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Social functions of cetacean acoustic communication

FUMIO NAKAHARA

College of Community Development, Tokiwa University, Miwa, Mito, 310-8585, Japan ([email protected])

SUMMARY: Acoustic communication in animals serves many functions. Acoustic signals contain information about sender identity, sender location, and particular contexts. Until recently, studies of cetacean acoustic communication have been hindered by the difficulty of localizing sounds underwater and thus identifying vocalizing individuals. New techniques integrate studies of the cetacean communication with social interaction. Cetaceans produce a large repertoire of complex vocalizations. For cetaceans, conflict resolution calls, sexual calls, social integration calls, parent-offspring calls, and environmental calls have been reported. Many cetaceans show patterns of intra-specific variation in vocalizations by individual, by group, and by geographical area. These variations are considered to have a strong correlation with social systems of the species. The individual variations are observed in those species forming fission-fusion societies. The inter-group variations are seen in the species forming stable groups. The inter-population variations are thought to have importance in maintaining group cohesion and for reproductive purpose.

KEYWORDS: cetacean, , , acoustic communication, social function, vocalization

INTRODUCTION vocalizations are still unknown. The purpose of this review is to improve the understanding of the possible Acoustic communication in animals serves many functions of their acoustic communication. functions. Acoustic signals provide information about sender identity, sender location, and particular contexts. IMPROVEMENT ON SOUND SOURCE The questions asked by receivers that may be answered by LOCATION TECHNIQUE sender signals can be classified by the contexts in which they arise.1) Thus we can distinguish between conflict Understandingthe functions of communicationsignals resolution signals, territory defense signals, sexual requiresdetailed behavioral observations concurrent with interaction signals, social integration signals, acoustical recordings. To study social functionsof parent-offspring interaction signals, environmental signals, communication,we have to identifythe senderand the and autocommunication signals.' Environmental signals receiverof a given call. Until recently,studies of the provide information to conspecifics about condition functionsof cetaceanacoustic communication have been external to either sender or receiver. Autocommunication hinderedby the difficultyof localizingsounds underwater is also categorized in acoustic communication. Animals and thus identifying vocalizing individuals. New use the differences between the emitted and received techniques integrate studies of the cetacean acoustic version of its own signal to extract information about communicationwith social interaction. Thus identification ambient conditions. of vocalizingindividuals and the groupsof cetaceanshas Cetaceans have developed a means of communication been facilitate.Multiple- recording system, using sounds, as a result of adaptation to the demand of includingmultiple-dimensional hydrophone array linked their underwater habitat.2 Sound emissions by toothed with underwater observation) and underwatermobile can be classified into two broad categories of video/acoustic system,7,8) and acoustic recording frequency: varying continuous tonal sounds referred to as attachment,such as the acoustic data logger,9,1(1) were whistles and broadband clicks including burst pulse appliedfor the recordingof cetaceansignals. sounds. 3, 4) The vocalization of baleen whales can be divided into two categories: songs and calls.4,5) Cetaceans SENDER IDENTITY produce a large repertoire of complex vocalizations. However, the communicative functions of cetacean Acoustic signals of cetaceans contain information about 299 severallevels of sender identity:individual, social group, with taxonomic relations. population,and species.Some of the best-studiedspecies includebottlenose (Tursiopstruncates), killer whales (Orcinus orca), sperm whales (Physeter macrocephalus),and humpback whales (Megaptera navaeangliae). Caldwell and Caldwell first reported on the function of cetacean acoustic signals in 1965.11) They have studied the acoustic characteristics of over one hundred captive bottlenose dolphins of all ages and both sexes, and reported that each individual dolphin in a group produces an individual-distinct whistle contour.12) These sounds came to be called "signature whistles". These "signature calls" are believed to identify the sender. Most signature calls are whistles.12) Recent studies have Fig. 1 Signature whistle exchanges in bottlenose dolphins. suggestedthat bottlenose dolphins can recognize group Sequence of two consecutive whistle where the second whistle (b) members on the basis of their signature whistles.8,13) was uttered by a different caller from the first (a). Watkins and Schevill14) found that sperm whales repeatedly produce unique click sounds, called codas, and suggest that these codas may serve as recognition codes USE OF IDENTITY INFORMATION IN for individual whales. CETACEANS Ford and his colleagues15,16)have studied group-specific dialectsof resident killer whales over a prolonged period. Animals that range widely in cohesive groups use combination of signal to coordinate group movement. Killer whales have been found to produce repetitious calls "Social integration calls" are observed in some cetacean that are now considered to be "group dialects".16) Each individual whale within pod is thought to produce the species. Signature whistles of bottlenose dolphins are used entirecall repertoire of that pod. Different pods may share to maintain group cohesion. Bottlenose dolphins use some discrete calls, but none share the same entire call signature whistles to maintain contact with individuals repertoire. McCowan et al.17) suggested that social from which they have been separated? Signature whistle familiarity influences whistle acoustic structure in adult exchanges may permit dolphins to make specific contact femalebottlenose dolphins. The coda repertoires of sperm with group members (Fig. 1).s) The basic function of whales are also reported to have dialects in the differing vocal exchange is consider to be to locate group members proportionalusage of coda types by different group.18) and to maintain within-group contact. Discrete calls of Geographical differences are associated with wide killer whales probably function as intra-group contact separated populations that do not normally mix.4) signals to maintain pod cohesion.26) "Geographic variations" are generally considered to result There are often specific signals exchanged between from acoustic adaptations to different environments, or parents and offspring("parent-offspring calls"). In most functionless by-product of isolation and genetic mammalian species, when mother and offspring are divergence caused by isolation.16) The characteristics of separated,the young produceisolation calls that are used whistle structures of bottlenose dolphins could tentially for regaining contact. During voluntary separation be used to distinguish among populations.19,20 Songs of between wild mother and calve bottlenose dolphins, humpback whales and codas of sperm whales are also whistlingcan be heardafter the calf turnedtoward reported to have geographical variations. Humpback mother. "Sexual calls" mediate a two-step process whale songs were first described by Payne and McVey.21) . Humpback whales from different ocean basins sing Mate attraction calls provide information on different songs. The structure of song changes location and availability that allows members of progressively with time, yet all whales within a breading two sexes to find and approach each other. population sing the same song and keep up with these Whereas, courtship calls determine whether changes.22) Weilgart and Whiteheadls described subsequent mating will occur and effect its geographical variation in the proportional usage of coordination.') For the sender, the function of sending a differentcodas. signal is to increase the chances that the receiver will "Species -specific characteristics" exist in whistle select that action most beneficial to the sender; for the structures of several dolphin species.23'24) The degree of receiver, the function of responding to a signal is to whistle structure differences among species correlated increase its own chances that it chooses the action best for 300

