The ecology of singing in Kloss gibbons (Hylobates klossii) on Siberut Island,Indonesia

Data are presented from a study of habituated Kloss gibbons on Siberut Island, Indonesia. Male Kloss gibbons can sing at any time from 0100 to 1300 hr, but the majority of songs is concentrated in the hour before dawn. Female Kloss gibbons sing only after dawn and the song bout includes a dramatic visual display. Neither countersinging nor coordinated chorusing has been proved in either sex. Males sing before dawn as often as possible but are inhibited by wet nights and by minimum temperatures below 21.5°C; postdawn songs of both sexes are inhibited by rain. The occurrence of any particular type of song bout is independent of the occurrence of the other types. Song trees used by males and those used by females do not differ in height. Song trees emerged from the neighboring canopy more than other available trees of similar height in the gibbons’ home range. Female song trees were most abundant on the slopes and where the trees were tallest. Almost all the male’s night trees could have been used for singing from had the weather been suitable. There was a greater likelihood of the male’s traveling a long way to the day’s first fruit source on mornings when he sang before dawn than on mornings when he did not. Considerations of sound transmission through tropical rain forest reveal that the times and frequencies used for singing by Kloss gibbons are optimal for communicating with neighboring groups.     

Calling in wild silvery gibbons (Hylobates moloch) in Java (Indonesia): behavior, phylogeny, and conservation

Hardly any behavioral data are available for the silvery gibbon (Hylobates moloch), an endangered primate that is endemic to the island of Java, Indonesia. We studied the singing behavior of the easternmost population of this species in the Dieng mountains, central Java, in 1998-1999. We aimed to document the timing of singing, quantify the amount of singing by the respective sexes, and explore the role of bioacoustics in density estimation. A total of 122 song bouts in at least 12 groups were monitored. No duet songs were heard. Most of the song bouts (91.5%) were female solo song bouts or female scream bouts. In contrast to an earlier study on the westernmost population of silvery gibbons, during which few if any male songs were heard, at least 8.5% of the song bouts in our study were male solo song bouts. They were significantly longer in duration than the female songs. All male song bouts uttered before dawn (0520 hr) were produced in a chorus fashion, with at least three individuals participating. Choruses occurred about once every 8.5 days, and lasted longer and occurred earlier than female solo song bouts. Most male songs (60%) started between 0355-0440 hr, when it was still dark. All female songs, in contrast, started after 0500 hr, and female singing activity peaked around 0600. Regular male singing, male chorusing, and regular predawn singing have not previously been reported for silvery gibbons. Similarly separated periods of male and female solo songs and the absence of duetting have been observed in Kloss's gibbons (H. klossii) on the Mentawai Islands, and may represent synapomorphies shared by both species. The pronounced individual-specific song characteristics of silvery gibbons allow accurate mapping of groups. The density of gibbons at our study site was established to be 1.9-3.7 groups/km2, corresponding to 6.7-13.1 individuals/km2. We reassess the suitability of gibbon songs as a means of estimating the density and size of gibbon populations, and discuss the proximate causes for the absence of duetting in silvery gibbons.     

Countersinging as a signal of aggression in a territorial songbird

Neighbouring territorial songbirds often interact through countersinging, where birds sing in response to the singing of neighbours such that their song bouts are temporally related. Complex forms of countersinging such as song type matching or song overlapping appear to be correlated with aggressiveness and readiness to escalate confrontations. Less attention has been paid to the importance of simpler forms of countersinging, where matched song types are not used and where individual songs do not temporally overlap. I examined countersinging behaviour in male Carolina wrens, Thryothorus ludovicianus, which countersing regularly. Why they countersing and how countersinging is perceived by neighbours is unknown. By comparing singing behaviour before and after simulated intrusions, I determined that subjects countersing with their neighbours more readily when highly aroused. Comparing responses to countersinging and noncountersinging playbacks showed that countersinging elicited more aggressive responses than did noncountersinging. Carolina wrens appear to exchange aggressive signals regularly through countersinging. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

