Male Gibbon ( Hylobates agilis ) Singing Behavior: Natural History, Song Variations and Function

Male gibbons engage in elaborate singing performances during which they emit a variety of song types. Field observations, acoustic analyses and experimental playbacks were conducted to investigate the communicative significance of song variations and to test the hypothesis that unmated male gibbons use song to attract potential mates. Singing is progressively elaborated during the course of a performance, with louder, longer, syllabically more complex songs produced at the end compared to the start. Similarly, song complexity increases during vocal interactions with territorial neighbors. Field playbacks showed that complex songs elicit significantly faster approaches than the simple songs used to initiate performances. Additional observations and experiments revealed few differences between the singing performances of mated and unmated males, and that the reproductive status of singing males did not affect the responsiveness of gibbons to songs. These results suggest that increasing song complexity during performances represents a series of graded signals used intrasexually in territorial defense; current evidence does not support the hypothesis that male song performs an intersexual mate attraction function.