The relationship of social vocalizations to surface behavior and aggression in the Hawaiian humpback whale (Megaptera novaeangliae)

GREGORY K. SILBER'

Moss Landing Marine Laboratories, Moss Landing, CA, U .S.A . 95039 Received January 7 , 1986

SILBER, G. K. 1986. The relationship of social vocalizations to surface behavior and aggression in the Hawaiian humpback whale (Megaptera novaeangliae) . Can. J. Zool. 64: 2075-2080.

Humpback whale (Megaptera novaeangliae) social vocalizations (nonsong sounds) were clearly related to whale group size and surface activity. Social sounds occurred almost exclusively in groups containing three or more whales and were rarely heard near single whales, pairs, or cow-calf groups. Large groups (3 to 20 individuals) vocalized at an overall mean rate of 43.1 + 55.52 sounds per whale/h. Group size changed frequently and a dramatic increase in vocalization rate resulted when a new whale entered a group. Large groups engaged in flurries of surface activity, such as breaching, flipper- and tail-slapping, and under- water bubbling. Aggressive encounters resulted from male-male interaction. Social sounds probably acted to demonstrate aggression or agitation as adult males competed for temporary social dominance within the group and for proximity to the female. Likewise, visual displays may have been used as threats in close quarters and were apparently produced in conjunction with sounds to convey levels of aggression. Although other studies suggested that surface activity increased with group size, I found a negative correlation between activity and group size, both in the group as a whole and per individual. In contrast, social vocali- zations per group increased with group size while the vocalization rate per individual did not vary significantly with increasing group size.

SILBER, G. K. 1986. The relationship of social vocalizations to surface behavior and aggression in the Hawaiian humpback whale (Megaptera novaeangliae). Can. J. Zool. 64: 2075-2080.

Les cris a fonction sociale (sons qui ne font pas partie de chants) chez le rorqual megaptere (Megaptera novaeangliae) sont clairement relies a la taille du groupe et a l'activitk de surface. Les cris sociaux sont produits presque exclusivement chez les groupes contenant trois rorquals ou plus et ne s'en tendent que rarement chez les rorquals isoles, les couples ou les groupes formCs par des mere et ses petits. Les groupes importants (3 a 20 individus) produisent des cris a un taux moyen de 43,l * 55,52 cris par rorqual/h. Le nombre d'individus dans un groupe change frequemment et il se produit une importante augmentation du taux d'kmission de cris lorsqu'un rorqual se joint au groupe. Les grands groupes entreprennent des phases d'activite de surface: sauts, claquements des nageoires et de la queue en surface et production de bulles sous l'eau. Les comportements agressifs sont le resultat d'altercations mile-mile. Les cris sociaux servent probablement a manifester de l'aggressivite ou de l'agitation quand les miles adultes entrent en competition pour la dominance sociale temporaire du groupe ou pour une place pres de la femelle. De meme, les manifestations visuelles servent peut-itre de menaces dans des territoires restreints et s'accompagnent de produc- tion de cris de fason a donner une impression d'aggressivite. Bien que d'autres etudes aient indiquk une augmentation de I'acti- vite de surface en relation avec l'augmentation du nombre d'individus dans un groupe, j'ai constate une correlation negative entre I'activitC et la taille d'un groupe, chez les individus et chez le groupe dans son ensemble. En revanche, les cris sociaux dans un groupe augmentent lorsque le groupe devient plus grand, alors que le taux de production de cris chez un individu ne varie pas significativement en fonction d'une augmentation du nombre d'individus dans le groupe.

[Traduit par la revue]

Introduction In the winter months, humpback whales (Megaptera

novaeangliae) aggregate in the waters surrounding the Hawaiian islands. During this period the humpback produces a complex series of sounds called "songs" (Payne and McVay 1971). The humpback also emits another very different type of vocalization, called "social sounds" (Payne 197 8; Tyack 1982). Social sounds differ from songs in several ways. Unlike songs, social sounds are produced in both the northern and southern latitudes (Thompson et al. 1977; Payne 1978). Songs are rhythmic and continuous (Payne and McVay 197 1) and singers are lone stationary adult whales (Winn and Winn 1978; Tyack 1982; Darling 1983). In contrast, social sounds are variable through time and do not exhibit a consistent or continuous pattern. Individual humpbacks frequently interrupt activities, such as singing, and travel considerable distances to join groups that are producing social sounds (Tyack 1983; personal observa- tion). Tyack (1983) demonstrated the dramatic positive response of humpbacks to social sounds during playback experiments.

