Hylobates moloch (Primates: Hylobatidae)

Abstract

Hylobates moloch (Audebert, 1797), the Javan or silvery gibbon, is a pair-living small ape which is exclusively found in the western and central regions of the Indonesian island of Java. It represents the southernmost occurring species of the genus Hylobates and inhabits the canopy of tropical rainforests. It is foremost characterized by its long silvery-gray fur in combination with a lack of duet songs in mated pairs. Hylobates moloch is threatened by habitat loss as well as the illegal wildlife trade and is listed as “Endangered” (EN) by the IUCN Red List of Threatened Species.

Hylobates Illiger, 1811

• Homo : Linnaeus, 1771:521. Part, not Homo Linnaeus, 1758.

• Simia : Schreber, 1775:66, pl.III. Part, not Simia Linnaeus, 1758, an unavailable name (International Commission on Zoological Nomenclature 1929, Opinion 114).

• Gibbon Zimmermann, 1777:405. Type species Homo lar Linnaeus, 1771 (= Hylobates lar (Linnaeus, 1771)), by monotypy. Unavailable name (International Commission on Zoological Nomenclature 1954, Opinion 257).

• Pithecus : Latreille, 1804:276. Part, not Pithecus Geoffroy Saint-Hilaire and Cuvier, 1795, an unavailable name (International Commission on Zoological Nomenclature 1929, Opinion 114).

• Hylobates Illiger, 1811:67. Type species Homo lar Linnaeus, 1771, by monotypy.

• Satyrus Oken, 1816:1225. No type species designated. Unavailable name (International Commission on Zoological Nomenclature 1956, Opinion, 417); preoccupied by Satyra Meigen, 1803, a genus of Diptera.

• Laratus Gray, 1821:297. Type species Homo lar Linnaeus, 1771.

• Cheiron Burnett, 1828:307. No type species designated.

• Hilobates Harlan, 1834:52. Incorrect subsequent spelling of Hylobates Illiger, 1811.

• Hybolates Geoffroy Saint-Hilaire, 1834:31. Incorrect subsequent spelling of Hylobates Illiger, 1811.

• Brachiopithecus Sénéchal, 1839:428. Part. No type species designated.

• Methylobates Ameghino, 1884:479. Part. No type species designated.

• Brachitanytes Schultz, 1932:369. Type species Symphalangus klossii, Miller, 1903 (= Hylobates klossii (Miller, 1903)) by original designation.

• Loratus Groves in Wilson and Reeder, 1993. Incorrect subsequent spelling of Laratus Gray, 1821.

Context and Content.

Order Primates, suborder Haplorrhini, infraorder Simiiformes, parvorder Catarrhini, superfamily Hominoidea, family Hylobatidae, genus Hylobates. Hylobates is polytypic, with nine extant species being commonly recognized (Chivers et al. 2013; Roos 2016): H. abbotti, H. agilis, H. albibarbis, H. funereus, H. klossii, H. lar, H. moloch, H. muelleri, H. pileatus. The Bornean species H. abbotti and H. funereus, which have been suggested to be distinct from H. muelleri (Thinh et al. 2010), remain insufficiently studied so that their current taxonomic status is not yet convincingly justified (Roos 2016). The following key is based on adult morphological traits, primarily coat color, as described by Geissmann (1995) and Groves (1972, 2001), as well as selected song characteristics (Geissmann 1995). Expression of facial ornaments might be reduced in senile animals (Geissmann 1995). The presence of sexual dichromatism, ontogenetic changes in fur color, and great intraspecific variability in pelage coloration in some taxa can notably complicate the diagnosis of some Hylobates species based on physical characteristics alone.

1. Both sexes jet black; small, sparsely haired throat sack present. Distribution: Mentawai islands west of Sumatra... ….................................................................………H. klossii

   • Pelage not pure black, no throat sack present.….....................2

2. Pelage sexually dichromatic, both sexes with white hands and feet, males black with white genital tuft, face ring, and a black cap typically framed by white to gray hair; females creamy white to gray with black cap and chest. Distribution: Eastern Thailand and western Cambodia…….........…...…H. pileatus

   • Pelage not sexually dichrozmatic (except facial ornaments), if color phases present, then unrelated to sex…..................3

3. Hands and feet always white; white face ring usually well developed (more so in males than in females). Light (cream-colored) and dark (brown to black) color phases in continental populations, invariably light brown on Sumatra. Distribution: Central Indochina, Malayan Peninsula, northern Sumatra (Aceh)…....................................................................…H. lar

