Geissmann, T., and Nijman, V. (2006). Calling in wild silvery gibbons (Hylobates moloch) in Java (Indonesia): Behavior, phylogeny, and conservation. American Journal of Primatology 68: 1-19.
Thomas Geissmann1 & Vincent Nijman2
1 Anthropological Institute,
University Zürich-Irchel, Zürich, Switzerland
2 Institute for Biodiversity and Ecosystem Dynamics, Zoological Museum,
University of Amsterdam, The Netherlands
Key words: Hylobates moloch; gibbon; duet;
song; vocalization; population density.
Abstract: Hardly any behavioral data are available for the silvery gibbon
(Hylobates moloch), an endangered primate that is endemic to the island of
Java, Indonesia. We studied the singing behavior of the easternmost population of
this species in the Dieng mountains, central Java, in 1998ó1999. We aimed to document
the timing of singing, quantify the amount of singing by the respective sexes, and
explore the role of bioacoustics in density estimation. A total of 122 song bouts
in at least 12 groups were monitored. No duet songs were heard. Most of the song
bouts (91.5%) were female solo song bouts or female scream bouts. In contrast to
an earlier study on the westernmost population of silvery gibbons, during which few
if any male songs were heard, at least 8.5% of the song bouts in our study were male
solo song bouts. They were significantly longer in duration than the female songs.
All male song bouts uttered before dawn (0520 hr) were produced in a chorus fashion,
with at least three individuals participating. Choruses occurred about once every
8.5 days, and lasted longer and occurred earlier than female solo song bouts. Most
male songs (60%) started between 0355-0440 hr, when it was still dark. All female
songs, in contrast, started after 0500 hr, and female singing activity peaked around
0600. Regular male singing, male chorusing, and regular predawn singing have not
previously been reported for silvery gibbons. Similarly separated periods of male
and female solo songs and the absence of duetting have been observed in Klossís gibbons
(H. klossii) on the Mentawai Islands, and may represent synapomorphies shared
by both species. The pronounced individual-specific song characteristics of silvery
gibbons allow accurate mapping of groups. The density of gibbons at our study site
was established to be 1.9-3.7 groups/km2, corresponding to 6.7-13.1 individuals/km2.
We reassess the suitability of gibbon songs as a means of estimating the density
and size of gibbon populations, and discuss the proximate causes for the absence
of duetting in silvery gibbons.
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