The male song of the Javan silvery gibbon (Hylobates moloch)

Contributions to Zoology, 74 (1/2) 1-25 (2005)The male song of the Javan silvery gibbon (Hylobates moloch)Thomas Geissmann 1 , Sylke Bohlen-Eyring 2 and Arite Heuck 21 Anthropological Institute, University Zürich-Irchel, Winterthurerstr. 190, CH-8057 Zürich, Switzerland;2 Institute of Zoology, Tierärztliche Hochschule Hannover, GermanyKeywords: Hylobates moloch, silvery gibbon, male song, individuality, calls, honest signalAbstractThis is the first study on the male song of the Javan silverygibbon (Hylobates moloch), and the first quantitativeevaluation of the syntax of male solo singing in any gibbonspecies carried out on a representative sample of individuals.Because male gibbon songs generally exhibit a higher degreeof structural variability than female songs, the syntacticalrules and the degree of variability in male singing have rarelybeen examined. In contrast to most other gibbon species,mated silvery gibbons do not appear to produce duet songbouts but solo song bouts only, and male singing is exceptionallyrare, making this study particularly challenging. For thepresent study, we tape-recorded and analysed several solosong bouts of eight silvery gibbon males, including both wildand captive individuals. Based on their frequency characteristics,song notes were classified into a total of 14 notetypes. These can be grouped into five groups (labelled Athrough E). The proportions of the various note types weredetermined individually for successive 50-note sectionsthroughout the whole song bout. Based on changes in theproportion of different note classes and note types, weroughly identify two phases in the male song: an introductoryphase, during which A and B1 notes are dominant, B3 notesare rare and C notes are absent, and a main song phase,during which B3 or C notes are dominant. The occurrenceand the proportion of various types apparently differ amongindividual males, however, both in the introductory and inthe main phase. We estimated song motivation by determiningthe “number of notes per phrase” for each 50-notesegment of the song bout. In each song bout, song motivationquickly increases during the introductory phase. Songmotivation may exhibit strong fluctuations during the mainphase of the song, but usually remains above values of 2notes per phrase and thus above the values observed duringthe introductory phase. Males appear to exhibit individualpreferences in the order of different note types used in theirphrases. Phrase structure was found to exhibit unusually lowdegrees of stereotypy and high degrees of variability. As asurprising finding of our study, male phrase variability bothwithin and between individuals appears to be higher in H.moloch than in most, perhaps all, other gibbon species. Thishigh variability appears to be a derived characteristic amongthe Hylobatidae. We discuss the implications of this findingfor the interpretation of song function and present new andtestable functional hypotheses. Our study demonstrates thatsong function cannot be identified for “the gibbon”. Gibbonsongs appear to be multifunctional, and the relevance ofthese functions appears to exhibit strong differences amonggibbon species.ContentsIntroduction ....................................................................1Material and methods.....................................................3Study animals .............................................................3Recording and analysis equipment ...........................3Acoustic terms and definitions ..................................3Data collection............................................................3Statistics ......................................................................5Results.............................................................................5Repertoire of note classes and types .........................5Frequency of note types.............................................8Inter-phrase interval .................................................12Song motivation .......................................................12Phrase composition ..................................................13Discussion.....................................................................17Male note repertoire .................................................17Composition of the song bout .................................17Phrase syntax ............................................................17Individuality .............................................................18Comparison with male singing in other gibbons....18Possible functions of male singing..........................19New functional hypotheses......................................21Conclusions ..................................................................22Acknowledgements......................................................22References ....................................................................22Appendix ......................................................................24IntroductionThe gibbons or small apes are distributed throughoutthe tropical rain forests of South-east Asia(Chivers, 2001; Geissmann, 1995; Marshall andSugardjito, 1986). They live in monogamous,territorial family groups typically consisting of anadult pair and 1-3 immature offspring (Chivers,1977, 2001; Leighton, 1987). All species ofgibbons are known to produce elaborate, loud,long and stereotyped patterns of vocalisationusually referred to as “songs” (Geissmann, 1993,1995, 2000; Haimoff, 1984; Marshall andMarshall, 1976). Preferentially, song bouts areproduced in the early morning and have a durationof about 10-30 minutes. In most gibbon species,

2 T. Geissmann et al. – The Male Song of the Javan Silvery Gibbonmated pairs utter their songs in the form of wellcoordinatedduets. In addition, mated males of thelar group (genus Hylobates) may also producemale solo songs (Geissmann, 2002a).The Javan silvery gibbon (Hylobates moloch) isone of only two gibbon species not known toproduce duet songs (the other non-duettingspecies being H. klossii) (Geissmann, 2002a). Inaddition, H. moloch is particularly unusual amonghylobatids because most of the singing isproduced by females (Geissmann and Nijman,2001; Kappeler, 1984a), and mated femalesappear to be the vocal “ ’representative’ of thefamily” (Kappeler, 1984a). In all other gibbons,either the opposite is true or the amounts of maleand female singing are about equal (Geissmann,2002a).Because male singing is exceptionally rare insilvery gibbons, studying these songs wasparticularly challenging. During 130 days inWestern Java, Kappeler heard none of his fivestudy group males produce a song and concluded“it thus appears that territorial male molochgibbons do not sing” (Kappeler, 1984a, p. 386).The only male song Kappeler heard in his studyarea was produced by a solitary male singing onthe common territory boundary of two residentgroups. Another similar song bout was heardoutside of the study area. More recent observationsin Central Java revealed that territorialmales do sing, albeit rarely (Geissmann andNijman, 2001). These authors heard choruses ofmale silvery gibbon song bouts occurring aboutonce each week on average and usually beforedawn. Male songs usually ended before thefemales started their song bouts at around dawn.Acoustic individuality has been demonstratedto exist in several gibbon species (Dallmann andGeissmann, 2001a, b; Haimoff and Gittins, 1985;Haimoff and Tilson, 1985), but these studies havebeen restricted to female singing. Female songs,as a rule, are more stereotyped than male songs(Geissmann, 1995, 2002a) and, therefore, easier toanalyse.The elusive male song of H. moloch has notpreviously been analysed quantitatively. All weknow of the song is based on short verbaldescriptions supplemented with few shortsonagrams (Kappeler, 1984a; Geissmann andNijman, 2001). An additional song of a captivemale has been described by Haimoff (1983), butthis individual was later identified as a Borneangibbon (H. muelleri) (Geissmann, 2002a).In this study, we describe the syntax of thesilvery gibbon male song based on tape-recordedsongs of several captive and wild individuals. Wecompare the variability of male song structurewithin and between individuals. Our results canbe used as a baseline for future studies onindividuality in male gibbon singing, because thesyntax and variability of male solo songs haveapparently not been studied quantitatively in arepresentative sample of individuals for any othergibbon species.Table 1. Origin, number of individuals and song bouts, and recording information of the material analysed in this study.Recording a Recording Date Individual Author of Recording Duration ofRecordingCompleteness ofSong BoutHalimun 23 Nov. 1998 “Halimun” B. Merker 14 min 43 s NoHowletts 1 17 Oct. 1988 “Omar” T. Geissmann 38 min 46 s YesHowletts 2 18 June 1995 “Hilo” T. Geissmann 35 min 52 s Yes bHowletts 3 28 Aug. 1999 “Lupau” S. Bohlen-Eyring 14 min 42 s YesJakarta 1a 8 June 1978 “Jakarta 1” M. Kappeler 7 min 31 s NoJakarta 1b 7 June 1978 “Jakarta 1” M. Kappeler 8 min 26 s NoJakarta 2 28 Jan. 2001 “Jakarta 2” T. Geissmann 40 min 45 s NoKalejetan 25 Aug.1978 “Kalejetan” M. Kappeler 2 min 17 s NoMunich 1a 21 June 1988 “Paul” T. Geissmann 12 min 15 s Yes cMunich 1b 17 Oct. 1991 “Paul” T. Geissmann 21 min 03 s YesMunich 1c 28 April 1995 “Paul” T. Geissmann 20 min 02 s YesabcRecording localities are described in the text.Song aborted upon the start of a solo song by the neighbouring female H. moloch.Song aborted upon the start of a solo song by the neighbouring female Nomascus leucogenys.

Contributions to Zoology, 74 (1/2) – 2005 3Material and methodsStudy animalsWe analysed a total of 11 male song bouts from 8different H. moloch males, as listed in Table 1.Tape-recordings were carried out at the followinglocalities: (1) Howletts Zoo Park, Canterbury,United Kingdom; (2) Hellabrunn Zoo, Munich,Germany; (3) Ragunan Zoo, Jakarta, Java, Indonesia;(4) Kalejetan, Ujung-Kulon National Park,West Java, Indonesia; (5) Cikaniki ResearchStation, Gunung Halimun, West Java, Indonesia.The latter two recordings were made on wildsilvery gibbons. In addition, one of us (TG) madeseveral tape-recordings of male songs in LinggoAsri (Dieng Plateau, Central Java). Because of thelarge distance between the observer and thegibbons, these latter tapes were not analysed indetail, but only used for qualitative comparisons,as were additional tape-recorded male songs fromMunich and Howletts.Recording and analysis equipmentGibbon songs were recorded with SONY WM-D5M and SONY WM-D6C cassette recordersequipped with Sennheiser ME80(+K3U) and JVCMZ-707 directional microphones by us, with aUHER REPORT 4200 tape recorder and a NIVICOIVC directional microphone by M. Kappeler, andwith a SONY TDC-D8 DAT-recorder and twoSONY elektret microphones equipped withparabolic reflectors by B. Merker.The sound material was digitised with a samplerate of 11 kHz and a sample size of 16 bit. Timeversus frequency displays (sonagrams) of taperecordedvocalisations were generated using theCanary software version 1.2.4 on an Applepersonal computer (Power Macintosh G3). TheFFT (Fast Fourier Transformation) size of thesonagrams was 2048 points with an overlap of75% and a frame length of 1024 points (frequencyresolution = 5.383 Hz) (Charif et al., 1995).Acoustic terms and definitionsA note is defined as “any single continuous soundof any distinct frequency (pitch) or frequency modulation,which may be produced by either an inhaledor exhaled breath” (Haimoff, 1984, p. 335).Song notes may occur either single or in moreor less brief series, which are termed phrases andfigures here. A phrase is a larger, loose collectionof several notes preferentially uttered in combination.Intra-phrase intervals are, by definition,shorter than inter-phrase intervals. Short, stereotypedsets of notes which may facultatively occurwithin phrases are termed figures (sensu Haimoff,1984).A song is what fulfils the criteria set forth byThorpe (1961, p. 15): “What is usually understoodkHz2Note durationNote duration10MaximumfrequencyStartfrequency= MinimumfrequencyEndfrequencyMaximumfrequencyStartfrequencyabEndfrequencyMinimumfrequency0 24 6 sFig. 1. Sonagram of two male song notes (a wa note and a chevron note), illustrating the measurements taken during this study.In the chevron note, “a” indicates the main or chevron part of the note, and “b” indicates the facultative end part.