it.1) Songs of humpback whales are thought to Bottlenose dolphins are skilled at imitating whistle-like have function of mate-attraction calls.228,29)Female sounds, and whistle matching appears to play a role in humpback whales have been observed to join their natural communication system.38) While vocal with singers.30) Female and male dolphins were learning is critical for development of language in the observed repetitively broadcasting their own primate, there is very little evidence that signature whistles during attempted courtship nonhuman terrestrial are capable of vocal and mating.30 learning.39) Even though vocal development has been little There is clear correlation between the types of studied in cetaceans, there is strong evidence for vocal social bonds and the types of communication learning in several species.40) Janik41 described that bottlenose dolphins use their learned whistles in matching signals seen in different cetacean group. interactions, in which an individual responds to a whistle Individual-specific signals have been reported for of a conspecific by emitting the same whistle type. species, such as bottlenose dolphins and sperm A popular philosophical tradition supports a conceptual whales, with strong individual social bonds. requirement that communication involve Group-specific vocal repertoires have been self-awareness.42) Communication takes place when there reported for species, such as killer whales, with is an act of meaning on the part of a sender and an act of stable groups. Population-specific signals have understanding on the part of a receiver. An act of meaning been seen reported among species, such as requires an intention to secure a belief by virtue of the humpback whales, in which adults appear to receiver's recognition of the sender's intention. Reiss and have neither stable bonds nor stable groups. Marino43) reported the potential ability of self-awareness Large number of individuals seasonally in bottlenose dolphins, led by the mirror self-recognition aggregate and use vocalizations themselves to experiment. mediate joining fluid groups. COCLUSION CALLS WITH SPECIFIC BEHAVIORAL CONTEXTS Little is known about how cetaceans use their acoustic signals. Data from captive animals can provide us some Signalsproduced in the contextof "conflictresolution" information on functions of their acoustic communication, help answer questions arising during aggressive but the pattern of vocalization may different in the wild. encounters between animals.1)They are often called Current studies of cetacean acoustic communication are agonisticsignals. Most of the agonisticsignals reported advancing by the new techniques of identifying vocalizing for dolphins are burst-pulsedsounds.32) Squawks are individuals. These techniques integrate studies of the considered to have the function of threat calls in cetacean acoustic communication with social interaction. bottlenose dolphins32,33)and in spotted dolphins.31,34) Future work analyzing cetacean acoustic communication Morton35) suggested that sounds used on aggressive with respect to behavioral contexts and social interactionsin a broad rangeof birds and mammalstend relationships is needed. to be low frequency(motivation-structural rules). Pops are the threat vocalizations,which induces the female to remainclose to the poppingmale during consortships. 36) REFERENCE Morisaka20)suggested the possibilityof existence of begging calls ("parent-offspringcalls") in neonate 1. Bradbury JW,Vehrencamp SL Principles of Animal bottlenosedolphins, for the firsttime. Communication.Sinauer Associates, Sunderland. 1998. "Environmental calls" provide information to 2. NakaharaF Vocaldevelopment and vocalbehavior of dolphins. conspecificsabout conditionexternal to either sender of IBI Rep. 1998;8:43-51. In Japanesewith English abstract. 3. Evans WE. Vocalizationamong marine mammals.In: Tavolga receiver.Alarm calls indicate the presenceof predatorsor WN (ed) Marine ,Vol. 2. PergamonPress, Oxford. other threats. Whistle-squawks are emitted during 1967:159-186. behavioralcontext of inspectionaggregation (Nakahara F 4. Au WWL Hearing in whales and dolphins:an overview.In: & MiyazakiN, unpubl. Data, 1998). Food calls alert Au WWL, Popper AN, Fay RR (eds) Hearing by Whalesand conspecificsto the presenceof a shareablefood source. Dolphins.Springer-Verlag, New York.2000;1.42. 5. Clark CW. Acousticbehavior of mysticetewhales. In: TomasJ, Brays are used to manipulate prey behavior and may Kastelein R (eds) Sensory Abilities of Cetaceans.Plenum Press, attractconspecifics as a by-product37 New York.1990:571-583. 6. Gerdamke J, Costa DP, Dunstan A. Localizationand visual CURRENT TOPICS verificationof a complex minke whale vocalization.J.Acous. Soc. Am. 2001;109: 3038-3047. 01

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