ACOUSTIC COMMUNICATION SIGNALS OF MYSTICETE WHALES

Transcription

1 Bioacoustics The nternational Journal of Animal Sound and its Recording, 1997, Vol. 8, pp /97 $ AB Academic Publishers ACOUSTC COMMUNCATON SGNALS OF MYSTCETE WHALES PEGGY L. EDDS-WALTON Mari.ne Biological Laboratory, Woods Hole, MA 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 NTRODUCTON 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, University of California, Riverside, CA, Department of Biology, Spieth Hall,

2 / / L. 48 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!llP2.1<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. n 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). n 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 " 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.

3 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. n 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. n 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. n 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). 49

4 50 CONTACf CALLS 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, 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 ( Hz, durations 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 ( Hz) with shorter durations ( 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 ( Hz; sec). The calls attributed to the calf were individual broadband pulses ( Hz; 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).

5 51 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 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. n 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 ( 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, msec duration) when released in the vicinity of a pod of migrating gray whales (Fish et a!. 1974), 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. n 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

6 / types, a lower "pulse" and a higher "click", occurred in contexts in which a contact call (or separation call) would be expected. WNTER (BREEDNG) VOCAL!7ATONS "! 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. n 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. n 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). t 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

7 53 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 ( sec) and higher frequencies ( 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 ( 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. ndividual 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). f 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). ndividual 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. t 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

8 54 1: '. 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). n 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. n addition, repetition makes a caller easier to locate from a distance because the sounds persist while the interested listener approaches. SOUNDS PRODUCED DURNG SOCAL BEHAVOR 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. n 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. f 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. n 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.

9 '! H li : li!! il ij,,! 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 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 Hz, 1-2 sec). Upsweeps, downsweeps and calls with various inflections in frequency are common. n 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. ndividual 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).

10 56 ' i j h,. li ' 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 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 ( 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).

11 CONCLUDNG 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. n the Balaenidae and Balaenopteridae, calls that can stimulate responses from conspecifics 9-10 km away have frequency sweeps and are repeated. n 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 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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