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Superregnum: Eukaryota
Cladus: Unikonta
Cladus: Opisthokonta
Cladus: Holozoa
Regnum: Animalia
Subregnum: Eumetazoa
Cladus: Bilateria
Cladus: Nephrozoa
Superphylum: Deuterostomia
Phylum: Chordata
Subphylum: Vertebrata
Infraphylum: Gnathostomata
Megaclassis: Osteichthyes
Cladus: Sarcopterygii
Cladus: Rhipidistia
Cladus: Tetrapodomorpha
Cladus: Eotetrapodiformes
Cladus: Elpistostegalia
Superclassis: Tetrapoda
Cladus: Reptiliomorpha
Cladus: Amniota
Cladus: Synapsida
Cladus: Eupelycosauria
Cladus: Sphenacodontia
Cladus: Sphenacodontoidea
Cladus: Therapsida
Cladus: Theriodontia
Cladus: Cynodontia
Cladus: Eucynodontia
Cladus: Probainognathia
Cladus: Prozostrodontia
Cladus: Mammaliaformes
Classis: Mammalia
Subclassis: Trechnotheria
Infraclassis: Zatheria
Supercohors: Theria
Cohors: Eutheria
Infraclassis: Placentalia
Cladus: Boreoeutheria
Superordo: Euarchontoglires
Ordo: Primates
Subordo: Haplorhini
Infraordo: Simiiformes
Parvordo: Catarrhini
Superfamilia: Hominoidea

Familia: Hylobatidae
Genus: Hylobates
Species: Hylobates klossii

Hylobates klossii (Miller, 1903)

Hylobates klossii in Mammal Species of the World.
Wilson, Don E. & Reeder, DeeAnn M. (Editors) 2005. Mammal Species of the World – A Taxonomic and Geographic Reference. Third edition. ISBN 0-8018-8221-4.

Vernacular names
dansk: Kloss' gibbon
Deutsch: Kloss-Gibbon, auch Mentawai-Gibbon, Biloh oder Zwergsiamang
English: Kloss Gibbon, Bilou
español: gibón de Kloos
français: Siamang de Kloss ou Gibbon de Kloss
magyar: Kloss-gibbon
italiano: gibbone delle Mentawai
日本語: クロステナガザル
Nederlands: Dwergsiamang
polski: Gibbon karłowaty
português: gibão-de-klos
русский: Карликовый гиббон
svenska: Kloss' gibbon

Kloss's gibbon (Hylobates klossii), also known as the Mentawai gibbon, the bilou or dwarf siamang, is an endangered primate in the gibbon family, Hylobatidae. It is identifiable in that it is all black,[4] resembling the siamang with its black fur, but is considerably smaller and lacks the siamang's distinctive throat pouch. Kloss's gibbon reaches a size 17 to 25 inches (44 to 63 cm) and weigh at most 13 pounds (6 kg). As is the case for all gibbons, they have long arms and no tail. Males and females are difficult to distinguish.[5]

Kloss's gibbon exclusively lives on the Mentawai Islands that lie to the west of Sumatra.[1] The Mentawai Islands consist four main islands that are part of an island archipelago.[6] These islands are hearths of biodiversity, and are the homes of many endemic species.[6] It is a diurnal inhabitant of the rain forest that hangs in the trees from its long arms and rarely comes to the ground. Because Kloss's gibbon rarely comes out of the canopy, these gibbons use tree limbs to cross obstacles like bodies of water.[7] Like all species of gibbons it lives together in pairs that stake out a territory from approximately 49 to 74 acres (20 to 30 hectares) of size. This area is defended vehemently against other gibbons. Its diet consists mainly of fruits, occasionally also eating different plant parts, bird eggs, insects and small vertebrates.

Kloss's gibbon are unique gibbons because the two sexes do not vocalize songs in unison.[8] Male Kloss's gibbon sing before the sun rises and female Kloss's gibbon vocalize songs once the sun has risen.[8] Female Kloss's gibbon calls are unique and have slight variations in the different stages of the call.[9] The calls emitted by the females could offer other members of the species information about their position in the canopy.[9]

The reproductive cycle of Kloss's gibbon is similar to that of other gibbons. Every two to three years the female may give birth to a single young (with a gestation period of seven months). The young is weaned in the middle of its second year, and is fully mature in about seven years. Their life expectancy is about 25 years in the wild, and up to 40 years in captivity. Kloss's gibbons are monogamous, and male Kloss's gibbons use territory to attract and court possible mates.[10] Territories are often contested and Kloss's gibbons defend their territories with aggression and threats.[10] During courtship, males and females defend territory together, and mating only occurs after the females deems the male capable of defending a suitable territory.[10]

Kloss's gibbon are picky eaters, and never consume over-ripened fruit.[5] Kloss's gibbons diet consists of fruit, leaves, shoots, and insects.[5] Kloss's gibbons favorite fruit is figs, but these gibbons do not spend a lot of time eating figs because figs are scarce on the Mentawai Islands.[5]