4 T. Geissmann et al. – The Male Song of the Javan Silvery GibbonTable 2. Note measurements determined during the present study. Each cell includes mean value ± standard deviation, range,number of individuals and (in parenthesis) the total number of notes measured (Individuals included: Halimun, Howletts 1,Howletts 2, Jakarta 1, Jakarta 2, Kalejetan, Munich).NotetypeDuration (s) Start frequency (kHz) End frequency (kHz) Minimum frequency(kHz)A 0.178 ± 0.075 0.647 ± 0.066 0.675 ± 0.133 0.593 ± 0.0750.106 - 0.450 0.529 - 0.758 0.388 - 0.952 0.388 - 7037 (20)7 (20)7 (20)7 (20)B1 0.252 ± 0.068 0.630 ± 0.040 0.884 ± 0.072 0.627 ± 0.0390.151 - 0.410 0.551 - 0.700 0.777 - 0.998 0.551 - 07006 (18)6 (18)6 (18)6 (18)B2 0.294 ± 0.0450.222 - 0.3866 (18)B3 0.305 ± 0.0560.240 - 0.4855 (15)B4 0.259 ± 0.0600.142 - 0.3745 (15)C1 0.486 ± 0.1270.309 - 0.7226 (16)C2 0.511 ± 0.1190.317 - 0.7156 (17)C3 0.834 ± 0.1370.560 - 1.0966 (18)C4 0.778 ± 0.1560.616 - 1.0715 (15)C5 0.712 ± 0.0870.541 - 0.7703 (7)C6 0.757 ± 0.0930.608 - 0.9144 (12)C7 0.652 ± 0.1110.475 - 0.8596 (16)D 0.621 ± 0.2130.339 - 0.9124 (10)E 0.238 ± 0.0930.114 - 0,4046 (16)0.642 ± 0.0460.539 - 0.7006 (18)0.659 ± 0.0480.537 - 0.7005 (15)0.772 ± 0.0520.714 - 0.8905 (15)0.705 ± 0.0940.529 - 0.8496 (16)0.748 ± 0.1130.558 - 0.9806 (17)0.715 ± 0.1030.558 - 0.9106 (18)0.725 ± 0.1220.606 - 1.0245 (15)0.672 ± 0.0840.611 - 0.8493 (7)0.692 ± 0.1040.551 - 0.9504 (12)0.68 ± 0.1010.510 - 0.8366 (16)0.718 ± 0.0720.600 - 0.8024 (10)0.968 ± 0.160.710 - 1.2326 (16)1.152 ± 0.0811.010 - 1.2586 (18)1.298 ± 0.1091.070 - 1.5145 (15)1.110 ± 0.2090.824 - 1.5375 (15)0.673 ± 0.0940.520 - 0.8446 (16)0.719 ± 0.1100.558 - 0.9806 (17)0.787 ± 0.1490.558 - 1.0826 (18)0.692 ± 0.0970.534 - 0.8395 (15)1.135 ± 0.1740.922 - 1.3533 (7)0.880 ± 0.1990.581 - 1.1774 (12)0.674 ± 0.1210.530 - 0.9566 (16)0.682 ± 0.1160.443 - 0.8144 (10)0.810 ± 0.1540.577 - 1.1146 (16)0.641 ± 0.0460.539 - 0.7006 (18)0.657 ± 0.0480.537 - 0.7005 (15)0.771 ± 0.0530.714 - 0.8905 (15)0.654 ± 0.0790.520 - 0.7996 (16)0.701 ± 0.0980.558 - 0.9186 (17)0.672 ± 0.0740.558 - 0.8096 (18)0.655 ± 0.0780.534 - 0.8105 (15)0.668 ± 0.0850.611 - 0.8493 (7)0.672 ± 0.0760.551 - 0.7724 (12)0.632 ± 0.0850.510 - 0.8296 (16)0.649 ± 0.0930.443 - 0.7654 (10)0.808 ± 0.1530.577 - 1.1146 (16)Maximum frequency(kHz)0.714 ± 0.0940.529 - 0.9527 (20)0.896 ± 0.0710.784 - 0.9986 (18)1.173 ± 0.0881.010 - 1.2786 (18)1.380 ± 0.0731.301 - 1.5225 (15)1.151 ± 0.1990.875 - 1.5375 (15)0.955 ± 0.0330.890 - 1.0006 (16)1.199 ± 0.2800.756 - 2.0016 (17)1.264 ± 0.2940.758 - 1.9906 (18)1.233 ± 0.1710.994 - 1.5515 (15)1.321 ± 0.3270.982 - 1.9703 (7)1.136 ± 0.3610.102 - 1.4734 (12)1.077 ± 0.1740.879 - 1.4446 (16)0.801 ± 0.0940.600 - 0.8954 (10)1.064 ± 0.1690.794 - 1.3256 (16)by the term song is a series of notes, generally ofmore than one type, uttered in succession and sorelated as to form a recognisable sequence orpattern in time”, or, a song is a succession ofphrases with non-random succession probability(“Strophenfolgen mit nicht-zufälliger Folgewahrscheinlichkeit”,Tembrock, 1977, p. 33). Songbouts are separated from each other by anarbitrarily defined interval of at least 5 minutes.Some authors have referred to individualphrases within a song bout as “songs” (e.g.Marshall and Sugardjito, 1986). We follow Geissmann’s(2000) recommendation using the term“phrase” instead, in order to make it clear whetherone is talking about a mere collection of notes(usually shorter than 1 minute), or about a songbout (i.e. a collection of phrases, with an overallduration of several minutes).

6 T. Geissmann et al. – The Male Song of the Javan Silvery Gibbonfined and described below. Duration andfrequency measurements were taken on threerepresentative notes of each note type per individual.Summary measurements (mean, standarddeviation, range) are presented in Table 2. Inaddition, the table lists the number of gibbons andthe number of notes that were available for themeasurements. Because not all individuals useevery note type in their song bouts, the number ofindividuals studied may differ between note typesin the table. Typical sonagrams of all note typesand classes are shown in Figure 2.Based on their different mechanisms ofutterance, we first differentiate between expirationnotes and inspiration notes (i.e. notes which areproduced during exhalation and during breathintake, respectively). As there is no intermediateoption between these two forms of vocalisation,the two forms cannot, by definition, grade intoeach other and are, therefore, identified as twodifferent note classes. Expiration notes representthe large majority of the song repertoire.Expiration notes can be further divided in anumber of note types. They represent a gradedsystem, and intermediate forms can be foundamong the different types. Yet, because someforms are particularly common, we identifiedthem as different note types, although thesedivisions remain arbitrary. Based on similarity,expiration note types are combined into four basicgroups: oo notes (1 type), wa notes (4 types),chevron notes (7 types) and variable notes (1type). Only one type of inspiration note isrecognised in this study.Oo notesThe oo notes (Type A) are the most simple expirationnotes in the male song repertoire, exhibitingthe lowest intensity, the shortest duration (0.2±0.1s) and the smallest amount of frequencymodulation (Figure 2). The fundamental frequencyis below 1 kHz. Oo notes exhibit only amoderate amount of frequency modulation, butthe note shape in the sonagram is very variable:We found oo notes of increasing, decreasing orstable frequency. More complex sonagramstructures – such as u-shaped or n-shaped notes ornotes which begin with stable frequency butsharply go down at the end of the note – alsooccur. Oo notes are preferentially uttered at thebeginning of the male song bout, but also occurduring mild disturbances and moderate excitementoutside of the song context.During song bouts, notes may gradually changefrom oo notes into wa notes (see below).Wa notesFollowing Kappeler (1984a), the most commongroup of expiration notes are termed wa notes.They are short like, but louder than, oo notes andcharacterised by a fast frequency increase. Startingfrequency and minimum frequency alwayscoincide in wa notes. After reaching the maximumfrequency near the end of the note, a terminalfrequency decrease is absent or slight, amountingto less than half of the preceding frequencyincrease. Wa notes are very variable in their endand maximum frequencies. We distinguish betweenfour different note types (Figures 2), whichwe term B1, B2, B3 and B4 in the following text.Note type B1 comprises wa notes of relativelymoderate frequency modulation. Notes have astarting frequency of about 0.7 kHz or below, andreach a maximum frequency of 1 kHz or less.Note type B2 resembles B1, but the maximumfrequency ranges from above 1 kHz up to 1.3 kHz.Note type B3 comprises the wa notes with thehighest amount of frequency modulation. Thisnote type resembles B1, but the maximumfrequency ranges from above 1.3 kHz to about 1.5kHz. Note type B4 includes wa notes with arelatively high starting frequency (i.e. above 0.7kHz). The maximum frequency is variable and canrise to about 1.5 kHz.The trill is a particularly stereotyped figure (i.e.a song unit intermediate between single notes andphrases). Trills are short bursts of three to five B1notes with very short intervals. Often, the trillexhibits a distinct decrease in minimum frequency,maximum frequency and frequency rangefrom the first to the last trill note. Individual trillnotes may or may not be followed by inspirationnotes (described below). Trills usually are precededby a B2 or B3 note, also with facultativeinspiration note.Chevron notesThe third group of note types comprises thosenotes which have been termed wa-oo and wa-oowanotes by Kappeler (1984 a) and are referred toas chevron notes in the present study. Allchevrons begin with rising frequency, then, afterreaching a peak, frequency decreases again.Chevron notes differ in their frequency structure

