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Nomascus siki

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Taxonomy [top]

Kingdom Phylum Class Order Family
ANIMALIA CHORDATA MAMMALIA PRIMATES HYLOBATIDAE

Scientific Name: Nomascus siki
Species Authority: (Delacour, 1951)
Common Name(s):
English Southern White-cheeked Gibbon
Taxonomic Notes: This taxon is variously considered a subspecies of N. concolor, N. leucogenys and N. gabriellae (M. Richardson pers. comm.). This may not be a genuine species, but rather, a natural hybrid of N. leucogenys and N. gabriellae. According to Delacour (1951) and Groves (1972), this species may possibly interbreed with N. gabriellae in Saravane and Savannakhet, Lao.

The limits of distribution of this species are unclear, both with regards to N. gabriellae in the south and N. leucogenys in the north. Identity of gibbons in a large area covering parts of central Viet Nam, southern Lao and northeastern Cambodia is unclear, as these gibbons differ in their song from both N. gabriellae in the south and N. siki in the north, but phenotypically resemble N. gabriellae (Geissmann et al. 2000, Konrad and Geissmann 2006, T. Geissmann pers. comm. 2007). Here, gibbons that, at least phenotypically (colouration), look like N. gabriellae, are included in N. gabriellae. Similarly, phenotype information for the type locality of N. siki (Thua Luu, Thua Thien Hue province, central Viet Nam) is contradictory (Geissmann et al. 2000), but these gibbons are tentatively identified as N. gabriellae here.

Assessment Information [top]

Red List Category & Criteria: Endangered A2cd ver 3.1
Year Published: 2008
Date Assessed: 2008-06-30
Assessor(s): Manh Ha, N., Rawson, B., Geissmann, T. & Timmins, R.J.
Reviewer(s): Mittermeier, R.A. & Rylands, A.B. (Primate Red List Authority)
Justification:
Listed as Endangered as there is reason to believe the species has declined by at least 50% over the past 45 years (three generations) due primarily to logging, agricultural encroachment, and hunting.
History:
2000 Data Deficient

Geographic Range [top]

Range Description: This species is distributed in southern Lao PDR and north-central Viet Nam east of the Mekong River, and is found between 17°N and about 19.3°N (Geissmann et al. 2000). There is an apparent overlap or interdigitation between the ranges ofN. sikiand N. leucogenys between about 19 and 20°N. Traditionally, the range of N. siki includes central Viet Nam as far south as Bach Ma, but it here extends less to the south to exclude animals that, at least phenotypically (coloration), are N. gabriellae, such that the range extends only as far south as Savannakhet in southern Lao PDR and Quang Binh province, and possibly Quang Tri province in central Viet Nam (Geissmann et al. 2000).
Countries:
Native:
Lao People's Democratic Republic; Viet Nam
Range Map: Click here to open the map viewer and explore range.

Population [top]

Population: Many populations in Viet Nam are supported by too little contiguous habitat and experience too high a degree of human disturbance to remain viable, especially those in forested areas that are not shared with Lao PDR (Geissmann et al. 2000). In Pu Mat, numbers are believed to have declined where encounter rates declined from 22.6 per 100 survey days in 1999 to 14 in 2004 (Grieser Johns et al. 2004). In a status survey report, Geissmann et al. (2003) recorded six localities where traditionalN. sikiwere previously known to occur, of which all but one still had surviving populations at the time. During a survey covering about 15 km2 of the Phong Nha-Ke Bang National Park (Quang Binh province, central Viet Nam; total area: 858 km2), 13 gibbon groups were heard, suggesting a density of about 0.7 groups/km2 (Ruppell 2007). There are no available population estimates in Lao PDR, but the species is widespread and common in all large forest blocks within its distribution area (T. Geissmann pers. comm.).
Population Trend: Decreasing

Habitat and Ecology [top]

Habitat and Ecology: It lives in tall primary broadleaf evergreen forest, and is found in the lowlands at elevations of 30–100 m, in a typical wet tropical climate, with no influence of the north east monsoon and no conspicuous dry season (Dao Van Tien 1983). In some areas such as Phong Nha-Ke Bang National Park in central Viet Nam, these gibbons live in steep karst forest (Ruppell 2007). In Lao PDR, gibbons are found from the Mekong plains up to 1,800 m in Nakai-Nam Theun National Biodiversity Conservation Area (Duckworth et al. 1999).

Gibbons are strictly arboreal, diurnal and mainly frugivorous (Geissmann et al.2000), but there are virtually no field data on the behavioral ecology of N. siki. Dao Van Tien (1985) studied the contents of the stomach of six wild-shot crested gibbons (genus Nomascus) from Viet Nam, including one N. siki, and found 90–100% fruits, associated with some leaves and insects. These data cannot be compared directly to field observations, which usually measure the time spent eating various food items (Geissmann et al. 2000). Average group size is unknown. A gibbon group with three individuals was reported from Khe Giua State forest enterprise (Quang Binh province, central Viet Nam), and in another gibbon group in Nui Giang Man area (also Quang Binh province), five individuals were counted (Le Khac Quyet 2004; Nguyen Manh Ha et al. 2005).
Systems: Terrestrial

Threats [top]

Major Threat(s): Unlike in Lao PDR, the forest habitat of Nomascus siki in Viet Nam is heavily fragmented due to logging and agricultural encroachment, and a high population density of humans continues to threaten this species’ habitat and population numbers. Hunting, in particular, is a major threat in both Viet Nam and Lao PDR where they are used both in traditional “medicine,” food, and in the pet trade (Duckworth et al. 1999; Geissmann et al. 2000; Nguyen Manh Ha et al. 2005).

Conservation Actions [top]

Conservation Actions: This species is listed on CITES Appendix I. Although it occurs in a mixture of protected areas and national parks throughout its range and is legally protected in Viet Nam (Appendix 1b of Decree 32, 2006), there is inadequate enforcement against forest encroachment and poaching. Recommended conservation measures include prevention of hunting and wildlife trade; minimization of habitat disturbance; and research and field surveys throughout the range, specifically tape recordings, genetic analysis and photographic recordings to help better define the distribution area of the taxon relative to N. gabriellae and N. leucogenys.

In Viet Nam, the most important populations are in Phong Nha-Ke Bang National Park and Pu Mat National Park (Nguyen Manh Ha et al. 2005). In Lao PDR, the most important population is in Nakai Nam Theun National Biodiversity Conservation Area (Timmins and Evans 1996).

Citation: Manh Ha, N., Rawson, B., Geissmann, T. & Timmins, R.J. 2008. Nomascus siki. The IUCN Red List of Threatened Species. Version 2014.2. <www.iucnredlist.org>. Downloaded on 19 September 2014.
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