The effect of neighbours and females on dawn and daytime singing behaviours by male chipping sparrows

Bird song may play an important role for communication among territorial neighbours, but the effect of neighbours on song use is still not well known. My previous field observations suggested that male chipping sparrows, Spizella passerina, use the dawn chorus for interactions among neighbouring males, and use day song for female attraction. To determine how these social factors may influence dawn and daytime singing behaviour, I conducted a series of experiments in which I removed the male neighbours or the female mate of territorial males during 1998-2000. Following removal of all neighbouring males, the solitary male either stopped or reduced his dawn chorus (N=9), but did not change his daytime singing behaviour. After one of the neighbouring males was returned to his territory, the focal male resumed and increased his dawn bout, accompanied with close-range countersinging. Following the removal of a territorial male's mate, the widowed male did not change the dawn chorus, but significantly increased his day song. This study thus revealed that, in chipping sparrows, the presence or absence of neighbouring males has a significant effect on the dawn chorus singing behaviour of territorial males. The presence or absence of a male's mate, in contrast, has a strong influence on a male's daytime singing behaviour. This study also supports the hypothesis that the dawn chorus and daytime song have different functions.     

Cautious response of inexperienced birds to conventional signal of stronger threat

Several studies demonstrated that bird song functions as a first line of territorial defence. The efficiency of deterring rivals depends strongly on the strategy of singing used (e.g. alternating/overlapping singing, singing with low/high rate, matching song type of a rival or singing different type). Causes of between males variation during countersinging are still not fully understood, especially when different signals have similar production costs and their meaning is assigned by arbitrary convention (conventional signalling). We tested whether an oscine bird with small repertoire size, the ortolan bunting Emberiza hortulana , differentiate strategy of responding to song of an intruder in relation to its age and threat value of signals. We performed playback experiments to measure response of second year (SY) and after second year (ASY) males to a song of low (eventual variety singing) and high (immediate variety singing) threat value. We found substantial differences in response to playback, which were related both to the type of stimuli used and age of responding males. Both SY and ASY males gave more calls than songs in response to immediate variety playback, which suggest stronger vocal response to the signal of higher threat value. Approaching loudspeaker was similar for both age classes when lower threat value signal was played back, while simultaneously SY males clearly avoided approaching loudspeaker when stronger threat values signal was played back. We conclude that ortolan bunting differentiate response to signal of different threat value and that the strength of response depends on the age of a male. This study provides experimental evidence that age of receiver affects its response to a territorial intruder. It also demonstrates that observed in many studies variation in response to playback may be an effect of age differences between males, which rarely is controlled.     

Social facilitation of male song by male and female conspecifics in the zebra finch, Taeniopygia guttata

Zebra finches are a ubiquitous model system for the study of vocal learning in animal communication. Their song has been well described, but its possible function(s) in social communication are only partly understood. The so-called ‘directed song’ is a high-intensity, high-performance song given during courtship in close proximity to the female, which is known to mediate mate choice and mating. However, this singing mode constitutes only a fraction of zebra finch males’ prolific song output. Potential communicative functions of their second, ‘undirected’ singing mode remain unresolved in the face of contradicting reports of both facilitating and inhibiting effects of social company on singing. We addressed this issue by experimentally manipulating social contexts in a within-subject design, comparing a solo versus male or female only company condition, each lasting for 24 h. Males’ total song output was significantly higher when a conspecific was in audible and visible distance than when they were alone. Male and female company had an equally facilitating effect on song output. Our findings thus indicate that singing motivation is facilitated rather than inhibited by social company, suggesting that singing in zebra finches might function both in inter- and intrasexual communication.     