Contributions to Zoology, 74 (1/2) – 2005 7and duration. Based on their shape in thesonagram, some chevron notes exhibit two distinctparts. The first part of the note (a) consists of thechevron proper described above. The second part(b) is the drawn-out tail of the chevron (Figure 1).The tail’s frequency structure is particularlyvariable. Chevron notes are introduced relativelylate during the song bout, after the wa notes.Chevron notes represent the most characteristicvocalisations in the song of male silvery gibbons,but are absent or very rare in female song bouts.Individual males appear to differ strongly in theiruse of chevron notes in general and of certaintypes of chevron notes in particular. Wedifferentiate between seven types of chevron notesin H. moloch (Figure 2), which we term C1, C2,C3 and so on in the following text.Note type C1 is of a simple chevron form in thesonagram; a tail end is absent. This note typereaches a maximum frequency of 1 kHz or less.Note type C2 resembles C1, but its maximumfrequency is situated above 1 kHz. Note type C3 isa chevron with a tail end. The latter is of chevronform, too, with its own peak frequency. After thetail peak, the frequency descends to about thestarting level for the second time. Note type C4resembles C3, but the frequency remains nearlyunmodulated in the tail end. Note type C5 also is achevron with a tail end. After reaching the firstpeak of the chevron, the frequency descends onlymoderately and does not reach the level of thestarting frequency. In the tail end, the frequencyrises again and reaches or even surpasses themaximum frequency of the first peak. Note typeC6 resembles C5, but the frequency rise in the tailend is more moderate and does not reach themaximum frequency of the first peak. Note typeC7 is a simple chevron resembling C2, but thefrequency decrease occurs much more slowly thanthe initial increase.Variable notesThe fourth group is termed variable notes andincludes only one note type, D. It comprises allexpiration notes which are neither wa notes norchevron notes. Notes of this type are extremelyvariable (Figure 2). Further partitioning of thisgroup would easily be possible but was not founduseful considering its rarity in the song bout.Variable notes are long (0.6±0.2 s) and of verymoderate frequency modulation (0.3 kHz or less).The direction of the frequency modulation isvariable; the frequency may rise, drop or changerepeatedly.The variable figure (Figure 2) is not a singlenote, but a cluster of more than three notes, withextremely short intervals and with a total durationof 1.5±0.3 s. All note types of the male songrepertoire may occur in these rapid notesequences, but variable notes and inspiration notesare most prominent. Both the degree and thedirection of frequency modulations are veryvariable in these figures. Intervals may be so shortthat the number of individual notes uttered in aseries can sometimes not be determined reliably.Therefore, variable figures were not broken downinto single notes for the note counts of this study(see below) but simply recorded as one unit.Inspiration notesThis note class also is variable in its frequencystructure, but most commonly exhibits an eitherdecreasing or chevron shape. Despite thestructural variability of inspiration notes, only onetype is recognised here (type E). A furtherpartitioning of this class did not appear to beuseful because of its rarity. Often, an inspirationnote immediately follows a wa note (forming a socalledwa-aa figure), or appears to link twoexpiration notes together (forming a wa-aa-wafigure). In these figures, intervals between notesare very short (the second sonagram of an E notein Figure 2 shows a typical wa-aa-wa figure).Particularly fast alternations of B1 and E notes canoccur in trill figures (described above). Inspirationnotes are often less loud than the accompanyingwa notes. The so-called variable figures (seeabove) may also include inspiration notes.Individuality in the note repertoireNot all note types were used by all males (at leastnot in the song material available for this study),as listed in Table 3. For instance, the Howletts 2male was lacking types C4 and C5, Howletts 3was lacking C3, C4, C5, and C6, and the songbout of the Halimun male did not include notetype D. Similarly, the variable figure did not occurin the song of the Howletts 3 and Halimun males.Trill figures were even less common. As a regularcomponent of the song repertoire, they onlyoccurred in the song of the Jakarta 2 male. They

8 T. Geissmann et al. – The Male Song of the Javan Silvery Gibbonoccurred infrequently in the songs of the Munichmale, and only one poorly-expressed trill wasfound in the song of the Howletts 1 male.Frequency of note typesThe frequency distributions of the various notetypes provide first insights into the rules by whichmale silvery gibbons build up their song bouts. Asshown below, at least two distinct phases canroughly be distinguished in the male song bout: abuild-up or introductory phase and a main phase.In order to monitor changes in the proportion ofnotes types in various stages of a male song bout,we divided each song bout in 50-note segmentsand determined the proportion of all note types foreach segment. Not all segments comprise exactly50 notes, because we avoided splitting phrases(described below) into different segments. A andB1 notes were pooled in this part of the analysis.Figure 3 shows the relative frequency of allnote types in the course of song bouts of our studyanimals. The song bouts of the Halimun, Jakarta1, Jakarta 2 and Kalejetan males were nottape-recorded in their entirety and may lack thebuild-up phase of the song bout. In order to facilitatecomparison, the frequency distributions of theincomplete song bouts are shown in separate partsof Figure 3.The proportion of most note types changesmarkedly during the song bout. The amount anddirection of these changes differ among notetypes. The proportion of note types A+B1, forinstance, tends to decrease during song bouts.Whereas these two note types make up between37 - 92% of the first 50 notes in the introductoryphase of all completely-recorded song bouts, theproportion decreases during the course of the songto levels below 20% and eventually approaches oreven reaches zero, although the proportion may goup again towards the end of the song (Howletts 3)or fluctuate around 15% during the second half ofthe song bout (Howletts 2). In the incompletelyrecorded song bouts, this trend cannot berecognised clearly, because the build-up part ispartly or completely missing. Yet, a decrease inthe proportions of note types A+B1 can also beobserved in each of the incomplete song boutsexcept in the Jakarta 2 male, which exhibits strongfluctuations around 15% in these note proportionsduring the whole tape-recording.The proportion of note type B2 also tends todecrease somewhat during the course of the songbout, but the decrease is less pronounced than innote types A+B1, and the proportion appears toexhibit strong individual differences. During theTable 3. Occurrence of note types and some figures in the songs of each study animal, based on all available tape-recordedmaterial. aNote Halimun Howletts 1 Howletts 2 Howletts 3 Jakarta 1 Jakarta 2 Kalejetan c Munichtype bA + + + + + + + +B1 + + + + + + + +B2 + + + + + + + +B3 + + + + + + + +B4 + + + + (+) – + +C1 + + + + + + + +C2 + + + + + + + +C3 + + + + + + + +C4 (+) + (+) + + (+) + +C5 (+) + – – (+) + + –C6 + – + – + + + +C7 + + + + + + + +D – + + + + (+) + (+)E + + + + + + + +FiguretypeTrill f. – (+) – – – + – (+)Variablef.– + + – + + – (+)a Symbols: + = present, (+) = very rare, less than 0.5% of the song notes, – = absentb f. = figurec Absence of some note types in this male may be due to the shortness of the tape-recorded song excerpt (2min 17s, see Table 1).

Contributions to Zoology, 74 (1/2) – 2005 9first 200 notes, the average proportions for Howletts1, Howletts 3 and Munich are at 30 - 42%,whereas the averages are lower in the remainderof the song bout (27-34%). In the Howletts 2male, however, this note type remains virtuallyabsent during the first quarter of the song bout.Only later, the proportion rises to an average levelof about 10%, and a maximum value of 22% isreached only in the last third of the song bout. Inthe available song excerpts of the Jakarta 1 male,on the other hand, B2 notes proportions exhibit anaverage level of about 50% and never decreasebelow 45%.In contrast to A, B1 and B2 notes, the proportionof B3 notes continually rises during thecourse of most song bouts. Whereas this note typemakes up less than 2% at the beginning of thesong bout, its proportion usually rises to a value ofmore than 20% within the first 400 notes of thesong bout (Howletts 1, Howletts 3, Munich) orlater (Howletts 2). The increase in note type B3appears to correlate with the decrease in note typeB1.The appearance of chevron notes appears tocoincide with the increase of the B3 note proportionand the decrease of the B1 note proportion.Chevron or C notes are never produced at thebeginning of a song bout (exception: 4% inHowletts 1). The proportion of C notes rises tohigher levels within the first 250-400 notes. Thesehigher levels appear to be strongly individualspecific,ranging from about 10% (Howletts 1,Howletts 2, Halimun), 17% (Munich), 20%(Howletts 3), 25% (Jakarta 1), 57% (Kalejetan), to70% (Jakarta 2). The respective proportions of thevarious variants of chevron notes (C1 - C7) do notsurpass 15%, except C3 notes in the Jakarta 2male (20%).The other note types defined in this study (B4,D and E) are much less common and, therefore,exhibit less obvious trends in their frequencydistribution during the song bout. In contrast tonote types A+B1, B2 , B3 and C, their proportionsrarely increase above 20%. Notable exceptions arethe high proportions of B4 notes in Howletts 2 andHalimun, and of inspiration or E notes in theHalimun male, which amounts to an average of19% for the whole tape-recording and whichreaches a maximum value of 34% before the endof the song bout. In the complete song bouts,inspiration notes are rare or absent at the beginning,and their frequency slightly increases withincreasing song activity.Males also appear to differ in their preferencesfor certain chevron notes (Table 4). For example,note type C6 was a preferred chevron type ofHowletts male 2 but was never produced byHowletts males 1 and 3.10080Howletts 1Howletts 2Howletts 3Munich (Mean)8060Howletts 1Howletts 2Howletts 3Munich (Mean)6040Percent note type A+B1402006040Jakarta 1 (Mean)Jakarta 2HalimunKalejetanPercent note type B2200806040Jakarta 1 (Mean)Jakarta 2HalimunKalejetan202000 200 400 600 800 1000 1200 1400 1600Rank of note in song boutFig. 3. Changing proportions of different note types in consecutive50-note sections of the male song bouts of Hylobatesmoloch. The Halimun, Jakarta 1, Jakarta 2 and Kalejetanmales are plotted separately, because the beginning of theirsong bouts was not recorded on tape. The resulting twingraphs show proportions of note types A+B1, B2, B3, B4, C,D, and E, respectively.0Fig. 3. (ctd.)0 200 400 600 800 1000 1200 1400 1600Rank of note in song bout