'present address: Institute of Marine Sciences, Applied Sciences Building, University of California, Santa Cruz, CA, U.S.A. 95064; and the West Coast Whale Research Foundation, c/o Long Marine Lab, University of California, Santa Cruz, CA, U.S .A. 95064.

In Hawaiian waters, humpbacks form groups varying in size and composition, including large groups (3 to 20 individuals) that are highly active at the surface. Numerous authors have characterized the structure, composition, and behavior of large, active groups (Baker et al. 198 1; Baker and Herman 1984; Glockner-Ferrari and Ferrari 198 1, 1983; Darling 1983; Mobley and Herman 1985; Tyack and Whitehead 1983). Agonistic be- havior attributed to male-male interaction has been documented in large groups (Baker et al. 198 1 ; Baker and Herman 1984; Glockner-Ferrari and Ferrari 1983; Darling 1983; Tyack and Whitehead 1983).

The function of social sounds and their relationship to surface behavior and group size are unknown. However, this paper explores these relationships in humpback whales wintering near Hawaii.

Materials and methods Social sounds were recorded off West Maui, Hawaii, in January-

April 198 1, and February- April 1982. Behavioral observations and underwater recordings were obtained from small craft. A Nakamichi 550 portable casette recorder was used in conjunction with Gould CH- 17U, Aquadyne AQ- 17, or sonobouy hydrophones and a Barcus Berry preamplifier. The hydrophones were deployed at depths ranging from 10 to 16 m depending upon the individual hydrophone in use. One channel of the casette was used for underwater recordings while the

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2076 CAN. J . ZOOL. VOL. 64, 1986

second was used to report time, group size estimates, and surface behavior using a hand-held microphone. Local weather conditions, the approximate distance of the whales from the hydrophone, and the presence of cetaceans other than humpback whales were noted.

Since groups of active humpbacks moved rapidly, observations and recordings were obtained by frequently repositioning the boat. The craft was stationed in front of a moving group and recordings were made as the whales approached. All observations and recordings were made when the whales were within 200 m of the boat.

A social sound was defined as any phonation that did not possess the rhythmic and continuous patterning of song. Each surface activity other than normal respiration was tabulated as a single event. The following definitions were used to categorize surface activities.

1. Head-up: While swimming at the surface, the whale rapidly arched its back dorsally and lifted its head from the water as it continued forward.

2. White water: This included any activity producing water turbu- lence that resulted in splashing. Many activities fell into this category when the exact behavior was not seen or when the whale or whales were mostly submerged.

3. Underwater blow: The whale produced an exhalation before the nares cleared the water.

4. Vertical flukes: The whale was positioned parallel to the surface, but was oriented on its side, so that a single fluke emerged from the water. The whale waved its flukes rapidly, describing an "S" arc.

5. Tail lob: The whale was oriented with its head down; it raised the posterior third of the body from the water and pounded the flukes onto the surface.

6. Tail lash: The whale was positioned parallel to the surface with its flukes horizontal to the surface; the flukes were waved quickly from side to side.

7. Loud blow: A wheezing sound that accompanied an otherwise normal respiration. This sound had both surface and underwater components (Watkins 1967) which were similar in character.

8. Breach: The whale jumped head first from the water, landing on its side or back.

9 . F l ipp ing : The whale waved its pectoral flipper in the air and slapped it repeatedly onto the water surface.

10. Bubble-stream: The whale released air from the mouth or blow- hole in a continuous trail.

11. All other behavior not included above. All data were divided into 5-min blocks during analysis. A model I

analysis of variance (ANOVA) (Zar 1974) was used to determine the significance of differences in phonation and surface activity rates relative to group size. A Student-Newman-Keuls (Zar 1974) test was used to determine if significant differences existed in vocalization rates between groups of differing size. A Spearman rank correlation (Zar 1974) was used to determine the correlation between vocalization rates and group size.

