
singer and should be able to distinguish one singer among many even
at a distance. In addition, repetition makes a caller easier to locate from
a distance because the sounds persist while the interested listener
approaches.
SOUNDS PRODUCED DURING SOCIAL BEHAVIOR
Social behavior is used here to indicate the presence of two or more
animals whose activity appears to be co-ordinated or at least interrelated, as in large active groups. In general, the animals are within
close proximity, typically less than a few body lengths. Social should
not be assumed to mean "friendly" interactions, since many observers
believe that the calls are produced during competitive interactions for
food or access to reproductive females.
Morton (1977) described motivational-structural "rules" that
appear to be common among social vocalizations of both birds and
mammals. Since these calls are produced when animals are nearby, the
constraints of long distance communication (Wiley and Richards 1978)
are not a selective factor. If one compares calls within a species
repertoire, the higher frequency, more tonal calls (like whistles) are
produced in friendly or appeasing contexts and harsher, lower
frequency calls (like growls) are produced by aggressive individuals. In
general, the harshness of a call increases as the arousal level of the
caller increases (Morton 1977). The species repertoire is not well known
for most baleen whales, but there are indications that these marine
mammals have similar motivational-structural rules to their social
vocalizations. For example, the harshest vocalizations, which are
generally pulsed sounds with growl-like qualities, have been recorded
during agonistic encounters.
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Of the Balaenidac, the vocalizations and behavior of the
southern right whale on its breeding grounds will be considered, since
little work has been published on right whale vocalizations from
feeding areas. The vocalizations reported by Clark (1982) vary in their
frequency content and temporal characteristics. The simplest vocalizations can be placed in discrete categories, while the more complex
calls form a continuum. Overall, the calls cover frequencies from 50 Hz
to over 1,000 Hz, with most energy below 500 Hz. Durations of
southern right whale vocalizations range from 0.5-3.5 sec, but only the
harsher social sounds have the longer durations. The tonal, upsweeping contact call, described above, is in the lower region of the
frequency range, between 50 and 200 Hz, as would be expected for a
long distance signal (Clark i982): Other tonal calls, e.g. downsweeping
or constant frequencies, occurred when animals were swimming or
mildly active. The harshest,. most complex sounds were recorded during
observations of very active groups of conspecifics. Growl-like sounds
were recorded when an adult approached a mother-calf pair (Clark
1983). Although little bioacoustical work has been published on
northern right whales Eubalaena glacialis, the evidence to date
indicates that their vocalizations are very similar in both frequency and
temporal characteristics to those of their southern hemisphere
congener, with the tonal "up call" being predominant in the
vocalizations of single animals and more complex, pulsed calls occurring
during social interactions (Clark 1990).
The vocalizations of bowhead whales have been well-studied in
Alaskan waters during the spring migration to their more northern
feeding areas and the southward fall migration (Ljungblad eta!. 1982,
Clark and Johnson 1984). A study in Canadian waters, also a feeding
area, correlated social activity and vocalizations in more detail than the
migration ·studies that were conducted primarily for population
censusing (Wursig et a!. 1982, 1985a,b). Like the other balaenids, the
most common calls of single animals are tonal (primarily 50-400 Hz, 12 sec). Upsweeps, downsweeps and calls with various inflections in
frequency are common. In social situations, more pulsed sounds are
produced, which have higher frequency components (1-2 kHz). Clark
and ,Johnson (1984) reported that the bowhead repertoire seems to
have a higher proportion of pulsive calls than the southern right
whale, but acoustic sampling was skewed to surface-active groups, so
the data may not be representative.
The best studied member of the Balaneopteridae is the Pacific
population of the humpback whale. Sounds have been recorded during
social interactions in both feeding and breeding areas. The social
sounds, like the song (see above), are very varied in both frequency
and temporal characteristics. Individual humpbacks 9 km away will
approach the playback site if sounds recorded from surface active or
feeding humpbacks are broadcast (Tyack 1983, Mobley et a!. 1988).
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Humpback social sounds recorded in the Pacific breeding areas off
Hawaii do not have a structure like song (Silber 1986). Their production varies with the level of interaction of animals, i.e. larger more
active groups produce more sounds. Tyack and Whitehead (1983) have
suggested that these large surface active groups may be males
competing for access to a single female also in the group. Listening for
a singer versus surface-active social groups may provide an interesting
choice for a reproductive female.
