Individuality in the female songs of wild Silvery Gibbons (Hylobates moloch) on Java, Indonesia

This is the first study comparing individuality in the songs among several gibbon species. All gibbon species produce loud, long and elaborate song bo uts in the early morning. Silvery gibbons (Hylobates moloch) differ from ot her hylobatids, however, in that duet song bouts are absent, male singing a ppears to be uncommon and most song bouts are female solo songs. Consistent individual differences easily distinguish neighboring females in the field , and it has been suggested that female individuality is particularly high in H, moloch in order to compensate for the lack of a family-labeling male song. The aim in this study is to test this hypothesis by quantifying indiv iduality in H. moloch h and comparing it with data on song individuality in two other gibbon species, H. agilis and H. klossii, available from earlier studies (Haimoff and Gittins, 1985: Haimoff and Tilson, 1985). The focus i n those studies had been on the great call(i.e. the most stereotypical song phrase produced by gibbon females) and individual variation of several var iables (such as duration and frequency range of selected great call notes) had been determined. We exactly replicated each of those studies with great calls of H. moloch, which were tape-recorded in Jung Kulon and Gunung Pang rango. According to the working hypotheses, individuality should be highest in H. moloch, lower in H. klossii(male singing is common) and lowest in H. agilis (male singing and duets are common). Results: We found a statistica lly-significant degree of inter-individual variability in most great call v ariables of H. moloch. which is higher than that of H. klossii, but lower t han that of H. agilis. Our results do not support the hypothesis that H. mo loch females compensate for the rarity of male song contributions with an e levated degree of individuality in their singing. Instead, we suggest that the amount or great call individuality may be compromised by the amount of a trill component exhibited by various species of the lar group of gibbons.