The Conservation Status of Gibbons in Vietnam - Gibbon Research ...

The Conservation Status of Gibbons in Vietnam9.4 Characteristics of the Crested Gibbons in Vietnam9.4.1 Eastern Black Gibbon Nomascus nasutusAdult males and juveniles are almost completely black with a brownish tinge on the chest andfrontal areas, and an only moderately developed crown crest. Adult females are pale yellow,yellow, orange or beige brown, exhibit a broad whitish face ring, and have a black cap that variesin size. Chest and sometimes the belly are adorned with a patch of varying size consisting of grey,brown or blackish hairs. The black crown is connected to the black facial area. It is narrow in thefrontal area, becomes wide at the top of the head, and extends past the nape and across theshoulders where it then tapers toward the centre of the spine above or past the scapula.Description after Geissmann et al. (2000), Groves (2001), Mootnick (2006) and Mootnick and FanPengfei (2011).Fully developed male vocalizations consist of only two of the three note types described for malecrested gibbons (Geissmann 1993, 1995; Geissmann et al. 2000): staccato phrases and multimodulatedphrases; boom notes are absent (Figure 13). Staccato notes are short and consist of arapid up-down sweep in frequency. The multi-modulated phrase is much more simple than inother crested gibbons. The first note usually differs from the other notes of the phrase and beginswith a slow and moderate increase in frequency, followed by a more pronounced and long downsweepin frequency. The following notes exhibit a different design and consist mainly ofincreasing frequency. The up-sweep is slow and moderate in the first part of the note, and shortand pronounced in the second part. These notes sound like “cao-vit” or “cao-vat”, which gave thegibbon its local name in Bac Kan Province (Geissmann, unpublished information). Adult malesproduce a special multi-modulated phrase, the “coda”, during or immediately after the climax ofthe female great call. The coda phrase may include notes consisting of a frequency down-upsweep, which appear u-shaped in sonograms. Complete great call phrases of adult femalesconsist of a series of 8-12 notes. All great call notes except the first 2-3 consist of very rapid vibratosounds. These notes somewhat resemble the twitter-like vocalizations which follow at the end ofthe great call in other crested gibbons (Geissmann 1989, 1997; Geissmann et al. 2000; Van NgocThinh et al. 2010d; Van Ngoc Thinh et al. 2011). Great calls of N. nasutus (and N. hainanus) do notreach the high frequency levels of great calls of other crested gibbons: all fundamentalfrequencies are below 2.8 kHz, whereas in other crested gibbons, great call frequencies typicallygo up to above 3 kHz (Geissmann et al. 2000).Figure 13. Sonogram showing call sequence of Nomascus nasutusSeveral male phrases continued during great call and directly followed by male coda near the end of the great call.Adapted from Van Ngoc Thinh et al. (2011).103