10 T. Geissmann et al. – The Male Song of the Javan Silvery Gibbon8060Howletts 1Howletts 2Howletts 3Munich (Mean)6040Howletts 1Howletts 2Howletts 3Munich (Mean)4020Percent note type B32006040Jakarta 1 (Mean)Jakarta 2HalimunKalejetanPercent note type C0806040Percent note type B42000 200 400 600 800 1000 1200 1400 160040Howletts 1Howletts 2Howletts 320Munich (Mean)04020Jakarta 1 (Mean)HalimunKalejetanPercent note type DJakarta 1 (Mean)20Jakarta 2HalimunKalejetan00 200 400 600 800 1000 1200 1400 160040Howletts 1Howletts 2Howletts 320Munich (Mean)04020Jakarta 1 (Mean)Jakarta 200 200 400 600 800 1000 1200 1400 1600Rank of note in song boutFig. 3. (ctd.)00 200 400 600 800 1000 1200 1400 1600Rank of note in song boutFig. 3. (ctd.)C5 was a preferred chevron type of Jakarta male1, but was never produced by Jakarta male 2(Table 4).Inspiration notes (type E) are introduced atabout the same time as, but remain less commonthan, B and C notes (except in the Halimun male,which uses E notes more frequently than C notes).Trills also occur only during the main part of thesong bout, but among our study males, only theJakarta 2 male frequently produced these conspicuousfigures. This figure was produced veryrarely by the Munich male (only 4 occurrences in4 song bouts) and once by the Howletts male 1. Inall other males, the trill was not heard at all.In summary, the note repertoire of the malesong of H. moloch is structured as follows: Thesong bout begins with soft oo notes (type A)which gradually change into low frequency wanotes (types B1 and B2). We use the term build-upphase or introductory phase for this part of thesong bout, which consists almost entirely of oonotes and low frequency wa notes. After about150 to 250 notes, high frequency wa notes (typesB3 and B4) and chevron notes (type C) are graduPercent note type E402004020Howletts 1Howletts 2Howletts 3Munich (Mean)Jakarta 1 (Mean)Jakarta 2HalimunKalejetan00 200 400 600 800 1000 1200 1400 1600Rank of note in song boutFig. 3. (ctd.)ally introduced, and after about 400 notes, theproportions of note types appears to stabilisesomewhat. We use the term main phase for thispart of the song bout. During the main phase, thedominant note types are B3 notes (25-50%) in 3males (Halimun, Howletts 1 and 2), B2 notes (30-50%) in 3 males (Howletts 3, Jakarta 1, Munich),and C notes (60-70%) in two males (Jakarta 2,Kalejetan). The changes in the note repertoire aresummarised in Figure 4.

Contributions to Zoology, 74 (1/2) – 2005 11Table 4. Comparison between the note type proportions in the song bouts of the study males. Stars indicate significantdifferences.f g p significant maleNote Number of notes produced by each individual (a-g) ANOVA Scheffé post-hoc testtype a b c d emun) 1 letts 1) letts 2) letts 3) 1) 1, 2 2) 1 nich) 3(Hali-(How-(How-(How-(Jakarta (Jakarta (Mu-comparisons(a) Introductory phase:A +152 292 66 307

12 T. Geissmann et al. – The Male Song of the Javan Silvery GibbonNote TypeAProgress of song bout (time)10Howletts 1Howletts 2Munich (Mean)B1, B28B3, B4, C, E6Build-upphaseMain phaseFig. 4. Diagram of the changes in the note repertoire usedduring various phases of the song bout of male silvery gibbons.Inter-phrase intervalThe male songs of H. moloch appear to be highlyvariable. We found virtually no stereotypic repeatedstructures comparable to the so-called great callphrases of the female songs (Dallmann and Geissmann,2001a, b). This impedes the recognition ofphrases (i.e. of elements which are typically utteredin recognisable sequences). Because the note intervalswithin phrases are, by definition, shorter thanthose between phrases, it should be possible toidentify a critical interval duration which discriminatesparticularly well between inter-phrase andintra-phrase intervals. For this purpose, we determinedthe frequency distribution of note intervaldurations in steps of 0.02 s for the songs of eachmale. We expected that both inter-phrase intervalsand intra-phrase intervals would both occur morefrequently than intermediate note intervals. Thelatter should be recognisable as a trough in the frequencydistribution curve and represent a suitablecritical interval duration distinguishing betweeninter-phrase intervals and intra-phrase intervals.Each song appears to exhibit several localminima in the frequency distribution of the noteintervals. Most minima occur in some individualsonly, but not in others. A through near 0.75 s consistentlyoccurred in the frequency distribution ofall study animals, however. Therefore, we defined0.75 s as the critical interval. In the following analysis,notes separated by an interval of more than0.75 s are considered as belonging to differentphrases. Likewise, non-phrase notes (i.e. singlenotes) must be isolated from all other notes byintervals of more than 0.75 s.Song motivationSong motivation (i.e. the gibbon’s drive to sing)cannot be measured directly. As an estimate forMean number of notes per phrase4201086420Jakarta 1 (Mean)Jakarta 2HalimunKalejetan0 200 400 600 800 1000 1200 1400 1600Rank of note in song boutFig. 5. Song motivation (number of notes per phrase) inconsecutive 50-note sections of the male song bouts ofHylobates moloch. The Halimun, Jakarta 1, Jakarta 2 andKalejetan males are plotted separately, because the beginningsof their song bouts were not recorded on tape.song motivation, we use the average number ofnotes per phrase, determined over consecutive songsections of 50 notes. Inspiration notes were ignoredin this calculation.In Figure 5, song motivation is shown as afunction of song bout progression, as determined ingroups of 50 consecutive notes. At the beginning ofa song bout, the average number of notes per phraseis below a value of 1.7 notes (Figure 5, uppergraph). In each case, song motivation increasesearly during the song bout (i.e. during about thefirst 200 - 250 notes). During the main part of thesong bout, song motivation may still exhibit considerablefluctuations, but tends to remain within arange of 2.5 - 4.0 notes/phrase.The beginning of the song bouts from Halimun,Jakarta 1, Jakarta 2 and Kalejetan (Figure 5, lowergraph) was not recorded on tape. An initial increasein song motivation is not visible in these songbouts. The songs of the Jakarta 1 and 2 malesexhibit a range of song motivation resembling the

Contributions to Zoology, 74 (1/2) – 2005 13Table 5. Kendall rank-order correlations between note types and song motivation (defined as notes per phrase, see text). Eachcell below row 4 includes the correlation coefficient (Tau) and the error probability (p). Stars and bold print indicate significantcorrelations.NoteIndividual atype Howletts 1 Howletts 2 Munich Halimun Jakarta 1 Jakarta 2Number ofnotes b 1159 1657 483935950615 6587661667Number of 295 (453) 379 (546) 106 (178)phrases b 205 (301)190 (341)81 (98) 143 (222)139 (195)A+B1 – 0.2630.078–0.3720.002 *– 0.673 0.999B4 0.1000.0280.2950.1160.2670.5060.8180.0780.5810.165C1 – 0.278 –0.069 –0.007 –0.404 0.0000.0630.5720.9650.055>0.999C2 0.0580.2660.129–0.041 0.3680.6980.029 * 0.4420.8440.080C3 0.2300.1660.3410.053–0.1160.1240.1750.041 * 0.8020.581C4 0.184–0.104 0.3180.2580.4700.2190.3930.0570.2200.025 *C5 0.341does not occur does not occur 0.0480.0000.023 *0.821>0.999C6 does not occur 0.2760.0130.1510.2750.024 * 0.9370.4730.191C7 – 0.122 0.3250.0520.264–0.3250.4160.008 * 0.7550.2090.122C, combined 0.0600.3870.3820.208– 0.2490.6870.002 * 0.022 * 0.3230.196D – 0.360 –0.133 0.109does not occur – 0.0620.016 * 0.2750.5150.746E – 0.173 –0.027 0.4010.4780.1530.2470.8250.016 * 0.022 * 0.426a373 (592)–0.0160.8960.1380.2660.2030.103does not occur–0.2000.1080.3520.047 *–0.2310.0630.0120.926–0.0160.896–0.0860.4880.1400.2610.0910.4630.0050.9700.1240.319Kalejetan was excluded because the tape-recorded song fragment was to short to calculate a correlation (108 notes, 24phrases if single notes are excluded, 48 phrases if single notes are included).bMore than one song was analysed for the Munich and Jakarta 1 males, respectively; the number of phrases and number ofnotes are listed for each song separately. The number of phrases excludes single notes (i.e. phrases with just one note), the valuein parentheses includes them.main part of the complete song bouts describedabove (i.e. 2.5-4.0 notes/phrase). Song motivationin the Kalejetan male is low (around 1.8 notes perphrase), suggesting that the available part of thesong bout represents an initial section, but thefragment is too short to draw reliable conclusions.The song bout of the Halimun male, however,exhibits unusually high values, mostly in the rangeof 4.0-7.0 notes/phrase, and reaching peak levelsof 10-11 notes/phrase.The frequency of several note types correlateswith song motivation. A total of 16 significantcorrelations were found (p < 0.05), and 3 additionalcorrelations are close to significance (p

