12 T. Geissmann et al. – <strong>The</strong> Male Song <strong>of</strong> <strong>the</strong> <strong>Javan</strong> Silvery GibbonNote TypeAProgress <strong>of</strong> <strong>song</strong> bout (time)10Howletts 1Howletts 2Munich (Mean)B1, B28B3, B4, C, E6Build-upphaseMain phaseFig. 4. Diagram <strong>of</strong> <strong>the</strong> changes in <strong>the</strong> note repertoire usedduring various phases <strong>of</strong> <strong>the</strong> <strong>song</strong> bout <strong>of</strong> <strong>male</strong> <strong>silvery</strong> <strong>gibbon</strong>s.Inter-phrase interval<strong>The</strong> <strong>male</strong> <strong>song</strong>s <strong>of</strong> H. <strong>moloch</strong> appear to be highlyvariable. We found virtually no stereotypic repeatedstructures comparable to <strong>the</strong> so-called great callphrases <strong>of</strong> <strong>the</strong> fe<strong>male</strong> <strong>song</strong>s (Dallmann and Geissmann,2001a, b). This impedes <strong>the</strong> recognition <strong>of</strong>phrases (i.e. <strong>of</strong> elements which are typically utteredin recognisable sequences). Because <strong>the</strong> 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 determined<strong>the</strong> frequency distribution <strong>of</strong> note intervaldurations in steps <strong>of</strong> 0.02 s for <strong>the</strong> <strong>song</strong>s <strong>of</strong> each<strong>male</strong>. We expected that both inter-phrase intervalsand intra-phrase intervals would both occur morefrequently than intermediate note intervals. <strong>The</strong>latter should be recognisable as a trough in <strong>the</strong> frequencydistribution curve and represent a suitablecritical interval duration distinguishing betweeninter-phrase intervals and intra-phrase intervals.Each <strong>song</strong> appears to exhibit several localminima in <strong>the</strong> frequency distribution <strong>of</strong> <strong>the</strong> noteintervals. Most minima occur in some individualsonly, but not in o<strong>the</strong>rs. A through near 0.75 s consistentlyoccurred in <strong>the</strong> frequency distribution <strong>of</strong>all study animals, however. <strong>The</strong>refore, we defined0.75 s as <strong>the</strong> critical interval. In <strong>the</strong> following analysis,notes separated by an interval <strong>of</strong> more than0.75 s are considered as belonging to differentphrases. Likewise, non-phrase notes (i.e. singlenotes) must be isolated from all o<strong>the</strong>r notes byintervals <strong>of</strong> more than 0.75 s.Song motivationSong motivation (i.e. <strong>the</strong> <strong>gibbon</strong>’s drive to sing)cannot be measured directly. As an estimate forMean number <strong>of</strong> notes per phrase4201086420Jakarta 1 (Mean)Jakarta 2HalimunKalejetan0 200 400 600 800 1000 1200 1400 1600Rank <strong>of</strong> note in <strong>song</strong> boutFig. 5. Song motivation (number <strong>of</strong> notes per phrase) inconsecutive 50-note sections <strong>of</strong> <strong>the</strong> <strong>male</strong> <strong>song</strong> bouts <strong>of</strong><strong>Hylobates</strong> <strong>moloch</strong>. <strong>The</strong> Halimun, Jakarta 1, Jakarta 2 andKalejetan <strong>male</strong>s are plotted separately, because <strong>the</strong> beginnings<strong>of</strong> <strong>the</strong>ir <strong>song</strong> bouts were not recorded on tape.<strong>song</strong> motivation, we use <strong>the</strong> average number <strong>of</strong>notes per phrase, determined over consecutive <strong>song</strong>sections <strong>of</strong> 50 notes. Inspiration notes were ignoredin this calculation.In Figure 5, <strong>song</strong> motivation is shown as afunction <strong>of</strong> <strong>song</strong> bout progression, as determined ingroups <strong>of</strong> 50 consecutive notes. At <strong>the</strong> beginning <strong>of</strong>a <strong>song</strong> bout, <strong>the</strong> average number <strong>of</strong> notes per phraseis below a value <strong>of</strong> 1.7 notes (Figure 5, uppergraph). In each case, <strong>song</strong> motivation increasesearly during <strong>the</strong> <strong>song</strong> bout (i.e. during about <strong>the</strong>first 200 - 250 notes). During <strong>the</strong> main part <strong>of</strong> <strong>the</strong><strong>song</strong> bout, <strong>song</strong> motivation may still exhibit considerablefluctuations, but tends to remain within arange <strong>of</strong> 2.5 - 4.0 notes/phrase.<strong>The</strong> beginning <strong>of</strong> <strong>the</strong> <strong>song</strong> bouts from Halimun,Jakarta 1, Jakarta 2 and Kalejetan (Figure 5, lowergraph) was not recorded on tape. An initial increasein <strong>song</strong> motivation is not visible in <strong>the</strong>se <strong>song</strong>bouts. <strong>The</strong> <strong>song</strong>s <strong>of</strong> <strong>the</strong> Jakarta 1 and 2 <strong>male</strong>sexhibit a range <strong>of</strong> <strong>song</strong> motivation resembling <strong>the</strong>
Contributions to Zoology, 74 (1/2) – 2005 13Table 5. Kendall rank-order correlations between note types and <strong>song</strong> motivation (defined as notes per phrase, see text). Eachcell below row 4 includes <strong>the</strong> correlation coefficient (Tau) and <strong>the</strong> error probability (p). Stars and bold print indicate significantcorrelations.NoteIndividual atype Howletts 1 Howletts 2 Munich Halimun Jakarta 1 Jakarta 2Number <strong>of</strong>notes b 1159 1657 483935950615 6587661667Number <strong>of</strong> 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 <strong>the</strong> tape-recorded <strong>song</strong> 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 <strong>song</strong> was analysed for <strong>the</strong> Munich and Jakarta 1 <strong>male</strong>s, respectively; <strong>the</strong> number <strong>of</strong> phrases and number <strong>of</strong>notes are listed for each <strong>song</strong> separately. <strong>The</strong> number <strong>of</strong> phrases excludes single notes (i.e. phrases with just one note), <strong>the</strong> valuein paren<strong>the</strong>ses includes <strong>the</strong>m.main part <strong>of</strong> <strong>the</strong> complete <strong>song</strong> bouts describedabove (i.e. 2.5-4.0 notes/phrase). Song motivationin <strong>the</strong> Kalejetan <strong>male</strong> is low (around 1.8 notes perphrase), suggesting that <strong>the</strong> available part <strong>of</strong> <strong>the</strong><strong>song</strong> bout represents an initial section, but <strong>the</strong>fragment is too short to draw reliable conclusions.<strong>The</strong> <strong>song</strong> bout <strong>of</strong> <strong>the</strong> Halimun <strong>male</strong>, however,exhibits unusually high values, mostly in <strong>the</strong> range<strong>of</strong> 4.0-7.0 notes/phrase, and reaching peak levels<strong>of</strong> 10-11 notes/phrase.<strong>The</strong> frequency <strong>of</strong> several note types correlateswith <strong>song</strong> motivation. A total <strong>of</strong> 16 significantcorrelations were found (p < 0.05), and 3 additionalcorrelations are close to significance (p