Finback behavior has been studied primarily in feeding areas in
the western North Atlantic (Watkins 1981, Edds and Macfarlane 1987,
Edds 1988). Finbacks produce a variety of frequency modulated tonal
calls below 100 Hz in general (Watkins 1981, Edds 1988). The most
common calls (80%) are frequency downsweeps approximately 1 sec in
duration in both the northern feeding areas (Watkins 1981, Edds 1988)
and potential breeding areas in the southern portion of their range (see
previous section, Watkins eta!. 1987). The low frequency downsweeps
(described above as the contact call) are produced intermittently by
single animals. Finbacks in pairs or large groups produce a mixture of
downsweeps with low (initial frequencies below 40 Hz) and higher
frequency downsweeps (initial frequencies of 40-100 Hz) and other call
types (e.g. constant frequency calls, upsweeps) during social interactions (Edds 1988). A finback may be able to detect existing social
groups by the variety of their calls. Also in feeding areas, harsh, grow!like calls have been recorded from finbacks in potentially agonistic
interactions by both Watkins (1981) and Edds (1988).
We do not have sufficient data to suggest that we know the
repertoire of the Bryde's whale, but there are indications that different
call types may occur in different contexts, as has been found in other
baleen whale species. Two studies in the Gulf of California have
revealed that frequency modulated, tonal moans as well as pulsed
sounds are produced by adults of this species (Cummings et al. 1986,
Edds et a!. 1993). Simple frequency downsweeps or upsweeps were
rare, but sounds with two or more frequency inflections (wavers) were
common. Since the frequency range of Bryde's whale vocalizations
overlaps those of the sympatric minke and fin back, these differences in
call structure rriay be important for species recognition. Differences in
the relative occurrence of tonal sounds (Cummings et al. 1986) versus
pulsed sounds (Edds et al. 1993) need further study. Complex pulsed
calls with varying pulse repetition rates (60-160 pulses/sec) and some
tonal components were recorded from surface-active groups (Edds et al.
1993); perhaps complex pulse sounds are indicative of active social
behavior, as is the case for right whales (Clark 1990).
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CONCLUDING REMARKS
Baleen whales produce vocalizations to initiate or maintain social
interactions like other vertebrates. The vocalizations that have been
described as contact calls among baleen whales have the characteristics
one would predict for a contact call. In the Balaenidae and
Balaenopteridae, calls that can stimulate responses from conspecifics 910 km away have frequency sweeps and are repeated. In the case of
blue whales, the very low frequency vocalizations (< 20 Hz) have
frequency sweeps of only a few Hertz, but the calls are sustained for
10-20 sec (Cummings and Thompson 1971, Edds 1982, Mellinger and
Clark 1995). Songs of humpback whales have frequency sweeps and/
or sharper, broadband components, both of which should stand out
against biological background noise and allow the listener to locate the
singer. The pulses of gray· whales are relatively brief, but stand out
from the biological background noise because of their bandwidth and
sharp onsets. Vocalizations recorded from separated cow-calf pairs (see
above) also have characteristics that make the caller easy to locate.
Responses to conspecific sounds have been demonstrated in
several mysticete species, and there is no reason to believe that other
species, for whom information is lacking (e.g. blue whales), do not
communicate similarly. The ability to communicate over long distances,
during migration and in breeding areas, has important implications for
social behavior and reproductive success for most baleen whale species.
Although we do not know the functional significance of humpback song
or patterned calls produced by fmbacks in breeding areas, the ability of
these whales to hear these vocalizations may mean the difference
between successful and unsuccessiul breeding seasons.
Mas}j:ing of baleen whale vocalizations by human generated
sound is liKely for many species (NRC Report 1995), but until we know
more about what components of their vocalizations are critical for
baleen whales to hear, we cannot assess the magnitude of that impact.
Laboratory experiments on perception are not possible with adult
bateen whales, but as Marler (1976, p. 17) wrote: "... careful study of
the structure of stimuli generated by the signalling behavior of a
species should indicate requirements that the sensory side must satisfy.
At least, it should help in asking the right question of a sensory
preparation." Careful study of the structure of vocalizations that are
important in the social behavior of baleen whales should help us to
suggest what they "need" to hear and what questions to ask, for
example, during field playback experiments.
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ACKNOWLEDGEMENTS
The author would like to thank Arthur Popper for his continued
encouragement and the Office of Naval Research for its financial
support. Discussions with David Mellinger concerning unpublished
data were appreciated. The comments of D. Mellinger and two
reviewers on an earlier version of the manuscript were also
appreciated.
REFERENCES
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Busnel, R.~G. (1963}. On certain aspe.cts of animal acoustic signals. In Acoustic Behaviour
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