Inheritance of Song Parameters in the Gibbon Song, Analysed in 2 Hybrid Gibbons (Hylobates pileatus

The song of a male and a female hybrid gibbon (Hylobates pileatus x H. lar) is described. Comparison with the song of both parental species revealed that at least some of the hybrid’s song characteristics were inherited. Although several studies have tentatively suggested a genetical determination of gibbon song patterns, there have been no previous analyses of gibbon singing behaviour which conclusively documented this phenomenon. Attention is drawn to similarities between the song of the hybrids and gibbon species other than the parental ones, and their significance is considered. Introduction maintenance of pair and family bonds [for a list of references see Geissmann, 1983]. Among most gibbon species, mates vocalize together in a relatively rigid pattern, and at least in part, with a sex-specific repertoire so that a so-called duet evolves [Deputte, 1982; Deputte and Leclerc-Cassan, 1981; Geissmann, 1983; Gittins, 1978a; Goustard, 1979a, b, 1980, 1982a, b; Haimoff, 1981, 1984, in press a, b, c; Lamprecht, 1970; Marler and Tenaza, 1977; Marshall, 1981; Marshall et al., 1972; Marshall and Marshall, 1976; Schilling, 1980; Schröpel, 1977; Tembrock, 1962, 1964, 1974; Ziegler All gibbon species on which investigation has been carried out live in tropical rain forests, in monogamous, strictly territorial family groups [Carpenter, 1940; Chivers, 1972, 1974; Ellefson, 1968, 1974; Gittins, 1980; Gittins and Raemaekers, 1980; Kappeler, in press; Tenaza, 1975a; Tilson, 1979, 1981; Whitten, 1982a, b]. Their conspicuous vocalizations, mainly uttered at specifically established times of day, probably play an important role in territorial behaviour, and possibly also in the Inheritance in the Song of 2 Hybrid Gibbons 217 and Knobloch, 1968]. All gibbon songs show a similarly determined sequential structure [Haimoff, in press a; Haimoff et al., 1982]. This includes solo songs of those species where, up to now, no duets have been described in the wild [Kappeler, in press, pers. commun. ; Marler and Tenaza, 1977; Tenaza, 1975b, 1976; Whitten, 1982c]. Yearbook, 1962-1974, 1977-1981); the most spectacular case describes a gibbon-siamang hybrid, H. muelleri abbotti x H. syndactylus [Myers and Shafer, 1979; Rumbaugh et al., 1976; Wolkin and Myers, 19801. In addition, from several species with adjoining or overlapping distribution, individual feral hybrids have been reported [Brockelman, 1978; Gittins, 1978b]. The song repertoire is notably constant in structure and organization for each species, and according to several authors believed to be largely genetically determined [Boutan, 1913; Brockelman, 1978; Carpenter, 1940; Marler and Tenaza, 1977; Tembrock, 19701. However, proof or disproof of inherited song characteristics could not be conclusively determined from previous research. The observation that captive gibbons retain their species-specific song even in heterogenous groups [Carpenter, 1940] does not explain how these animals acquired their particular song. Boutan [1913] raised a young gibbon in isolation from other gibbons; also this animal finally could utter (according to Boutan) the song typical of this species. The possibility cannot be precluded, however, that this animal learned the song from his parents prior to separation; moreover, it is not clear from this otherwise detailed description, to what extent the song of this animal was in fact speciesspecific. Nevertheless, only fragmentary details are available regarding the songs of gibbon hybrids. Brockelman [1978] reports ‘odd vocalizations’ in connection with naturally occurring hybrids between H. lar and H. pileatus. Marler and Tenaza [1977] describe a pair of H. lar x H. muelleri hybrids where the females’ song was ‘structurally intermediate between songs of the parental species’, whereas ‘both song structure and duetting behavior’ of the male hybrid resembled that of the father. One should note that in the description of hybrids presented here, the father’s species is mentioned before the mother’s. Maples and Haraway [1982] stated that a female H. muelleri x H. agilis hybrid vocalized in a way that was typical for H. muelleri females, and ‘quite different from the song of either of her parents’. The authors claim, moreover, that the daughter produced this song without ever having heard it before. Hybrids such as those examined here, namely crosses between the pileated or capped gibbon, H. pileatus, and the lar or whitehanded gibbon, H. lar, have been repeatedly born in captivity and have become the object of scientific studies [Ibscher, 1964, 1967; MeyerHolzapfel, 1950; Steiner, 1949]. Backcrosses have also been reported [e.g. Groves, 19721. The distribution of both parental species overlaps slightly in the area of Khao Yai National Park in northeast Thailand, about 200 There remains the question of how the species-specific song traits can be passed on from one generation to the next. Critical evidence could, under particular circumstances which exclude the possibility of parental teaching, be expected from the analysis of songs from hybrid gibbons compared with their parents. In fact, several gibbon species have been crossed in captivity (see e.g. the records of ‘mammals bred in captivity’, in: Int. Zoo