14 T. Geissmann et al. – The Male Song of the Javan Silvery Gibbonand 5 correlations, respectively) than in the incompletelyrecorded songs of the Halimun, Jakarta1 and Jakarta 2 males (1 significant correlationeach).As shown in Table 5, no two males haveexactly the same note types among their set ofsignificant correlations. Only five note typesexhibit matching correlations in two males (i.e.A+B1, B3, C2, C combined, E), six note types aresignificant in only one male (C3, C4, C5, C6, C7,D) and three note types (B2, B4, C1) do notexhibit any correlation with song motivation.Phrase compositionTypical excerpts of the main phase of each studyanimal are shown in the sonagrams of Figure 6.The males appear to differ strongly not only in theproportions of the various note types, but also inthe structure of the phrases.In order to understand the sequential structureof note types in male phrases, we compared allphrases from the main song phase among allavailable song bouts as well as among individuals.Only phrases of at least 3 notes were included, andinspiration notes were ignored for this part of thekHz2a10101banalysis. The calculated note sequence probabilitiesfor each gibbon are shown in Figure 7.Data from multiple song bouts of the sameindividual were pooled.The Halimun male (Fig. 7a) begins 52% of hisphrases in the main song phase with a note of thetype B2. This note is followed by a B3 note in42% of the cases, which is followed by two morenotes of the same type in succession. All othernote sequences also occurring in this male’sphrases only play a subordinate role.Male 1 from Howletts (Fig. 7b) begins 83% ofhis phrases with a note of the type B3. This note isfollowed by a B3 note in 63% of the cases, whichis followed, again, by two more notes of the sametype in succession. Male 2 from Howletts (Fig. 7c)exhibits the same preferred phrase structure, butthe relative frequencies for each position in thephrase are less narrowly determined.The tape-recorded song bouts of the Jakarta 1male (Fig. 7d) reveal a much different preferredorder of phrase notes. The first note is of the B2note type in 59% of the cases, similar to theHalimun male. Unlike the phrases of any otherstudy male, the first note is followed by a note oftype B2 in 49% of all phrases (or – in 43% – bynote type C), followed by C and B3 in third andfourth positions, respectively.The Jakarta 2 male (Fig. 7e) starts 78% of allphrases with note type C and preferentially insertstype C in positions 2-5 of his phrases as well. Thisgibbon exhibits the most pronounced preferencefor note type C of all study males. Moreover, thephrase structure of this male appears to be theleast variable of the study males and, therefore,01010101010cd0 2 4 6 8 10 12 14 16 skHz2e1010f1010g1Fig. 6. Representative excerpts from song bouts of silverygibbon males, showing individual differences and low degreeof stereotypy in song structure. All sonagrams stem from themain phase of song bouts. The individual males are: (a)Halimun, (b) Howletts 1, (c) Howletts 2, (d) Jakarta 1, (e)Jakarta 2, (f) Kalejetan, (g) Munich.0100 2 4 6 8 10 12 14 16 sFig. 6. (ctd.)

Contributions to Zoology, 74 (1/2) – 2005 1544%B 2n = 48 %B 2n = 715 %43%B 3n = 36 %B 1n = 918 %B 2n = 2652 %33%35%42%19%B 3n = 36 %B 2n = 918 %B 3n = 1122 %B 4n = 510 %33%44%45%27%B 2n = 66 %B 3n = 48 %B 3n = 510 %B 4n = 36 %80%B 3n = 48 %B 3n = 4083 %15%63%17%B 2n = 613 %B 3n = 2552 %Cn = 715 %12%16%64%43%43%B 1n = 36 %B 2n = 48 %B 3n = 1633 %B 1n = 36 %B 2n = 36 %25%44%B 2n = 48 %B 3n = 715 %(a)B 3n = 1530 %60%40%B 3n = 918 %B 4n = 612 %44%44%50%50%B 3n = 48 %B 4n = 48 %B 3n = 36 %B 4n = 36 %75%B 4n = 36 %1 2 3 4Rank of phrase note1 2 3 4Rank of phrase note(b)Fig. 7. (ctd.)B 3n = 2958 %10%48%38%B 2n = 36 %B 3n = 1428 %B 4n = 1122 %100%64%21%45%45%B 3n = 36 %B 3n = 918 %B 4n = 36 %B 3n = 510 %B 4n = 510 %67%33%100%80%B 3n = 612 %Cn = 36 %B 3n = 510 %B 3n = 48 %Fig. 7. Phrase structure in the male song of Hylobatesmoloch: (a) Halimun, (b) Howletts 1, (c) Howletts 2, (d)Jakarta 1, (e) Jakarta 2, and (f) Munich. Data of several songbouts of the same individual (Munich, Jakarta 1) were pooled.Fifty phrases were analysed of each song (including onlyphrases with at least three notes). The preferred phrasestructure is drawn in bold lines. The letters in the boxesindicate the note type. The numbers in the boxes indicate theabsolute (n) and the relative frequency (%) of a note typeoccurring in this particular position in the phrase, and thenumbers accompanying the arrows represent the relativefrequency of two particular note types following each other inthese particular positions (ranks) within a phrase. In order tokeep the figures simple, we omitted relative frequenciesbelow 5% and absolute frequencies below n=2.Cn = 2142 %1 2 3 4Rank of phrase note(c)Fig. 7. (ctd.)43%57%B 3n = 918 %Cn = 1224 %33%50%33%B 3n = 612 %B 3n = 612 %Cn = 48 %50%50%B 4n = 36 %B 3n = 36 %

B 1Cn = 475%n = 38 %6 %CCCCn = 3284%n = 2774%n = 2080%n = 1682% 64 %54 %40 %32 %Cn = 3978 %B 110% n = 48 %1 2 3 4 5Rank of phrase note16 T. Geissmann et al. – The Male Song of the Javan Silvery GibbonB 2n = 5159 %B 3n = 3338 %(d)Fig. 7. (ctd.)B 1n = 48 %Cn = 3978 %49%43%15%24%61%B 2n = 2529 %Cn = 2226 %B 2n = 56 %B 3n = 89 %Cn = 2023 %36%52%45%27%60%75%55%25%20%B 2n = 910 %Cn = 1315 %B 2n = 1012 %B 3n = 67 %B 2n = 33 %Cn = 67 %B 2n = 1113 %B 3n = 56 %Cn = 44 %31%50%45%55%B 3n = 45 %B 2n = 56 %B 3n = 56 %Cn = 67 %1 2 3 4Rank of phrase note75%82%10%Cn = 36 %Cn = 3264 %B 1n = 48 %84%Cn = 2754 %74%Cn = 2040 %80%Cn = 1632 %1 2 3 4 5Rank of phrase note(e)Fig. 7. (ctd.)exhibits the highest transition probabilities betweensuccessive notes.The Kalejetan male was not included in thisanalysis because the shortness of the taperecordedsong fragment precluded the calculationof reliable transition probabilities. The high proportionof type C notes in the available songfragment (as described above, see Figure 3)suggests, however, that this male’s phrasestructure may resemble that of the Jakarta 2 male.Similar to the Howletts 1 and 2 males, 75% ofB 2n = 2620 %B 3n = 9775 %31%23%42%14%31%52%B 2n = 86 %B 3n = 65 %Cn = 118 %B 2n = 1411 %B 3n = 3023 %Cn = 5038 %67%45%45%57%36%57%37%40%12%42%Cn = 43 %B 2n = 54 %B 3n = 54 %B 2n = 86 %B 3n = 54 %B 3n = 1713 %Cn = 118 %B 2n = 2015 %B 4n = 65 %B 3n = 2116 %35%55%40%25%38%28%B 3n = 65 %B 3n = 65 %B 2n = 86 %B 3n = 54 %B 3n = 86 %Cn = 65 %50%50%B 3n = 43 %B 3n = 43 %1 2 3 4 5Rank of phrase note(f)Fig. 7. (ctd.)all phrases of the Munich male (Fig. 7f) beginwith a note of type B3. In contrast to other males,a C note follows on the second position (52%).Notes three, four and five of the phrases aremostly of type B3.The preferred note sequences in the phrases ofeach male are summarised in Table 6. In addition,the table also separately lists the results formultiple songs of the Jakarta 1 and Munich males.As can be seen in the list, the first note of thephrase remains of the same note type in all songbouts of the same male, but the phrase structure ofneither male is completely stable. At least in theMunich male, the differences in phrase structureare relatively moderate, considering that the thirdtape-recording (Munich 1c) was made 8 yearsafter the first one (Munich 1a).The proportions of the most common note sequences(one of each male) occurring in songphrases were compared among individual gibbonsusing minimum discrimination information statistics,revealing a strongly significant differenceamong the males (2I = 114.7, df=25, p

Contributions to Zoology, 74 (1/2) – 2005 17Table 6. The most frequently used order of note types in song phrases of each tape-recorded song bout.Tape-recording 1. note of phrase 2. note of phrase 3. note of phrase 4. note of phrase 5. note of phraseHalimun B2 B3 B3 B3 –Howletts 1 B3 B3 B3 B3 –Howletts 2 B3 B3 B3 B3 –Jakarta 1a B2 C B3 – –Jakarta 1b B2 B2 C B3 –Jakarta 1 total B2 B2 C B3 –Jakarta 2 C C C C CMunich 1a B3 C B3 C –Munich 1b B3 C B2 B2 –Munich 1c B3 B3 or C B3 B3 –Munich total B3 C B3 B3 B3DiscussionMale note repertoireOur study revealed that the note repertoire ofsilvery gibbon males appears to be much morecomplex than previously thought. The three notetypes defined by Kappeler (1984 a) are notsufficient to describe their vocal diversity. Wegrouped the male note repertoire of H. moloch intwo classes (expiration and inspiration notes). Theexpiration notes are further divided into fourgroups (labelled A through E), which are furtherdivided into distinct note types, resulting in a totalof 14 note types.Even this much larger number represents a conservativeapproach which cannot fully describe tothe vocal variability of the male songs of thisgibbon species. Especially the variable andinspiration notes (types D and E) could easily besplit into a larger number of distinct variants.Because these note types occur only rarely in thesongs that we analysed, a further subdivisionwould probably not have revealed additionalinsights in the song syntax.Wa notes (type B) are the dominant song notesin most males of this study. Similar wa notes arealso produced by other species of the genus Hylobates– such as H. lar (Raemaekers et al., 1984),H. muelleri and H. agilis (Haimoff, 1983) andusually represent the most common note type inthe repertoire of these species (Geissmann, ownobservation). The wa notes appear to be so typicalof the genus Hylobates (least of all H. pileatus)that this group might also be named the “wa notegibbons”.Composition of the song boutDespite their variability, male songs appear tofollow certain syntactical rules. Song boutstypically begin softly with several single oo notes(type A), which gradually develop into single wanotes (mostly type B1). The first phrasesoccurring in the song bout are short and simple,consisting of wa notes only. As the song boutcontinues, phrases become longer and more notetypes are included. This gradual developmentfrom simple oo notes to complex phrases can beseen in the completely sonagraphed (but unusuallyshort) song bout in Appendix 1. The progressiveelaboration appears to be typical of male solosongs of most (possibly all) gibbon species (e.g.Demars and Goustard, 1978; Goustard, 1976;Haimoff, 1984b, 1985; Mitani, 1988; Raemaekerset al., 1984; Tenaza, 1976; Whitten, 1982).Especially in the wa notes, a fine division intonote types is important: Whereas most wa notes atthe beginning of the song bout exhibit a smallrange of frequency modulation (note type B1), therange of most wa notes during the main songphase is more than twice as large (type B3).Phrase syntaxIt is difficult to identify a clearly preferred numberof notes per phrase – here termed song motivation– in a song bout. In the introductory phase, theaverage number of notes per phrase is usuallyshort and below 2 notes / phrase (single notephrases are included in this count, but inspirationnotes are not). The average phrase length in

