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

This is the first study on the male song of the Javan silvery gibbon (Hylobates moloch), and the first quantitative evaluation of the syntax of male solo singing in any gibbon species carried out on a representative sample of individuals. Because male gibbon songs generally exhibit a higher degree of structural variability than female songs, the syntactical rules and the degree of variability in male singing have rarely been examined. In contrast to most other gibbon species, mated silvery gibbons do not appear to produce duet song bouts but solo song bouts only, and male singing is exceptionally rare, making this study particularly challenging. For the present study, we tape-recorded and analysed several solo song bouts of eight silvery gibbon males, including both wild and captive individuals. Based on their frequency characteristics, song notes were classified into a total of 14 note types. These can be grouped into five groups (labelled A through E). The proportions of the various note types were determined individually for successive 50-note sections throughout the whole song bout. Based on changes in the proportion of different note classes and note types, we roughly identify two phases in the male song: an introductory phase, during which A and B1 notes are dominant, B3 notes are rare and C notes are absent, and a main song phase, during which B3 or C notes are dominant. The occurrence and the proportion of various types apparently differ among individual males, however, both in the introductory and in the main phase. We estimated song motivation by determining the “number of notes per phrase” for each 50-note segment of the song bout. In each song bout, song motivation quickly increases during introductory phase. Song motivation may exhibit strong fluctuations during the main phase of the song, but usually remains above values of 2 notes per phrase and thus above the values observed during the introductory phase. Males appear to exhibit individual preferences in the order of different note types used in their phrases. Phrase structure was found to exhibit unusually low degrees of stereotypy and high degrees of variability. As a surprising finding of our study, male phrase variability both within and between individuals appears to be higher in H. moloch than in most, perhaps all, other gibbon species. This high variability appears to be a derived characteristic among the Hylobatidae. We discuss the implications of this finding for the interpretation of song function and present new and testable functional hypotheses. Our study demonstrates that song function cannot be identified for “the gibbon”. Gibbon songs appear to be multifunctional, and the relevance of these functions appears to exhibit strong differences among gibbon species.