Variation in singing style use in the reed bunting Emberiza schoeniclus: influencing factors and possible functions

The two main functions of bird song are territory defence and mate attraction. Considerable progress has been made in understanding how species adjust the use of songs to serve these and other (presumed) functions of bird song, but the striking variety of singing behavior observable in wild birds remains enigmatic. Some species make do with simple songs and small repertoires, while others show large, complex repertoires and still others have evolved several distinct singing styles. In most species with distinct singing styles, however, the functions of singing styles are poorly understood. Two distinct singing styles (type I and II, respectively) have long been known in the reed bunting Emberiza schoeniclus, while a new third one has recently been reported to exist. We first quantitatively investigated the evidence for the existence of three singing styles. Then, we tested predictions of the mate attraction hypothesis, the mate guarding hypothesis and the territory defence hypothesis by examining the relations between singing style use with social and temporal factors. Cluster and discriminant analyses supported the existence of three (instead of two) singing styles, which could be differentiated based on four variables referring to song structure and complexity. Use of singing styles was related to male mating status (consistent with the mate attraction hypothesis), but not to female breeding stage (no support for the mate guarding hypothesis). Finally, use of singing styles differed in relation to time of day, with the dawn chorus of paired reed buntings consisting almost exclusively of songs of the recently discovered type III singing style and daytime singing primarily consisting of songs of long‐known type I (in unpaired males) or II singing styles (in paired males). Our findings suggest that one singing style (type I) primarily serves to attract a social mate, although an additional territorial function of this singing style cannot be dismissed. The function(s) of the other two singing styles, both only sung by paired males, are not related to attraction of a social mate or to the own female's fertility, but appear to be important in the context of territory defence and extra‐pair matings.     

Sedentary life style of Neotropical sedge wrens promotes song imitation

To what extent has the style of song development among songbirds coevolved with other life history strategies? Among Cistothorus wrens in North America, it seems that sedentary or site-faithful habits of marsh wrens, C. palustris, favour song imitation, but seminomadic habits of sedge wrens, C. platensis, favour song improvisation, whereby each male generates a large but unique song repertoire. In this study, we tested whether more sedentary populations of sedge wrens in the Neotropics would imitate songs. At our primary study site near Cartago, Costa Rica, breeding birds were colour-banded during 1995 and 1996, and follow-up surveys revealed that the birds remained at this site the year round. Extensive tape recording and analysis of songs showed that males had large song repertoires (200-300+ songs), and that many songs were shared among neighbouring males. In addition, males only 27 km distant, at La Pastora, used different songs. Furthermore, matched countersinging, in which two males answer each other with identical song types, was recorded near Brasilia, in Brazil. The sharing of songs among permanent neighbours, microgeographical variation in song, and matched countersinging can be achieved only through song imitation, thus revealing a striking difference in the style of song development among different populations of the sedge wren. In the Neotropics, having predictable neighbours throughout life appears to have favoured song imitation, so that individuals can interact using a common, learned code typical of the local population; among more mobile populations in North America, however, individuals improvise large repertoires of species-typical songs, thereby enabling singing males to communicate with any individual, no matter what the population of origin. Strategies of song development must correlate with life history features, and further surveys are needed to make sense of the great diversity of singing behaviours among songbirds. Copyright 1999 The Association for the Study of Animal Behaviour.     

Duetting behavior in a Neotropical ovenbird: sexual and seasonal variation and adaptive signaling functions

Duetting is a collective behavior and might have multiple functions, including joint territory defense and mate guarding. An important step toward understanding the adaptive function of bird song is to determine if and how singing behavior varies seasonally. However, seasonal patterns for duetting species are different from the pattern described for species in which only the male sings, because song function may vary according to sex, singing role (initiator vs responder) and level of duet organization (individual vs pair). We investigated whether patterns of seasonal variation in duetting depends on these factors, which would suggest different interpretations of song function. We studied social pairs of a Neotropical bird species (rufous hornero Furnarius rufus) for seven consecutive months, recording vocal and territorial behaviors. Overall, partners coordinated 61% of their songs into duets and many song traits (song initiation rate, song output and duet rate) peaked in territorial contexts. Males engaged in territorial interactions with strangers more often, initiated more songs, and answered proportionately more of their partners’ songs than females. Male song initiation rate peaked during the pre‐ and post‐breeding stages, whereas females initiated more songs during the non‐breeding season. Both sexes answered partner songs faster and at higher rates during the pre‐breeding and female fertile stages. Partners duetted at a higher rate during the pre‐ and post‐breeding stages. Finally, song initiation rates and duet rate, but not song answering rates, correlated with frequency of territorial interactions with strangers. Although our findings indicate that song function may vary with sex, singing role and level of duet organization, our results suggest that in general duet functions to defend common territories and as a mutual mate guarding strategy in the rufous hornero.     

