Female crested gibbons (genus 'Nomascus') sing male song

Abstract

Crested gibbons (genus 'Nomascus') belong to a group of gibbons that is still relatively unexplored. Their vocal behaviour, in particular, raises a number of important questions. One question is whether the loud calls, also known as 'songs', are species- and/or sex-specific. Past studies have suggested that the song repertoire of crested gibbons is sex-specific, particularly in adult individuals (Geissmann 2002). In this study (Geissmann 2002), only a few male individuals were reported to imitate the song phrase of a female. This report was presented as evidence that, on some occasions, female crested gibbons can sing male song phrases. In the present study, it was found that six of sixteen yellow-cheeked ('Nomascus gabriellae') and white cheeked crested gibbons ('Nomascus leucogenys'), housed in zoos of the Zoological Park Organization, Thailand and of the Endangered Primate Rescue Center, Vietnam, produced male song phrases. Six females were found to be able to produce at least the multi-modulated call phrases as thought to be exclusively belonging to the male song repertoire, and one female white-cheeked crested gibbon even produced several types of male song phrases. The structure of the first element of these multimodulated call phrases was examined and found to be similar between males and females, but the females scored were found to have a larger frequency range than the males. Most females also produced first elements of longer duration than the males. These results lead us to believe that crested gibbons generally may have the ability to produce song phrases of either sex. However, it is unknown what exact factors may contribute to males or females producing elements of song by the opposite sex. Further study is needed. Resolving this question may have an impact on the current methods used in population surveys of crested gibbons.