When Kloss's gibbons are observed by human researchers, they adapt and become less afraid, a process called habituation. Kloss's gibbons can become habituated to humans and will not flee when they are observed.[11] Unhabituated Kloss's gibbons sometimes exhibit a warning behavior in which they act as a lure toward people.[11] One male gibbon will produce warning calls and attract attention to himself while other members of his group make their escape.[11] Kloss's gibbons behavior help it stay cryptic, possibly to help them avoid hunting pressure.[11] Female Kloss's gibbons sing less frequently than males, and males only sing in the predawn when visibility isn't optimal for hunting.[11] Kloss's gibbons also exhibit less aggregate behavior like grooming and playing, possibly to reduce their visibility and stay hidden from hunters.[11]

Kloss's gibbon is in jeopardy of going extinct.[8] Kloss's gibbon is classified as endangered by the IUCN (2021).[2] Recent estimates conclude that there are around 20,000 to 25,000 Kloss's gibbons alive in the wild, and its numbers are declining.[12] Over the last two and a half decades, the population size of Kloss's gibbon has fallen by 50%.[12] There are several threats that pose significant risk to Kloss's gibbon. Kloss's gibbon has no natural predators, and humans are the main threat to the existence of this primate.[7] Native peoples of the Mentawai Islands kill Kloss's gibbon and other endemic primates for subsistence, and also participate in poaching activities.[6] Globalization and industrialization in the Mentawai Islands are contributing toward the degradation of high quality habitat needed by Kloss's gibbon.[13] Road development and the adoption of air rifles are allowing natives to kill Kloss's gibbon easier and at higher rates.[13] Kloss's gibbon spends the majority of its time in the tree canopy, and as a result, this species requires undisturbed, old-growth forest habitats to sustain itself.[6] Kloss's gibbon is at risk due to habitat loss, as its homeland islands are suffering from deforestation.[6] Conservationists focused on improving Kloss's gibbons endangered status need to protect and preserve the high quality habitat needed by these gibbons.[6] Fragments of habitat need to be connected to allow movement without risk of exposure in highly modified areas.[6] Local government has been cooperating with global organizations such as UNESCO to raise awareness as well as increase the amount of protected land in the Mentawai Islands.[13]

Groves, C. P. (2005). "Order Primates". In Wilson, D. E.; Reeder, D. M (eds.). Mammal Species of the World: A Taxonomic and Geographic Reference (3rd ed.). Johns Hopkins University Press. p. 179. ISBN 978-0-8018-8221-0. OCLC 62265494.
Liswanto, D.; Whittaker, D.; Geissmann, T.; Whitten, T. (2020). "Hylobates klossii". IUCN Red List of Threatened Species. 2020: e.T10547A17967475. doi:10.2305/IUCN.UK.2020-2.RLTS.T10547A17967475.en. Retrieved 19 November 2021.
"Appendices | CITES". Retrieved 2022-01-14.
Kloss Gibbon at the zoo
Whitten, Anthony J. (1982-02-08). "Diet and Feeding Behaviour of Kloss Gibbons on Siberut Island, Indonesia". Folia Primatologica. 37 (3–4): 177–208. doi:10.1159/000156032. ISSN 0015-5713. PMID 7076055.
Yanuar, Ahmad (2018). "The Status of Primates in the Southern Mentawai Islands, Indonesia". Primate Conservation. Issue 32: 193–203 – via EBSCOhost.
Tenaza, R.R. (1975). "Territory and Monogamy Among Kloss' Gibbons (Hylobates klossii) in Siberut Island, Indonesia". Folia Primatologica. 24 (1): 60–80. doi:10.1159/000155685. ISSN 1421-9980. PMID 1140755.
Höing, Andrea; Quinten, Marcel C.; Indrawati, Yohana Maria; Cheyne, Susan M.; Waltert, Matthias (February 2013). "Line Transect and Triangulation Surveys Provide Reliable Estimates of the Density of Kloss' Gibbons (Hylobates klossii) on Siberut Island, Indonesia". International Journal of Primatology. 34 (1): 148–156. doi:10.1007/s10764-012-9655-7. ISSN 0164-0291. PMC 3605491. PMID 23538477.
Haimoff, E.H.; Tilson, R.L. (1985-02-14). "Individuality in the Female Songs of Wild Kloss' Gibbons (Hylobates klossii) on Siberut Island, Indonesia". Folia Primatologica. 44 (3–4): 129–137. doi:10.1159/000156207. ISSN 0015-5713.
Tilson, R. L. (1981-01-31). "Family Formation Strategies of Kloss's Gibbons". Folia Primatologica. 35 (4): 259–287. doi:10.1159/000155979. ISSN 0015-5713. PMID 7196868.
Dooley, Helen M.; Judge, Debra S. (2014-10-08). "Kloss gibbon (Hylobates klossii) behavior facilitates the avoidance of human predation in the Peleonan forest, Siberut Island, Indonesia". American Journal of Primatology. 77 (3): 296–308. doi:10.1002/ajp.22345. ISSN 0275-2565. PMID 25296898. S2CID 1616932.
Whittaker, Danielle J. (2005-10-19). "New population estimates for the endemic Kloss's gibbon Hylobates klossii on the Mentawai Islands, Indonesia". Oryx. 39 (4): 458. doi:10.1017/s0030605305001134. ISSN 0030-6053. S2CID 84629632.
Whittaker, Danielle J. (May 2006). "A Conservation Action Plan for the Mentawai Primates". Primate Conservation. 20: 95–105. doi:10.1896/0898-6207.20.1.95. ISSN 0898-6207. S2CID 85765480.

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