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

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