Geissmann, T.; Bohlen-Eyring, S. & Heuck, A. (2005). The male song of the Javan silvery gibbon (Hylobates moloch). Contributions to Zoology 74: 1-25.
Thomas Geissmann1; Sylke Bohlen-Eyring2 & Arite Heuck2
1 Anthropological Institute,
University Zürich-Irchel, Switzerland
Email: thomas.geissmann@aim.unizh.ch
2 Institute of Zoology, Tierärztliche Hochschule Hannover, Germany
Email: thomas.geissmann@aim.unizh.ch
Key words: Hylobates moloch, silvery
gibbon, male song, individuality, calls, honest signal.
Abstract: 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 the 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 multi-functional, and the relevance of these functions appears to exhibit
strong differences among gibbon species.
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