||Silvery Gibbon, Javan Gibbon, Moloch Gibbon, Owa Jawa
Hylobates cinera Cuvier, 1798
Hylobates javanicus Matchie, 1893
Hylobates leucisca (Schreber, 1799)
Hylobates pongoalsoni Sody, 1949
||This taxon is monotypic (Geissman et al. 2002; T. Geissmann pers. comm.), although it has been suggested that there is evidence for two genetically distinct silvery gibbon populations (Andayani et al. 2001), leading to the subsequent recognition of two subspecies by several authors (Hilton-Taylor 2000, Supriatna 2006, Supriatna and Wahyono 2000), a recent review of the molecular evidence and a comparison of morphological and vocal data casts doubt on this claim (Geissman et al. 2002, T. Geissmann pers. comm.).
|Red List Category & Criteria:
||Andayani, N., Brockelman, W., Geissmann, T., Nijman, V. & Supriatna, J.
||Mittermeier, R.A. & Rylands, A.B. (Primate Red List Authority)
Listed as Endangered because its population size is estimated to number fewer than 2,500 mature individuals, there is an observed continuing decline in the number of mature individuals, and no subpopulation contains more than 250 mature individuals. The change in status from Critically Endangered to Endangered reflects the availability of better information and does not suggest that the threats have decreased; in fact, threats continue to increase but do not yet reach the level necessary to be classified as Critically Endangered. There is concern about the legal status of the largest populations; this species, therefore, should be periodically reassessed so that current status and persistent threats are monitored.
|Previously published Red List assessments:|
- 2000 – Critically Endangered (CR)
- 1996 – Critically Endangered (CR)
- 1994 – Endangered (E)
- 1990 – Endangered (E)
- 1988 – Endangered (E)
- 1986 – Endangered (E)
|Population:||From 1994-2002, Nijman (2004) assessed the entirety of the silvery gibbon’s population in its known areas of occurrence by using fixed-point counts and forest transect walks, as well as by a review of literature. Their presence was detected by listening for gibbon song, and affirmed by local park officers and residents. He estimates that between 4,000–4,500 individuals remain in over 15 different locations. Over 95% of the gibbons are in populations of more than 100 individuals, and the four largest areas support populations of more than 500 individuals each (Nijman 2004). Asquith (2001) reported that in 1995 nine local populations had gone extinct, though Nijman found two of these locales to still harbor silvery gibbons. This is attributed to the effects of habitat disturbance and low population density on calling frequency, and suggests an under-representation of gibbon abundance and number of remaining populations (Nijman 2004). Small populations of the species are likely to go extinct; however, this will not impact the overall population estimate in the immediate future (Nijman pers. comm.).|
Median population density ranges are 2.7 groups/km2 or 9.0 individuals/km2 in lowland forest (<500 m), 2.6 groups/km2 or 8.6 individuals/km2 for hill forest (500-1,000 m), and 0.6 groups/km2 or 1.5 individuals/km2 for lower montane forest (1000-1,750 m) (Geissmann and Nijman 2006; Nijman 2004).
|Current Population Trend:||Decreasing|
|♦ Number of mature individuals:||4000-5000|
|♦ Population severely fragmented:||No|