Results S u ~ a c e activity

Whales in large groups produced a wide variety of obvious surface and aerial displays (Fig. 1) that were rare in other social groups. While some surface behavior such as underwater blows and bubbling were relatively benign, others such as vertical flukes and tail lashes were clearly directed at the head and body of other individuals. Violent collisions were common between whales. White scar tissue and bleeding abrasions were observed frequently on the heads and backs of whales in large groups. The sounds of body contact were distinctly audible underwater and were interspersed with vocalizations. These groups were also characterized by short dive times and fast, unpredictable swimming patterns. Surface displays were produced at an overall mean rate of 1 1.7 + SD 1 1.50 per whale/h. There was no significant difference in the mean surface display rate relative to group size (F = 0.0052, p > 0.50); however, there was a negative (but not significant) correlation ( r = -0.60, p >

Frequency occurrence

1 2 3 4 5 6 7 8 9 1 0 1 1

Type of Surface D~splay

FIG. 1. Frequency of occurrence of surface displays in decreasing order of occurrence. See text for behavioral descriptions.

0 1 I I I I I 1

3 4 5 6 7 8 +

Group S ~ z e

Fig. 2. The number of surface displays relative to group size. Mean + one standard error and sample sizes are indicated.

0 1 I I I I I I 3 4 5 6 7 8 +

Group Size

FIG. 3. The number of surface displays per whale relative to group size. Mean + one standard error and sample sizes are indicated.

0.50) between surface displays and increasing group size (Fig. 2). Likewise, no significant relationship existed between displays per individual and group size (F = 0.125, p > 0.25), but there was a significant negative correlation of displays per individual with increasing group size ( r = - 1.00, p = 0.01 ; Fig. 3).

Some surface activities were interpreted as highly aggressive because they commonly involved striking another group mem- ber with the tail flukes or intentional collisions with the head or upper body. These activities included vertical flukes, tail lash, tail lob, white water, and head-up. Less aggressive behavior included underwater blows, bubble streams, loud blows, breaches, and flippering.

Social sounds Humpback social sounds differed from songs in various

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SILBER

TABLE 1. A comparison of humpback song versus social sounds

Type of comparison Song Social sounds

Social context

Group size Behavioral context

Group composition Geographic location

Acoustic features

Vocalization rate (sounds per whale/h)

Little or no interaction with others while singing*

1 whalet Submerged; little movement;

no surface activity*

Male* Heard only in wintering regions11

Continuous; rhythmic, repeated, predictable* *

Continuous interaction with conspecifics

3-20 whales Long periods at surface;

constant movement; dramatic surface displays; aggression

Mostly males, a few females9 Heard in both summering and

wintering regions7 Variable; flurries of sound, long

silences; no known patterns

*Tyack 1982; Darling 1983. tin most cases singing whales are alone (Winn and Winn 1978; Tyack 1982; Darling 1983). A few exceptions have been noted

(Glockner-Ferrari and Ferrari 1983). $The gender of singing whales has been determined on numerous occasions (Winn et al. 1973; Darling 1983; Glockner-Ferrari and Ferrari

1983), and all have been males. There is at least one known exception in which a female was observed singing (Darling 1983). QWinn and Winn 1978; Glockner-Ferrari and Ferrari 1981, 1983; Darling 1983. ion occasion singing has been recorded in the northern latitudes of both North American coasts (L . Guifiee and D. McSweeney, personal

communication; C . Mayo, personal communication), but this is unusual. 7Payne 1978; Thompson et al. 1977; Chabot 1984. **Payne and McVay 1971. t tThis figure was derived from data included in Payne and McVay (1971) and Payne et al. (1983).

TABLE 2. The vocalization rate and time devoted to study of each humpback group type

Time under Vocalization rate, No. of groups % of N that observation no. per whale/h studied (N) vocalized (h) (.i k SD)

Large active group (3-20 individuals) 49

Cow and calf 7 Cow-calf and

escort 14 Two adults 49 Single adult

(nonsinging) 34

*Vocalization rate not obtained.

ways (Table 1). While some sounds were common to both types of vocalization, the two differed markedly in temporal struc- ture. Humpback song was continuous, rhythmic, and predic- table, while social sounds were erratic and occurred in sudden flurries of sounds interspersed by long silences. The two vocalization types also occurred in highly contrasting social contexts. The song was generally produced by lone, relatively stationary adults (Winn and Winn 1978; Tyack 1982; Darling 1983), whereas social sounds were heard almost exclusively in large groups (3-20 individuals; Table 2) that engaged in flurries of high activity at the surface. Cow-calf groups and single nonsinging adults were never heard to produce social sounds (Table 2). While the latter were probably not completely silent, it was evident that they vocalized at a much lower rate. Singing whales and groups of less than three whales generally exhibited very little surface activity.