Sound Spectrum Characteristics of Songs of Hainan Gibbon (Nomascus hainanus)

Gibbons are characterized by their species-specific calls, or songs. There are few studies of songs of Hainan gibbon (Nomascus hainanus). To study the sound spectrum characteristics and test for intergroup differences in Hainan gibbon song, we studied the singing behavior of Hainan gibbons in Bawangling National Nature Reserve, Hainan Province, China, intermittently from August 2002 to February 2013, collecting 184 recordings. Our results show that: 1) Hainan gibbon song bouts occur mainly 0–4 h after dawn. 2) The songs of adult males living in groups are composed mainly of one to three short notes and one to five long notes, while solitary adult male songs consist only of long frequency modulated notes and no short or single notes. 3) The song chorus is dominated by adult males, while females add a great call. Males do not have a great call, unlike those in other gibbon species. There are no female solos. 4) The sound spectrum frequency is similar in adult males living in two different groups, but the duration of the first long note differed significantly between the groups. The sonic frequencies of male and female songs are lower than those of other gibbons: no more than 2 kHz. Hainan gibbon sound structure is simple, although females participate in the chorus, reflecting their primitive status among gibbon species.     

Social context affects testosterone‐induced singing and the volume of song control nuclei in male canaries (Serinus canaria)

The contribution of social factors to seasonal plasticity in singing behavior and forebrain nuclei controlling song, and their interplay with gonadal steroid hormones are still poorly understood. In many songbird species, testosterone (T) enhances singing behavior but elevated plasma T concentrations are not absolutely required for singing to occur. Singing is generally produced either to defend a territory or to attract a mate and it is therefore not surprising that singing rate can be influenced by the sex and behavior of the social partner. We investigated, based on two independent experiments, the effect of the presence of a male or female partner on the rate of song produced by male canaries. In the first experiment, song rate was measured in dyads composed of one male and one female (M‐F) or two males (M‐M). Birds were implanted with T‐filled Silastic capsules or with empty capsules as control. The number of complete song bouts produced by all males was recorded during 240 min on week 1, 2, 4, and 8 after implantation. On the day following each recording session, brains from approximately one‐fourth of the birds were collected and the volumes of the song control nuclei HVC and RA were measured. T increased the singing rate and volume of HVC and RA but these effects were affected by the social context. Singing rates were higher in the M‐M than in the M‐F dyads. Also, in the M‐M dyads a dominance‐subordination relationship soon became established and dominant males sang at higher rates than subordinates in T‐treated but not in control pairs. The differences in song production were not reflected in the size of the song control nuclei: HVC was larger in M‐F than in M‐M males and within the M‐M dyads, no difference in HVC or RA size could be detected between dominant and subordinate males. At the individual level, the song rate with was positively correlated with RA and to a lower degree HVC volume, but this relationship was observed only in M‐M dyads, specifically in dominant males. A second experiment, carried out with castrated males that were all treated with T and exposed either to another T‐treated castrate or to an estradiol‐implanted female, confirmed that song rate was higher in the M‐M than in the M‐F condition and that HVC volume was larger in heterosexual than in same‐sex dyads. The effects of T on singing rate and on the volume of the song control nuclei are thus modulated by the social environment, including the presence/absence of a potential mate and dominance status among males. 2006 Wiley Periodicals, Inc. J Neurobiol, 2006     

