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Acoustic analyses and field playback experiments were conducted to investigate interspecific song differences and discrimination in male gibbons. Songs were recorded and analyzed from males of three populations: agile gibbons (Hylobates agilis) in West Kalimantan and South Sumatra and Müller's gibbons (Hylobates muelleri) in East Kalimantan. Male songs from each of the three populations were played back to West Kalimantan agile gibbons. Acoustic analyses indicated that the songs of the two Species differed in spectral features and in the temporal patterning of syllables. Agile gibbon songs from the West Kalimantan and South Sumatran populations varied in song durations and in the maximum frequencies of one song element. Field playbacks showed that West Katimantan agile gibbons respond in a similar manner to local songs and to conspecific songs from South Sumatra. In contrast, the animals responded differentially to conspecific and heterospecific Müller's gibbon songs. These results indicate that gibbons discriminate specific differences in songs, and support the division of agile and Müller's gibbons into distinct species. [ABSTRACT FROM AUTHOR]