Species Discrimination of Male Song in Gibbons JOHN C

American Journal of Primatology 13:413-423 (1987) Species Discrimination of Male Song in Gibbons JOHN C. MITANI The Rockefeller Uniuersity Field Research Center, Tyrrel Road, Millbrook, New York 12545 Acoustic analyses and field playback experiments were conducted to inves- tigate 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 Miiller’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 Kalimantan 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 Muller’s gibbon songs. These results indicate that gibbons discriminate specific differences in songs, and support the division of agile and Muller’s gibbons into distinct species. Key words: songs, gibbon taxonomy INTRODUCTION Distinctive loud vocalizations, used to mediate spacing between conspecifics, are characteristic elements of the vocal repertoires of many nonhuman primates [Mitani & Marler, in press]. To serve a spacing function, loud vocalizations must retain species-specific features while transmitted over long distances. Acoustic analyses routinely reveal variation in loud calls both within and between species [eg, Struh- saker, 1970; Hodun et al, 1981; Waser, 1982; Oates & Trocco, 1983; Macedonia & Taylor, 1985; Snowdon et al, 19861, but few field studies have investigated whether animals recognize this variability. Do primates discriminate between the vocalizations of conspecifics and those of closely related heterospecifics? Gibbons (Hylobatidae) are ideal animals in which to examine species differences in vocal behavior. These territorial monogamous apes are distributed widely throughout the remaining rain forests of Southeast Asia. Adult males and mated pairs defend their territories through the use of songs [Mitani, 1985a,b;Raemaekers & Raemaekers, 19851, which vary markedly between species [Marshall & Marshall, Received March 30,1987; revision accepted June 24, 1987. Address reprint requests to Dr. John C. Mitani, The Rockefeller University Field Research Center, BOX 38B, RR2, Tyrrel Road, Millbrook, NY 12545. 0 1987 Alan R. Liss, Inc. 414 I Mitani 19761. Playbacks of tape-recorded vocalizations have been employed successfully to elicit responses from gibbons under natural social and environmental settings [Mi- tani 1984, 1985a,b,c; 1987; Raemaekers & Raemaekers, 19851, thus providing an objective method to assess an animal’s ability to discriminate between the songs of different species. Research on song variation is of particular interest given its critical role in discussions of gibbon systematics. Controversy exists regarding the number of species composing the Hylobatidae [reviews in Preuschoft et al, 1984; Marshall & Sugardjito, 19861. While comparative anatomists recognize as few as five species of gibbons [eg, Creel & Preuschoft, 19841, ethologists maintain the existence of nine species, largely on the basis of population differences in female singing behavior [eg, Marler & Tenaza, 1977; Haimoff et al, 1984; Marshall & Sugardjito, 19861. Resolu- tion of this debate has been precluded, in part, by two factors. First, despite agree- ment over the use of vocal signals to determine taxonomic relationships among gibbons [Creel et al, 19841, there is a paucity of data regarding male gibbon singing behavior. Second, scant attention has been paid to the critical issue regarding the biological significance of population differences in gibbon singing behavior; this problem requires data on the singing patterns of different populations, the responses of gibbons to songs from different populations, and the function of singing behavior. In this paper, male song variation and discrimination are examined among gibbons from three Indonesian populations: Miiller’s gibbons (Hylobates muelleri) in East Kalimantan, and agile gibbons (Hylobates agilis) in South Sumatra and West Kalimantan. First, the acoustic basis of species differences in song is investigated through an analysis of male songs selected from the three populations. Second, a series of experimental field playbacks is used to test whether gibbons discriminate species differences in song. Results of this study confirm the role song plays in species identification in gibbons and clarify current problems in gibbon taxonomy. METHODS Observations of Singing Behavior Male singing behavior was studied at three locations in Indonesia from 1980 to 1986. Observations of male Muller’s gibbon singing behavior were conducted in East Kalimantan during September 1980-October 1982 and March-May 1984 at the Kutai Game Reserve. Observations of male agile gibbon singing were made in Lampung, South Sumatra, during June 1984 at the Waykambas National Park and in West Kalimantan during July-October 1985 and July-October 1986 at the Gun- ung Palung Nature Reserve. Male singing performances were recorded at Kutai and Waykambas with a Uher 4400 Report Stereo IC tape recorder; recordings at Gunung Palung were made with Sony TCD-5M and TCD-5PRO cassette decks. A Sennheiser ME 20 omnidirectional microphone attached to a 13-cm parabolic reflector was used to make all recordings analyzed here. The frequency response of these recording systems was flat over the range of interest (& 3 dB from 150-2500 Hi,cf. Fig. 1). Analysis of Singing Behavior Tape recordings of male singing performances were made opportunistically. From the available set of clear recordings, a singing performance was selected randomly from four males in each population. A sample of 12 consecutive songs was then drawn from each of these individuals for acoustic analysis. Male gibbon singing performances can last up to 2 hours and typically begin with a series of short songs consisting of a few simple syllable types. As performances continue, the songs become increasingly longer and complex in syllable type number and morphology. Songs were sampled from the end of performances, after males had been singing for Species Song Discrimination in Gibbons I 415 30 minutes. Interspecific variation in syllable morphology complicates the compari- son of song syllables among populations. One syllable type, a simple frequency upsweep, is found in the songs of all three populations, however. To compare song syllables among populations, frequency upsweeps were sampled and analyzed 12 samples consisting of 12 syllables were drawn by selecting one syllable from the 12 songs of each male. Songs were digitized at a sampling rate of 5 kHz and 128 point Fourier trans- forms were performed using a DEC PDP 11/23 microcomputer and a digital signal processing program developed by Dr. Christopher Clark and Kim Beeman at The Rockefeller University Field Research Center [Clark et al, in press]. The resulting frequency domain representations of songs spanned 0-2,000 Hz with a frequency resolution of 39 Hz at sampled intervals of 26 ms. Four variables-song duration, the number of syllables in each song, the maximum frequency of the song, and the minimum frequency of the song-were measured from each song. The durations, maximum frequencies, and minimum frequencies of frequency upsweep song syllables were also measured. The songs of male agile and Muller’s gibbons vary markedly in their temporal patterning to the human ear. The temporal patterning of song syllables within songs were examined by dividing each song into two halves and counting the number of syllables that were delivered per second in each half of the song. Variation in songs and song syllables within and among populations was evalu- ated by performing nested analyses of variance on the values of each measured variable. If significant variation existed among populations, the T method was used to examine whether means differed between populations. Descriptions of these statistical tests can be found in Sokal and Rohlf [1981]. Playback Site and Subjects Playback experiments were conducted during July-October 1986 at the Cabang Panti Research Station in the Gunung Palung Nature Reserve, West Kalimantan, Indonesia. The 15-km2study area comprises a mixture of lowland dipterocarp, peat swamp, and hill dipterocarp forests. Nine groups of agile gibbons found in lowland forest served as test subjects. These groups, ranging from three to seven individuals, consisted of mated pairs and their offspring. Playback Procedure The nine test groups were exposed once to three playback treatments: male agile gibbon songs from West Kalimantan and South Sumatra, and male Muller’s gibbon songs from East Kalimantan. Playback types were assigned numeric codes, and their order of presentation to the test subjects was determined with the use of a random numbers table. Playback experiments followed a design established in an earlier study [Mitani, 19871. In each experiment, the test group was located prior to playback and was followed for at least 10 minutes. Following this period, a field assistant played back a 10-minute segment of a male singing performance

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