Sex Differences in the Song of <Emphasis Type="Italic">Indri indri</Emphasis>

In some primate species, males and females within a social group emit loud calls in a coordinated manner or chorus. Indri indri emits a very conspicuous loud call that elicits the loud calls of neighboring groups. Previous investigations have hypothesized that the main functions of the indri chorus are related to territorial announcement, intergroup avoidance, and group cohesion. We investigated sex differences in indri song. We recorded and analysed songs given by 10 different groups over 160 d. Overall singing duration did not vary between the sexes. However, males emitted significantly fewer but longer notes. Adult males and females of each group participated in the song with sex-specific repertoires. Females had a song repertoire of 8 note types; males shared all of their 6 notes with females. Apart from the initial roars, in all note types shared by both sexes, male notes were significantly longer than female ones, whereas variations in frequency parameters differed according to the note type. These findings suggest that indri song may provide cues to conspecifics, such as group size and sex composition, which could influence interactions between groups.     

Singing by male and female Kloss gibbons (Hylobates klossii) in the Peleonan Forest, Siberut Island, Indonesia

Kloss gibbons (Hylobates klossii) are endemic to the Mentawai Islands in Indonesia and are one of only two gibbon species in which mated pairs do not sing duets. This is the first long-term study of the factors influencing the singing activity of Kloss gibbons within a northern Siberut Island population and follows two previous studies in central Siberut nearly 30 years ago. We collected data on the presence/absence of male and female singing within the study area on 198 days and within a focal group on 47 days. Rainfall during the time period in which they normally sing inhibits singing in both males and females. Our study supports the hypothesis that male and female songs function in intrasexual resource defence, as singing is associated with singing by same-sex neighbours, and same-sex choruses are more likely to occur after one or more days of silence (from that sex), suggesting there is pressure for individuals to communicate with same-sex neighbours regularly. Singing was not coordinated within a mated pair, suggesting that vocal coordination of the pair has been lost with the loss of the duet and that Kloss gibbon songs do not convey information to neighbours about the strength of the pair bond. On days when males sang predawn, females were more likely to sing after dawn and earlier in the morning. Additionally, the number of groups singing in female choruses was positively associated with the number of males that had sung in the predawn male chorus. We suggest that female songs have an intersexual territory defence as well as an intrasexual function.     

Assessing visual requirements for social context-dependent activation of the songbird song system

Social context has been shown to have a profound influence on brain activation in a wide range of vertebrate species. Best studied in songbirds, when males sing undirected song, the level of neural activity and expression of immediate early genes (IEGs) in several song nuclei is dramatically higher or lower than when they sing directed song to other birds, particularly females. This differential social context-dependent activation is independent of auditory input and is not simply dependent on the motor act of singing. These findings suggested that the critical sensory modality driving social context-dependent differences in the brain could be visual cues. Here, we tested this hypothesis by examining IEG activation in song nuclei in hemispheres to which visual input was normal or blocked. We found that covering one eye blocked visually induced IEG expression throughout both contralateral visual pathways of the brain, and reduced activation of the contralateral ventral tegmental area, a non-visual midbrain motivation-related area affected by social context. However, blocking visual input had no effect on the social context-dependent activation of the contralateral song nuclei during female-directed singing. Our findings suggest that individual sensory modalities are not direct driving forces for the social context differences in song nuclei during singing. Rather, these social context differences in brain activation appear to depend more on the general sense that another individual is present.     

Post-contest behaviour in black-capped chickadees (<Emphasis Type="Italic">Poecile atricapillus</Emphasis>): loser displays,not victory displays,follow asymmetrical countersinging exchanges

