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 bouts in the early morning. Silvery gibbons (Hylobales moloch) ,i differ from other hylobatids, however, in that duet song bouts are absent, male singing appears to be uncommon and most song bouts are female solo songs. Consistent individual differences easily distinguish neighboring females m 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 individuality in H. moloch 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 in those studies had been on the great call (i.e. the most stereotypical song phrase produced by gibbon females) and individual variation of several variables (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 Ujung Kulon and Gaining Pangrango. 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 statisticallysignificant degree of inter-individual variability in most great call variables of H. moloch, which is higher than that of H. klossii, but lower than that of H. agilis. Our results do not support the hypothesis that H. moloch females compensate for the rarity of male song contributions with an elevated degree of individuality in their singing. Instead, we suggest that the amount of great call individuality may be compromised by the amount of a trill component exhibited by various species of the lar group of gibbons.