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

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

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