Vocal Behaviour of White-Cheeked Crested Gibbons ('Nomascus leucogenys'): Vocal Production, Usage and Response

Abstract

White-cheeked crested gibbons ('Nomascus leucogenys'), one of the most endangered primate species in South-East Asia, produce a series of loud calls to communicate. This special vocal behaviour is termed "song" and was previously thought to be a fixed stereotyped system of communication. However, recent research has raised questions as to whether or not white-cheeked crested gibbons express their vocal individuality under such a fixed and stereotyped call structure and song organisation. Whether or not any additional factors are involved with changes and process of song development is also unclear. Moreover, the adaptive functions of song remain inconclusive. It is unknown whether the primary function of white-cheeked crested gibbon songs is the same as that of other previously studied lar group gibbons ('Hylobates spp'). The aims of the present study were to investigate the vocal individuality of adult white-cheeked crested gibbons along with patterns and change of song in terms of their social status. In association with this, preliminary playback experiments were conducted to investigate the possible primary function of song. This thesis investigated the vocalisations of nineteen white-cheeked crested gibbons housed at six different sites in Thailand and Vietnam between 13th August 2006 and 30th April 2008. A total of 178 complete song bouts with visual identity were recorded. A total of 14,101 call phrases were identified and analysed. Twenty-nine song variables were selected and analysed through sound analysis software (Raven Pro 1.4). Results were analysed using the general statistical method of analysis of variance (ANOVA). One of a selection of non-parametric tests (Friedman test, Cochran's Q test, Wilcoxon signed rank test and McNemar test) was used in the analysis if the transformed data were still not normally distributed. One-way repeated-measures ANOVA was used in a number of instances to test for differences in relation to age groups or social context across different sampling periods, and to also determine whether or not acoustic features changed during development of an individual's song.