(Symphalangus syndactylus)

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HABITAT AND ECOLOGY This species lives in primary and secondary semi-deciduous and tropical evergreen forest. All levels of the canopy are used, although emergent trees are required for resting and sleeping. Siamangs occur at lower densities in secondary forest, but can persist in secondary areas. They range from the lowlands up to 2,000 m in elevation in some areas of Kerinci Seblat National Park, Sumatra, Indonesia (Yanuar 2009), with preferences for lowland and submontane/montane forest types. Siamangs are rarely found in swamp forest habitat. During a short survey in southern Sumatra, siamangs appeared to be less sensitive to habitat degradation than sympatric agile gibbons (Hylobates agilis) (Geissmann et al. 2006). Siamangs were also noted to maintain relatively high densities at forest edges within the Bukit Barisan Selatan National Park, Sumatra, Indonesia (O’Brien et al. 2004). Nevertheless, compared to sympatric agile gibbons, this species was encountered infrequently at forest edges and within disturbed forest areas in the submontane forests of the Batang Toru Forest Complex, Sumatra, Indonesia (Nowak pers. obs.). Though this species is primarily folivorous in mainland Asia (Chivers 1974; MacKinnon and MacKinnon 1980; Raemaekers 1984), it is primarily frugivorous on Sumatra (West 1982; Palombit 1992, 1997; Nurcahyo 1998; Lappan 2009b). For both mainland Asia and Sumatra, their fruit/leaf diet is supplemented with moderate levels of flower and insect consumption. Palombit (1992, 1997) argues that these animals are flexible foragers, preferring fruit when available, but able to switch to leaves when necessary. Such flexibility may help reduce siamang vulnerability to habitat disturbance (O'Brien et al. 2003). Siamangs are strictly arboreal, highly territorial, and primarily monogamous (Chivers 1974). However, extra-pair copulations have been reported in Ketambe, Gunung Leuser National Park, Sumatra (Palombit 1994), and groups with more than one adult male have been reported in the Bukit Barisan Selatan National Park population, Sumatra (O'Brien et al. 2003; Lappan 2005, 2007, 2009d). Home range has been recorded at 15-47 ha on the Malayan peninsula (Chivers 1974; Raemaekers 1977; MacKinnon and MacKinnon 1980) and 20-50 ha in previously studied Sumatran populations (Palombit 1997; Nurcahyo 1998; O’Brien et al. 2003; Lappan 2009b; Yanuar and Chivers 2010). Within the submontane forests of the Batang Toru Forest Complex, siamangs have been documented to have home ranges >50 ha (Nowak pers. obs.). Dispersal distances are less than 3 km. O'Brien et al. (2003) found that monogamy and strict territoriality may limit the range of possible response to fire and other severe disturbances by this species. Wild siamangs are known to have relatively slow life histories, including long interbirth intervals (>3-4 years) (Palombit 1995; O’Brien et al. 2003). The infant to subadult survival rate in Bukit Barisan Selatan National Park for ‘normal’ groups was 61.7%, whereas for groups affected by ENSO-related wildfires, infant to subadult survival rate was substantially less (22%; O’Brien et al. 2003).

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Update : 06 April 2017