Victory displays are behaviours that occur after the conclusion of a signaling contest, performed solely by the contest winner. Victory displays may reinforce the dominance of the winner either to the loser or to other conspecifics within signaling range. Victory displays are poorly studied despite the significant consequences that post-conflict behaviour may have on the individuals involved. We examined the period immediately following 50 territorial countersinging contests between males in 10 neighbourhoods of black-capped chickadees (Poecile atricapillus) of known dominance rank. We characterized the post-contest singing behaviour of chickadees and evaluated whether post-contest behaviour is consistent with victory displays. Using a 16-microphone acoustic location system to simultaneously record entire neighbourhoods of breeding chickadees, we isolated 50 dyadic countersinging contests and measured the vocal behaviour of the contestants in the minutes following each interaction. Eighty-six percent of contests were followed by a period of solo singing by one of the contestants, while 14% were followed by silence. The post-contest singer was most often the contestant who held a subordinate dominance position in the previous winter’s dominance hierarchy; dominant males performed post-contest song bouts significantly less often. Asymmetry in overlapping between contestants did not predict which bird sang a post-contest bout. However, in a significant majority of cases, the post-contest singer was pitch-matched by his opponent during the contest more than he pitch-matched his opponent. Our results indicate that male chickadees do not perform acoustic victory displays after countersinging contests. In contrast, the post-contest behaviour of territorial chickadees is more consistent with a “loser display”.     

Social context affects testosterone-induced singing and the volume of song control nuclei in male canaries (Serinus canaria)

The contribution of social factors to seasonal plasticity in singing behavior and forebrain nuclei controlling song, and their interplay with gonadal steroid hormones are still poorly understood. In many songbird species, testosterone (T) enhances singing behavior but elevated plasma T concentrations are not absolutely required for singing to occur. Singing is generally produced either to defend a territory or to attract a mate and it is therefore not surprising that singing rate can be influenced by the sex and behavior of the social partner. We investigated, based on two independent experiments, the effect of the presence of a male or female partner on the rate of song produced by male canaries. In the first experiment, song rate was measured in dyads composed of one male and one female (M-F) or two males (M-M). Birds were implanted with T-filled Silastic capsules or with empty capsules as control. The number of complete song bouts produced by all males was recorded during 240 min on week 1, 2, 4, and 8 after implantation. On the day following each recording session, brains from approximately one-fourth of the birds were collected and the volumes of the song control nuclei HVC and RA were measured. T increased the singing rate and volume of HVC and RA but these effects were affected by the social context. Singing rates were higher in the M-M than in the M-F dyads. Also, in the M-M dyads a dominance-subordination relationship soon became established and dominant males sang at higher rates than subordinates in T-treated but not in control pairs. The differences in song production were not reflected in the size of the song control nuclei: HVC was larger in M-F than in M-M males and within the M-M dyads, no difference in HVC or RA size could be detected between dominant and subordinate males. At the individual level, the song rate with was positively correlated with RA and to a lower degree HVC volume, but this relationship was observed only in M-M dyads, specifically in dominant males. A second experiment, carried out with castrated males that were all treated with T and exposed either to another T-treated castrate or to an estradiol-implanted female, confirmed that song rate was higher in the M-M than in the M-F condition and that HVC volume was larger in heterosexual than in same-sex dyads. The effects of T on singing rate and on the volume of the song control nuclei are thus modulated by the social environment, including the presence/absence of a potential mate and dominance status among males.     

Song correlates with social context,testosterone and body condition in male barn swallows

Bird song, like many other male secondary sexual characters, may have evolved as intra- or inter-sexual signals of male phenotypic quality. The hypotheses that song rate and song features reflect androgen levels and body condition, qualities useful in male–male competition, and that they are also influenced by social context, was tested for the first time in the present correlational study. The relationships between song rate and 14 variables describing song structure, respectively, and absolute plasma testosterone levels, body mass, body condition, number of neighbouring males and distances between nest sites in male barn swallows,Hirundo rustica, were analysed. Song rate was not correlated with any of the song features nor with male or social context characteristics. By contrast, a harsh song syllable, the ‘rattle’, was positively related to plasma testosterone levels, and its peak amplitude frequency varied inversely with male body mass and condition. In addition, eight features of song varied according to the social environment of each male. In particular, males sang longer and more varied songs when they had few or no neighbours, whereas males in highly competitive contexts uttered short songs, interrupted them more frequently, and emphasized the rattle. Neighbouring males also sang more similar songs than distant males, and this resulted in matched countersinging. The quality of song output therefore reflects aspects of male competitive potential, and relationships between song structure and social context suggest that some features, such as the rattle, might have originally evolved to serve in male–male interactions; a female preference may have further promoted song evolution leading to complex syllable repertoires.     