18 T. Geissmann et al. – The Male Song of the Javan Silvery Gibbonthe main song phase is longer and amounts to 2notes/phrase. This value may increase, however,to averages of up to 5 notes / phrase in some partsof the song or in some individuals.The number of notes per phrase most stronglyincreases early in the song bout (i.e. during thefirst 150-400 notes). The initial increase in songmotivation may not be steady, and non-regularfluctuations of song motivation can occurthroughout the whole song bout.The frequency of some note types are significantlycorrelated with song motivation, but noneof these correlations are consistent across ourstudy males (see Table 5), again suggestingindividual preferrences.Some of the song bouts we studied were notcompletely tape-recorded. As none of themexhibits the increase of motivation, which istypical of the introductory phase, the introductoryphase of the song bout was apparently missedwhen the recording was carried out. This probablyexplains why we found fewer significant correlationsbetween note types and song motivationin these song bouts. Obviously, incomplete songbouts should not be used when calculating thistype of correlation.Because of the high variability within malesongs of H. moloch, it is difficult to recogniserecurrent patterns. In contrast to the female greatcalls, males produce much less stereotyped songphrases. Yet, all study animals appear to useindividually-preferred note sequences in the mainsong phase (see Table 6). The Halimun male, forinstance, prefers the note sequence B2 B3 B3 B3in his phrases. The Munich male, on the otherhand, prefers B3 C B3 B3 B3, and the Jakartamale 2 prefers C C C C C.IndividualitySongs of individual males differ significantlyamong each other in the note types they use and inhow often they use them, both during the introductoryphase and the main phase of their songbouts. Differences are particularly common in thedifferent types of chevron notes (type C). Inaddition, all study animals appear to exhibit individualpreferrences for note sequences in the mainsong phase.Our evidence for individuality should be regardedwith caution, however, because our samplesconsist of only one song bout for most studymales. Therefore, the significant differences innote repertoire and phrase composition could havebeen influenced by differences in the behaviouralcontext of the songs or the emotional state of thesingers, although no such differences were obviousto us. To properly demonstrate individualitywould require to find specific characteristicsconsistently occurring in several song bouts ofeach male. This appeared to be the case in themultiple song bouts we analysed of two of ourstudy males (3 of the Munich male and 2 of theJakarta 1 male).Comparison with male singing in other gibbonsThe frequency of singing varies between populationsand may be influenced by variables, suchas population density, availability of resources,sexual activity and weather. Yet, roughly, mostgibbon species duet almost daily (Leighton, 1987),except siamangs which duet about one day inthree (Gittins and Raemaekers, 1980). Among thenon-duetting gibbon species, male Kloss’sgibbons sing almost daily (Whitten, 1982), whereasmost male silvery gibbons appear to sing onlyrarely, about once a week (Geissmann and Nijman,2001). We cannot confirm Kappeler’s(1984a) conclusion that mated males of H. molochdo not sing, because the Munich, Howletts 2 andHowletts 3 males of this study were mated. Yet,the silvery gibbon appears to be exceptionalamong all gibbon species because of the rarity ofits male singing.In all examined gibbon species (e.g. H. klossii,H. lar, N. leucogenys) male song bouts usuallybegin with simple, single notes. During the courseof the song bout, additional, more complex notetypes are included and the phrases become longerand more complex (Schilling, 1984; Tenaza, 1976;Whitten, 1980, 1982; Raemaekers, Raemaekersand Haimoff, 1984). In this respect, the male songof H. moloch corresponds to that of the otherspecies.Male songs of many gibbon species such asH. klossii (Tenaza, 1976; Whitten, 1980, 1982), allspecies of the genus Nomascus (Schilling, 1984,Geissmann, unpublished data) and Symphalangussyndactylus (Geissmann, unpublished data) producerelatively stereotyped phrases exhibiting amoderate variability both within and between individuals.Figure 8 shows several sonagrams documentingthe repetitive structuring of the male songin H. klossii and N. leucogenys. Not only do individualmales of these species repeat their phrases

Contributions to Zoology, 74 (1/2) – 2005 19kHz2a1010210103210321032103210bcd0 2 4 6 8 10 12 14 16 sFig. 8. Sonagrams documenting the relatively stereotypedsong structure, both in intra- and inter-individual comparison,in males of Hylobates klossii and Nomascus leucogenysleucogenys, respectively. For each individual two consecutivephrases (H. klossii) or two consecutive phrase cycles (N.leucogenys) from the main phase of the song bout are shown.Intervals between lines have been cut in order to fit eachsonagram on one line. (a) H. klossii, Twycross Zoo, 8 Oct.1988; (b) H. klossii, Siberut, Mentawai, recorded 1973 byR.L. Tilson, published in Marshall and Marshall (1967); (c)N. leucogenys, Ménagerie du Jardin de Plantes, Paris, 17 May1988; (d) N. leucogenys, Hannover Zoo, 8 July 1997.in a relatively stereotyped way during the mainphase of the song bout, but other males of thesame species produce very similar phrases.In contrast to the taxa mentioned above, malephrases of H. moloch appear to exhibit very fewstereotyped structures and are characterised by ahigh degree of variability in their phrases. A relativelyhigh degree of intra-individual variabilitymay also occur in some other species of the genusHylobates, as well. In a study on male songs ofH. agilis, the authors concluded that male songphrases are extremely versatile. Precise copies ofphrases are rarely repeated consecutively, and therepertoire of individual males is very large(Mitani, 1988; Mitani and Marler, 1989). In H.moloch, however, intra-individual variability inphrase structure appears to be more variable thanwhat has previously been documented forgibbons. Although males may exhibit conspicuousrecurrent structures (e.g. the trill figure in Jakartamale 2 or the sequence of a high wa note of typeB3 followed by a very low note of type B1 inHowletts male 3), hardly two phrases are identical,as demonstrated in the sonagrams of a completesong bout in the Appendix. The differencebetween the highly variable male phrases ofH. moloch (Fig. 6) and the much more stereotypicmale songs of other gibbons (Fig. 8) could hardlybe more striking.Moreover, the unusually high variability ofsilvery gibbon male songs occurs on two levels:within and between individuals. Songs of differentsilvery gibbon males may differ so much in theirsong structure as to build up their phrases fromalmost completely different note types (e.g.compare the phrase structure of Howletts male 1and Jakarta male 2 in Fig. 7b and e). This has notpreviously been observed in other gibbon species.From a phylogenetic point of view, the highlyvariable male song of H. moloch appears to be aderived feature among gibbons. This is supportedby two independent lines of evidence. First, a highdegree of male song stereotypy is typical not onlyof H. klossii, but also of the genera Nomascus andSymphalangus, both of which have been found tooccupy more basal positions in hylobatidphylogeny than the genus Hylobates (e.g.Geissmann, 2002b; Roos and Geissmann, 2001).The variability of the male song of the genusBunopithecus has not been analysed in sufficientdetail to be included in the comparison. Second,gibbon songs have been proposed to be homologouswith loud calls typically produced by adultmales of most species of catarrhine primates(Geissmann, 2000; Geissmann and Dallmann,2002). Because loud calls of most species exhibita high degree of stereotypy (Geissmann, unpublishedobservations), this provides furthersupport for interpreting the highly variable malesong of H. moloch as a derived characteristicamong the Hylobatidae.Possible functions of male singingThe song is an important part of the gibbon behaviouralrepertoire. Various functions have been