The humpback call repertory was diverse in frequency range and duration (Silber 1986). Many calls were stereotypic while others exhibited considerable variability. The latter may be viewed as a continuum of sounds rather than as discrete units.

The sounds ranged in duration from 0.25 to over 5.00 s. They exhibited a frequency range from 50.0 Hz to 10.0 kHz (the latter being the upper limit of the recording equipment). The majority of sound energy was below 3.0 kHz. Most of the "fundamental" or concentrated energy was below 2.0 kHz. Sounds with simple structure, particularly frequency-modulated upsweeps, occurred most often (Silber 1986). Sounds were often produced in multicall sequences and there was variation in the number and type of sounds used in series. The most common format was a single sound preceded and followed by at least 10 s of silence. Sounds heard in series generally possessed brief intercall periods and were short in duration. It was common to hear whales vocalizing simultaneously (Silber 1986).

Whales in surface-active groups vocalized at a rate of 43.1 2 SD 55.52 sounds per whale/h. Vocalization rates differed significantly relative to group size (F = 5.27, p < 0.0005) and there was a significant correlation between vocalization rate and group size ( r = 0.829, p < 0.05) (Fig. 4). There was a significant difference in vocalization rate per individual relative to group size (F = 2.52, p < 0.05), and there was a slight (not

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CAN. J . ZOOL. VOL. 64, 1986

TABLE 3. Change in vocalization rate when new whales entered large active groups

Before After

Observation Vocalization rate Vocalization rate Example period (min) Group size (no. per whalelh) Group size (no. per whalelh)

*Vocalization rate decreased after whale joined group.

I I I I I I

2 3 4 5 6 7 8 +

Group Size

FIG. 4. Vocalization rate relative to group size. Mean -+ one standard error and sample sizes are indicated.

0 1 I I I I I I

2 3 4 5 6 7 8 +

Group S u e

FIG. 5. Vocalization rate per whale relative to group size. Mean -+ one standard error and sample sizes are indicated.

significant) correlation between individual vocalization rate and group size ( r = 0.029, p > 0.50) (Fig. 5). The vocalization rate often increased dramatically when a new whale or whales joined a large group (Table 3).

Discussion Recent evidence indicates that large groups of humpbacks

were composed of males which compete aggressively for access to an available female (Tyack 1982; Darling 1983; Tyack and Whitehead 1983; Baker and Herman 1984). Several authors demonstrated that active groups of humpbacks consisted of multiple males and very few females (Glockner-Ferrari and Ferrari 198 1; Darling 1983). Many surface activities were the

result of persistent agonistic attempts by the males to obtain a position near a female (Baker et al. 198 1; Darling et al. 1983; Tyack and Whitehead 1983; Baker and Herman 1984). There are numerous accounts in which an adult male successfully displaced another male in its role as the primary escort to the female (Darling et al. 1983; Glockner-Ferrari and Ferrari 1983; Tyack and Whitehead 1983).

The primary escort expended a great deal of energy attempt- ing to repel other males that approached the female. Nearby males were attacked or threatened with visual displays. Visual threats are an effective means of communication while indivi- duals are in close proximity and Herman and Tavolga (1980) argued that visual signals serve an important role in cetaceans. However, locomotor signaling behavior, especially when ex- pressed in combat, can represent a substantial energy expense (Marler 1968). Vocal threats were also produced. Vocalizations are less limited by distance, require less energy to produce, and may be coupled with visual threats to convey levels of aggres- sion. The simultaneous use of two or more sensory modalities to indicate the intensity or level of agitation is well documented in the study of animal communication (see Smith 1977; Lehner 1978). Social sounds were heard rarely in nonaggressive situa- tions and were probably produced by all competing males. In contrast, visual displays may have been limited principally to the primary escort.