Juvenile sparrows preferentially eavesdrop on adult song interactions

Recent research has demonstrated that bird song learning is influenced by social factors, but so far has been unable to isolate the particular social variables central to the learning process. Here we test the hypothesis that eavesdropping on singing interactions of adults is a key social event in song learning by birds. In a field experiment, we compared the response of juvenile male song sparrows (Melospiza melodia) to simulated adult counter-singing versus simulated solo singing. We used radio telemetry to follow the movements of each focal bird and assess his response to each playback trial. Juveniles approached the playback speakers when exposed to simulated interactive singing of two song sparrows, but not when exposed to simulated solo singing of a single song sparrow, which in fact they treated similar to heterospecific singing. Although the young birds approached simulated counter-singing, neither did they approach closely, nor did they vocalize themselves, suggesting that the primary function of approach was to permit eavesdropping on these singing interactions. These results indicate that during the prime song-learning phase, juvenile song sparrows are attracted to singing interactions between adults but not to singing by a single bird and suggest that singing interactions may be particularly powerful song-tutoring events.     

Duet-splitting and the evolution of gibbon songs

Unlike the great apes and most other primates, all species of gibbons are known to produce elaborate, species-specific and sex-specific patterns of vocalisation usually referred to as "songs". In most, but not all, species, mated pairs may characteristically combine their songs in a relatively rigid pattern to produce coordinated duet songs. Previous studies disagree on whether duetting or the absence of duetting represented the primitive condition in gibbons. The present study compares singing behaviour in all gibbon species. Various vocal characteristics were subjected to a phylogenetic analysis using previously published phylogenetic trees of the gibbon radiation as a framework. Variables included the degree of sex-specificity of the vocal repertoire, the occurrence of solo songs, and the preference for a specific time of day for song-production. The results suggest the following scenario for the evolution of gibbon songs: (1) The last common ancestor of recent gibbons produced duet songs. (2) Gibbon duets probably evolved from a song which was common to both sexes and which only later became separated into male-specific and female-specific parts (song-splitting theory). (3) A process tentatively called "duet-splitting" is suggested to have led secondarily from a duetting species to a non-duetting species, in that the contributions of the pair-partners split into temporally segregated solo songs. This appears to be the first time that a non-duetting animal can be shown to be derived from a duetting form. (4) The return to exclusive solo singing may be related to the isolated island distribution of the non-duetting species.     

Song Functions in Nonduetting Gibbons: Evidence from Playback Experiments on Javan Gibbons (<Emphasis Type="Italic">Hylobates moloch</Emphasis>)

Territorial, pair-living primates usually perform long-distance calls as duets in which adult males and females coordinate their calls. Previous studies using playback experiments have shown that gibbon duets convey information about the status of the caller (location, familiarity, sex of the caller, and paired status) and gibbons use this information to respond to achieve several nonmutually exclusive functions, including intragroup contact, territorial defense, and pair-bond advertisement and strengthening. However, not all pair-living gibbons duet, and it is unclear whether the same results should be expected in nonduetting species. We conducted song playback experiments (N = 47 trials) to test hypotheses about song functions in nonduetting gibbons on two groups of wild Javan gibbons (Hylobates moloch) in the Gunung Halimun-Salak National Park, Indonesia. Javan gibbons initiated movement toward the speaker more quickly in response to songs broadcast in the center of the territory, stranger songs, and songs of unpaired individuals than to songs at the border, neighbor songs, and songs from paired individuals. These results suggest that Javan gibbons can localize songs, and that Javan gibbon songs transmit information about the identity and paired status of the caller. Our results imply that Javan gibbon solo songs are likely to function for territorial defense and pair-bond advertisement like duets in other primates.