20 T. Geissmann et al. – The Male Song of the Javan Silvery Gibbonsuggested for gibbon songs. The most commonlyproposed functions include defence of resources(territories, mates), mate attraction, and strengtheningand advertisement of pair bonds (Geissmann,1993, 1995, 1999, 2000; Geissmann and Orgeldinger,2000; Haimoff, 1984; Leighton, 1987).If the song served to strengthen and/or advertisethe pair bond, mated pairs should engage in duetsinging. This is not the case in H. moloch(Kappeler, 1984 a, Geissmann and Nijman, 2001).Instead, the study males appeared to abort theirsong bouts each time when a female started tosing. This occurred not only in three of the songbouts that were analysed for this study, but also inat least three other male song bouts of captivesilvery gibbons and in one male song monitored inCentral Java (Geissmann, unpublished observations).Although male silvery gibbons may occasionallycall together with females in “screambouts” (Geissmann and Nijman, 2001) or “borderconflict call bouts” (Kappeler, 1984 a), these callshave a different organisation and are notrecognised as song bouts.Traditionally, territorial defence is cited as oneof the main functions of male gibbon songs, butother functions have been proposed, such as mateattraction, singing in order to defend mate and offspringagainst other males, and mediating spacingamong individuals (Cowlishaw, 1992; Geissmann,2000; Mitani, 1992). These functions would requiremore frequent vocal activity from malesthan what we find in H. moloch. Males ofH. moloch roughly sing about once a week (Geissmannand Nijman, 2001), which is much less thanother gibbons. For males of other gibbon specieswhich do sing several times each week, nosignificant relationship was found between the frequenciesof male solo song bouts and territorialconflicts; yet, these males are territorial and doprovide physical defence of their territories. Incontrast, territorial conflicts appear to correlatewith the frequency of song bouts of mated females(Cowlishaw, 1992).Cowlishaw (1996) suggested that gibbon males(but not females) use assessment signals toadvertise resource-holding potential. Whitten(1984a, b) suggested that male songs of H. klossii(but not female songs) follow the handicap principle(e.g. Zahavi, 1975; Zahavi and Zahavi,1998) by requiring a lot of energy and stamina ofthe singer. The songs would then be an honestsignal declaring the singer’s fitness and his con-fidence and willingness to defend his territory.This hypothesis is supported by the observationthat male Kloss’s gibbons tend to maximise theirvocal output by singing about twice as often asfemales (about once every 1.7-2.5 days) and byspending over seven times as much time singingas females. In addition, males sing earlier in themorning than females. They produce most of theirsongs before dawn (Tenaza, 1976; Whitten,1984b), at a time when most diurnal mammals tryto reduce their energy expenditure.In H. klossii and other members of the genusHylobates, male solo songs have a longer durationand tend to occur earlier in the morning thanfemale or duet songs (Geissmann, 2000). Theseobservations partially apply to H. moloch, as well.Whereas an average female solo song bout ofH. moloch has a duration of about 7 minutes(Geissmann and Nijman, 2001), the six completemale solo song bouts we analysed had an averageduration of 23 minutes. This average does noteven include the longest recording of our study(40 min 45 s), because that song bout was notcompletely recorded. Recent field observations inCentral Java also document that most male songsstart before dawn when it is still dark, whereasmost female songs start after dawn (Geissmannand Nijman, 2001).The analogy to the Kloss’s gibbon does notextend, however, to the frequency of male singing.The field observations in Central Java (Geissmannand Nijman, 2001), as well as our observations inHowletts, suggest that males sing only about onceper week, whereas females sing about ten times asoften. This appears to be a reversal of the rule that,in the majority of territorial species, it is the malethat calls and defends the territory (Clutton-Brockand Harvey, 1977). It is questionable whether 30minutes of singing once a week represent asuitable handicap by which a male gibbon canadvertise his fitness.None of the functions suggested for gibbonsongs so far appear to explain satisfactorily therarity and variability of the male song ofH. moloch. It is difficult to find a plausibleexplanation for an event which occurs so rarely. Itis also difficult to assess the biological relevanceof the high variability we found in note typestructure and phrase structure of male silverygibbons.Preferences for certain note types and for certainphrase syntax both appear to differ among

Contributions to Zoology, 74 (1/2) – 2005 21male silvery gibbons, although it is unknownwhether the gibbons individually recognise eachothers’ songs. Bornean gibbons (H. muelleri) didnot respond differentially to experimental playbacksof their own, neighbours’, and strangers’duets (Mitani, 1985). The author concluded,however, that failure to show vocal discriminationmay be due to factors associated with the experimentalprocedure (e.g. playback periods lastedonly 3 minutes) or may indicate that there is noselective advantage gained by respondingdifferentially under the playback procedure.New functional hypothesesEven if inter-individual variability serves to facilitateindividual recognition, this does not explainthe adaptive value of high intra-individual variability.Why should males produce stereotyped phrasesin some gibbon species and highly variablephrases in other species? Without knowing theadaptive values of variable versus stereotypedmale songs, it is plausible to assume that at leastone of the two strategies provides a selectiveadvantage to the singer. Therefore, two alternateviews can be compared: (1) There may be aselective advantage for gibbon males to producestereotyped phrases, but – for unknown reasons –selection became secondarily relaxed in thesilvery gibbon. (2) The opposite is true: intraindividualvariability has been highly selected forin the songs of silvery gibbons, but at unknowncosts. Of course, more complex intermediatesolutions are also possible, but less easy to test.Accordingly, a high degree of either songvariability or song stereotypy could be used toadvertise the singer’s fitness to conspecifics(potential mates or potential competitors). In orderto qualify as a handicap (Zahavi, 1975; Zahaviand Zahavi, 1997), the feature in question mustinvolve costs, otherwise it could not be used tosignalise fitness. Either song type could qualify asan honest “handicap” signal according to thehandicap principle, because neither variability norstereotypy could be “forged”, provided that theycome with a cost. But which of the two characterstates is the cost-intensive one? As a proximatemechanism, variability could be either result froman easily created randomness or from complexvariability generator. Likewise, stereotypy couldrequire a complex mechanism to countereffectentropy, or else it could represent formulaicrepetition of a simple code.Fitch et al. (2002) recently suggested thatnonlinearities in the peripheral vocal productionsystem of many primates and other mammals“allows individuals to generate highly complexand unpredictable vocalizations without requiringequivalently complex neural control mechanisms.”Although we do not think that theunusually high intra- and inter-individual variabilityof the male song in H. moloch are related tothe non-linear phenomena described by Fitch et al.(2002), their paper is important in the contextdiscussed here because it documents that vocalvariability and unpredictability is not necessarilyan “expensive” character state.Here we present two different hypothesis whichcould explain how either song variability or songstereotypy produce costs to the singer. Bothhypotheses can be used to generate predictionsthat can be tested in order to determine whethersong variability or song stereotypy produce morecosts.Hypothesis 1: Singing may require not onlymuscle activity, but also brain activity, both ofwhich consume energy. In this case, brain activitymay represent a decisive cost-factor differentiatingbetween a variable song and a stereotypedsong. If the energy requirements of singinggibbons were determined (e.g. by measuringoxygen consumption), it could be tested which ofthe two song types uses more energy.Hypothesis 2: It is well known to both gibbonhunters and gibbon observers that approachingwild gibbons is easier when they are singing thanwhen they are silent. The concentration requiredto produce song phrases may represent a decisiveinvestment provided by a gibbon. A more concentratedor focused singer should be morevulnerable because he could be surprised bypredators or competitors more easily because hemay pay less attention to potentially dangeroussounds or movements in his environment. Thequestion is: What requires more attention from asinger, producing stereotyped phrase or producingmore loosely organised variable phrases? The twoalternatives could be tested, for instance, bypresenting increasingly loud or long noise stimulito singing gibbons. The concentration of thesinger could be determined by the minimumstimulus duration or loudness required to benoticed by the gibbon (as estimated by hisresponses such as song interruption or songabortion). This would represent an estimate of thesinger’s concentration.Both hypothesis can, in principle, be tested, and

22 T. Geissmann et al. – The Male Song of the Javan Silvery Gibbonthe results would help to narrow down thepossible functions of singing in male gibbons. If,for instance, the extremely variable songs of themale silvery gibbon were found to be more costintensivethan the more stereotyped songs of othergibbon species, this could explain why silverygibbons sing less often and why their rare songscan still serve to advertise fitness. If the morestereotyped songs of male Kloss’s gibbons arecheaper to produce, this would explain why theyhave to be produced more often in order to beenough of a handicap.It has previously been suggested that gibbonsongs – even within one species – serve more thanone function (Geissmann, 1999; Geissmann andOrgeldinger, 2000). Results of the present studyreveal a differential song variability occurringamong the various gibbon species. This furthersupports the view that gibbon songs aremultifunctional and that the relevance of thesefunctions exhibits strong differences among thespecies. Expecting to discover “the song function”of “the gibbon” does not appear to be a realisticanymore.Conclusions1 The note repertoire of silvery gibbon malesappears to exhibit a higher degree of interindividualvariability than other gibbons.2 The note repertoire of silvery gibbon malesappears to exhibit a higher degree of intraindividualvariability than other gibbons (bothmales and females).3 The high variability of the male song in thesilvery gibbon appears to be a derivedcharacteristic among the Hylobatidae.4 Male songs of H. moloch may function more toadvertise individual fitness than ownership ofresources.5 Specific experiments are required in order todetermine whether different costs are involvedwith producing variable versus stereotypedsongs.6 Gibbon songs probably serve multiple functionsand these functions or their relevance differamong gibbon species.AcknowledgementsAdditional tape-recordings used in the present study werekindly made available by Dr. Markus Kappeler and Dr. BjörnMerker. We are grateful to Dr. Robert Dallmann and twoanonymous reviewers for reading and commenting on thismanuscript.ReferencesBrockelman WY, Srikosamatara S. 1984. Maintenance andevolution of social structure in gibbons. In: Preuschoft H,Chivers DJ, Brockelman WY, Creel N, eds. The lesserapes. Evolutionary and behavioural biology. Edinburgh:Edinburgh University Press, 298-323.Charif RA, Mitchell S, Clark CW. 1995. Canary 1.2 user’smanual. New York: Cornell Laboratory of Ornithology,Ithaca.Chivers DJ. 1977. The lesser apes. In: Prince Rainier III ofMonaco, Bourne GH, eds. Primate conservation. NewYork: Academic Press, 539-598.Chivers DJ. 2001. The swinging singing apes: Fighting forfood and family in far-east forests. In Chicago ZoologicalSociety, ed. The apes: Challenges for the 21st century.Brookfield Zoo, May 10-13, 2000, ConferenceProceedings. Brookfield, Illinois, U.S.A.: ChicagoZoological Society, 1-28.Cowlishaw G. 1992. Song function in gibbons. Behaviour121: 131-153.Cowlishaw G. 1996. Sexual selection and informationcontent in gibbon song bouts. Ethology 102: 272-284.Clutton-Brock TH, Harvey PH. 1977. Primate ecology andsocial organization. J. Zool. 183: 1-39.Dallmann R, Geissmann T. 2001a. Different levels ofvariability in the female song of wild silvery gibbons(Hylobates moloch). Behaviour 138: 629-648.Dallmann R, Geissmann T. 2001b. Individuality in thefemale songs of wild silvery gibbons (Hylobates moloch)on Java, Indonesia. Contrib. Zool. 70: 41-50.Demars C, Goustard M. 1978. Le “grand chant”d’Hylobates concolor leucogenys. Comparaison avec lesémissions sonores homologues d’H. concolor gabriellae etd’H. klossii (Îles Mentawei, ouest Sumatra). Behaviour65: 1-26 (French text, English summary).Fitch WT, Neubauer J, Herzel H. 2002. Calls out of chaos:The adaptive significance of nonlinear phenomena inmammalian vocal production. Anim. Behav. 63: 407-418.Geissmann T. 1993. Evolution of communication in gibbons(Hylobatidae). PhD thesis, Anthropological Institute,Philosoph. Faculty II, Zürich University, 374 pp.Geissmann T. 1995. Gibbon systematics and speciesidentification. Int. Zoo News 42: 467-501.Geissmann T. 1999. Duet songs of the siamang, Hylobatessyndactylus: II. Testing the pair-bonding hypothesisduring a partner exchange. Behaviour 136: 1005-1039.Geissmann T. 2000. Gibbon songs and human music from anevolutionary perspective. In: Wallin NL, Merker B, BrownS, eds. The origins of music. Cambridge, Massachusetts:MIT Press, 103-123.