Observed surface activity was the result of chases, clashes, and threats of aggressively interacting males. Some activities were directly agonistic, involving body contact. It is very likely that tail flukes, particularly the hard lateral edges, were used in direct aggression to strike another whale. This behavior has been observed in humpbacks (Chittleborough 1953) and right whales (Eubalaena australis) (Donne11 1967) when striking predators such as the killer whale (Orcinus orca). One whale lifting another out of the water (Tyack 1982; Tyack and Whitehead 1983) and collisions between whales (Baker and Herman 1984) have been observed. These activities often resulted in areas of severely chafed skin along the dorsum of interacting whales. Collisions and butting occurred when one whale attempted to interpose its body between two others. The remainder of the observed activities were interpreted as threats or displays and may have served to intimidate or confuse an adversary. These displays included bubble-streaming, under- water blows, loud blows, and breaching. Bubble-streams and underwater blows are effective visual and acoustic barriers and may screen one individual from another. The execution of head-ups results in surface turbulence and may serve to obscure or confuse another individual. The head-up also allows the whale to distend its throat pleats by engulfing air. Expanded

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SILBER 2079

throat pleats act to increase the whale's size underwater (Darling 1983; Glockner-Ferrari and Ferrari 1983) which creates a larger and perhaps more intimidating profile. This behavior also allows the whale to expel air from the mouth while underwater (personal observation). Loud blows have been heard in agonis- tic situations among humpbacks (Watkins 1967; Watkins and Wartzok 1985) and were correlated with apparent levels of agitation in right whales (Clark 1983). The function of breaching is unclear although it often had an underwater acoustic component. The underwater sound and white water produced by a breach may serve a communicative purpose, but this suggestion remains untested. Loud slapping sounds were heard when the entire group was submerged, and were attributed to underwater collisions or tail strikes, although the mechanism for their production is unknown. Similar sounds have been heard in conjunction with agonistic interactions in humpbacks (Tyack 1983) and underwater slapping in right whales (Clark 1983).

The surface activity rate and the activity rate per individual was negatively correlated with group size, although the latter exhibited a stronger correlation. These data differ from previous reports which indicated that activity increased with group size (Herman 1978; Tyack 1982; Tyack and Whitehead 1983). The data suggest that a single whale was responsible for producing most of the surface displays at a rate that was independent of group size. It is likely that the primary escort was responsible for most of the displays as it threatened and clashed with other males in an apparent attempt to preclude them from approaching the female. This would account for the high number of head-ups in which the principal male attempted to butt or intercept approaching males. Similarly head-ups, bubblestreams, under- water blows, and other displays were apparently used to confuse an adversary or obscure the path to the female. Other studies attributed the majority of such activities to the principal escort (Glockner-Ferrari and Ferrari 1983; Tyack and Whitehead 1983) and my qualitative observations are in agreement.

The vocalization rate was positively correlated with group size. This suggests that each group member contributed to the overall sound production, and each whale vocalized at about the same rate independent of group size. In some groups the vocalization rate increased markedly with the addition of new whales, supporting this contention. Overlapping sounds, or two or more whales vocalizing simultaneously, were heard often, suggesting that competing males emitted acoustic threats concurrently.

I also observed large groups that exhibited very few vocaliza- tions and little surface activity. It is possible that temporary social dominance among males had been established in these groups, and the number of threats were reduced. The introduc- tion of new whales may have upset a balance of social roles, which resulted in an increase in the number of vocalizations. Presumably some of the whales that leave large groups were displaced by other males. Departing whales often began to sing, suggesting that they resorted to a different strategy to attract females (Tyack 1982) or communicate to males (Darling 1983).

Acknowledgements This study received support from the American Cetacean

Society (Los Angeles and Monterey Bay chapters), the Packard Foundation, the Sigma Xi, the Maui Whale Watchers, and the West Coast Whale Research Foundation. Drs. J . Darling, R. Payne, and B. Wiirsig deserve special recognition for their constant encouragement. Drs. J . Ford and C . Clark

provided advice and equipment during analysis. T . Cutler and B. Mathews contributed immeasurably to the fieldwork and R. Roos provided logistical support. An earlier manuscript received critical editing from B . Wiirsig , J . Oliver, R. Wells, and K. Norris. Two anonymous reviewers made helpful suggestions.

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