Contributions to Zoology, 74 (1/2) – 2005 23Geissmann T. 2002a. Duet-splitting and the evolution ofgibbon songs. Biol. Rev. 77: 57-76.Geissmann T. 2002b. Taxonomy and evolution of gibbons.In: Soligo C, Anzenberger G, Martin RD. eds.Anthropology and primatology into the third millennium:The Centenary Congress of the Zürich AnthropologicalInstitute (Evolutionary Anthropology Vol. 11, Supplement1) (pp. 28-31). New York: Wiley-Liss.Geissmann T, Dallmann R. 2002. Die Gesänge der Gibbonsund die Evolution der Musik. Praxis derNaturwissenschaften – Biologie in der Schule 51(1): 21-29(German text).Geissmann T, Nijman V. 2001. Calling behaviour of wildJavan gibbons Hylobates moloch in Java, Indonesia. In:Nijman, V, ed. Forest (and) primates: Conservation andecology of the endemic primates of Java and Borneo.Tropenbos-Kalimantan Series Vol. 5. Wageningen, TheNetherlands: Tropenbos International, pp. 43-59.Geissmann T, Orgeldinger M. 2000. The relationshipbetween duet songs and pair bonds in siamangs, Hylobatessyndactylus. Anim. Behav. 60: 805-809.Gittins SP, Raemaekers JJ. 1980. Siamang, lar and agilegibbons. In: Chivers DJ, ed. Malayan forest primates –Ten years’ study in tropical rain forest. New York:Plenum Press, pp. 63-105.Goustard M. 1976. The vocalizations of Hylobates. In:Rumbaugh DM, ed. Gibbon and siamang, vol. 4. Baseland New York: Karger, pp. 135-166.Haimoff EH. 1983. Gibbon songs: An acoustical,organizational, and behavioural analysis. PhD thesis,University of Cambridge, pp. xxiii+412 pp.Haimoff EH. 1984a. Acoustic and organizational features ofgibbon songs. In: Preuschoft H, Chivers DJ, BrockelmanWY, Creel N, eds. The lesser apes. Evolutionary andbehavioural biology. Edinburgh: Edinburgh UniversityPress, pp. 333-353.Haimoff EH. 1984b. The organization of song in the agilegibbon (Hylobates agilis). Folia Primatol. 42: 42-61.Haimoff EH. 1985. The organization of song in Müller’sgibbon (Hylobates muelleri). Int. J. Primatol. 6: 173-192.Haimoff EH, Tilson RL. 1985. Individuality in the femalesongs of wild Kloss’ gibbons (Hylobates klossii) onSiberut Island, Indonesia. Folia Primatol. 44: 129-137.Haimoff EH, Gittins SP. 1985. Individuality in the songs ofwild agile gibbons (Hylobates agilis) of PeninsularMalaysia. Am. J. Primatol. 8: 239 - 247.Kappeler M. 1981. The Javan silvery gibbon (Hylobates larmoloch). PhD thesis, Philisophisch-Naturwissenschaftliche Fakultät, Universität Basel.Kappeler M. 1984a. Vocal bouts and territorial maintenancein the moloch gibbon. In: Preuschoft H, Chivers DJ,Brockelman WY, Creel N, eds. The lesser apes.Evolutionary and behavioural biology. Edinburgh:Edinburgh University Press, 376-389.Kappeler M. 1984b. The gibbon in Java. In: Preuschoft H,Chivers DJ, Brockelman WY, Creel N, eds. The lesserapes. Evolutionary and behavioural biology. Edinburgh:Edinburgh University Press, 19-31.Leighton DR. 1987. Gibbons: Territoriality and monogamy.In: Smuts BB, Cheney DL, Seyfarth RM, Wrangham RW,Struhsaker TT, eds. Primate societies. Chicago andLondon: University of Chicago Press, 135-145.Marshall JT, Marshall ER. 1976. Gibbons and theirterritorial songs. Science 193: 235-237.Marshall JT, Sugardjito J. 1986. Gibbon systematics. In:Swindler DR, Erwin J, eds. Comparative primate biology,vol. 1: Systematics, evolution, and anatomy. New York:Alan R. Liss, 137-185.Mitani JC. 1985. Responses of gibbons (Hylobates muelleri)to self, neighbor, and stranger song duets. Int. J. Primatol.6: 193-200.Mitani JC. 1988. Male gibbon (Hylobates agilis) singingbehavior: Natural history, song variations and function.Ethology 79: 177-194.Mitani JC. 1992. Singing behavior of male gibbons: Fieldobservations and experiments. In: Nishida T, McGrewWC, Marler P, Pickford M, deWaal FBM, eds. Topics inprimatology, vol. 1: Human origins. Tokyo: University ofTokyo Press, 199-210.Mitani JC, Marler P. 1989. A phonological analysis of malegibbon singing behavior. Behaviour 109: 20-45.Mitani JC, Gros-Louis J, Macedonia JM. 1996. Selectionfor acoustic individuality within the vocal repertoire ofwild chimpanzees. Int. J. Primatol. 17: 569-583.Raemaekers JJ, Raemaekers PM, Haimoff EH. 1984.Loud calls of the gibbon (Hylobates lar): Repertoire,organization and context. Behaviour 91: 146-189.Roos C, Geissmann T. 2001. Molecular phylogeny of themajor hylobatid divisions. Molec. Phylogen. Evol. 19:486-494.Snowdon CT. 1986. Vocal communication. In: Mitchell DR,Erwin J, eds. Comparative primate biology, vol. 2A:Behavior, conservation, and ecology. New York: Alan R.Liss, 495-530.Tenaza RR. 1976. Songs, choruses and countersingingamong Kloss’ gibbons (Hylobates klossi) in Siberut island,Indonesia. Z. Tierpsychol. 40: 37-52.Waser PM. 1982. The evolution of male loud calls amongmangabeys and baboons. In: Snowdon CT, Brown CH,Petersen MR, eds. Primate communication. Cambridge:Cambridge University Press, 117-143.Whitten AJ. 1980. The Kloss gibbon in Siberut rain forest.Ph.D. thesis, Sub-Department of Veterinary Anatomy,University of Cambridge.Whitten T. 1982. The ecology of singing in Kloss gibbons(Hylobates klossii) on Siberut Island, Indonesia. Int. J.Primatol. 3: 33-51.Whitten T. 1984a. Defense by singing. Great calls and songsof the Kloss gibbon. In: MacDonald D, ed. Theencyclopedia of mammals, vol. 1. London: Allen andUnwin, 420-421.Whitten AJ. 1984b. The trilling handicap in Kloss gibbons.In: Preuschoft H, Chivers DJ, Brockelman WY, Creel N,eds. The lesser apes. Evolutionary and behaviouralbiology. Edinburgh: Edinburgh University Press, 416-419.Zahavi A. 1975. Mate selection – a selection for a handicap.J. Theoret. Biol. 53: 205-214.Zahavi A, Zahavi A. 1997. The handicap principle: Amissing piece of Darwin’s puzzle. New York and Oxford:Oxford University Press.Received: 28 March 2003Accepted: 16 September 2004

24 T. Geissmann et al. – The Male Song of the Javan Silvery GibbonAppendixConsecutive sonagrams of a complete (andrelatively short) male solo song bout of male“Paul”. This is the song bout identified as Munich1a in Table 1. The song bout has a duration ofabout 12 minutes. It begins with simple, very softsingle hoots (oo notes). Later in the song bout,starting with line 3, the oo notes are graduallyreplace by louder wa notes. By the end of part 1,wa notes are gradually combined to phrases, andother note types are added. In lines 11-14, thegibbon’s song motivation appears to declinetemporarily. After this, the male builds up hisphrases anew, starting with soft hoots again (lines15-17). This second build-up phase proceedsfaster than at the beginning of the song bout. Thesong bout ends abruptly as a great-call phrase of afemale white-cheeked crested gibbon (Nomascusleucogenys) begins in the background (this is thebegin of a duet song bout of the crested gibbons).The female phrase is blurred in the sonagrambecause of echo effects occurring in the indoorcage. (The crested gibbon pair is kept in an indoorcage adjacent to the island inhabited by the silverygibbon pair.)Whenever the intensity of the gibbon calls wasvery low, the sensitivity of the tape-recorder wasincreased during the recording. As a result, thebackground noise increases in some sections ofthe tape-recording, resulting in a grey backgroundin some of the sonagrams.kHz2101010101010101010100 2 4 6 8 10 12 14 16 sAppendix part 1kHz2101010101010101010100 2 4 6 8 10 12 14 16 sAppendix part 2

Contributions to Zoology, 74 (1/2) – 2005 25kHz2101010101010101010100 2 4 6 8 10 12 14 16 sAppendix part 3kHz2101010101010101010100 2 4 6 8 10 12 14 16 sAppendix part 4kHz21010101010100 2 4 6 8 10 12 14 16 sAppendix part 5