   • Hands and feet never white...............................................….4

4. Pelage silvery gray, often with wooly texture. Light face ring often incomplete with distinct white beard on chin; dark cap at least in traces present and often distinct. Primitive molar morphology conforming to hominoid ground pattern. Distribution: West Java.…..................................…H. moloch

   • Pelage not silvery gray, molar morphology derived, no chin beard but with light brow band…..................………….5

5. Hair over ears markedly elongated, genital tuft in males short and dark. Usually no light beards in either sex. Female song climax with sequence of fast bubbling notes….............……6

   • Hair over ears not strongly elongated, male genital tuft prominent. Light beards at least present in most males. Female song climax consists of long notes……..................…....8

6. Pelage mousy gray to brownish with face ring reduced to a white brow streak and no chin beard. Dark cap weakly expressed if at all, hands not darkened. Distribution: West Borneo north of the Kapuas river……........………H. abbotti

   • Pelage brown with contrasting dark ventrum and cap………7

7. Hands or at least digits darker than limbs. Distribution: South eastern Borneo, east of the Barito river………….….........................................…….H. muelleri

   • Hand same color or lighter than limbs. Distribution: Northern Borneo, north to the Mahakam river..………H. funereus

8. Large light cheek patches and contrastingly colored pelage in both sexes, dorsum light brown to golden, cap and ventral portions of rump and limps dark brown. Distribution: Southern Borneo between the rivers Kapuas and Barito….…................................................……H. albibarbis

   • Pelage highly variable in color, ranging from creamy over brown to black, ventrum might be darkened or same as back. Face ring variably expressed, sometimes only supraciliary portion present. Males often with light cheek patches that are lacking in females. Dorsoventral contrast is weaker and the face ring in adults, especially in the cheek region, less prominent than in H. albibarbis. Distribution: Sumatra, south of Aceh, smaller zone of occurrence on the Malay Peninsula, roughly at the boarder of Thailand and Malaysia.......................……..…H. agilis

Hylobates moloch (Audebert, 1797)

Javan Gibbon

• Simia Nanodes Lichtenstein, 1791:31. Type locality “India praefertim in Java.” Nomen oblitum.

• Simia Moloch Audebert, 1797:pl. II. Type locality “les Moluques,” amended to “Tjianten, Mt. Salak, ca. 1100 m” by Sody (1949:121). Nomen protectum.

• Simia cinerea Cuvier, 1798:96 Type locality “Batavia.” Preoccupied by Simia cinerea Kerr, 1792 (= Mandrillus leucophaeus (F. Cuvier, 1807)).

• Simia Leucisca Schreber, 1799:pl. III B (dated after Sherborn (1892)). Type locality unknown.

• Pit[hecus]. cinereus : Latreille, 1804:277. Name combination.

• ? Simia hirsuta Forster, mentioned in Sonnerat, 1806:81. Type locality unknown. Nomen nudum.

• Pithecus leuciscus : Geoffroy Saint-Hilaire, 1812:89. Name combination.

• S[atyrus]. Leuciscus : Oken, 1816:1226. Name combination.

• Hylobates leuciscus : Kuhl, 1820:6. Name combination. Not Hylobates leuciscus Matschie, 1893:62 (= Hylobates abbotti).

• Cheiron Leuciscus: Burnett, 1828:307. Name combination.

• H[ybolates]. Leusiscus Geoffroy Saint-Hilaire, 1834:34. Incorrect subsequent spelling of leuciscus Schreber, 1799.

• H[ylobates]. leucurus Gray, 1861:136. Incorrect subsequent spelling of leuciscus Schreber, 1799.

• Hylobates javanicus Matschie, 1893:62. Type locality “Java.”

• Hylobates lar leuciscus : Pocock, 1927:727. Name combination.

• Hylobates cinereus cinereus : Kloss, 1929:119. Name combination.

• Hylobates moloch : Frechkop, 1934:23. First use of current name combination (already alluded to but not written out in Cabrera, 1930).

• Hylobates moloch moloch : Chasen, 1940:64. Name combination.

• Hylobates lar moloch : Sody, 1949:121. Name combination.

• Hylobates lar pongoalsoni Sody, 1949:123. Type locality ”Kali Kidang, Mount Slamat, C[entral]. Java, 800 m.”

• Hylobates moloch pongoalsoni : Supriatna, Andayani, Forstner, and Melnick in Supriatna and Manullang, 1999. Name combination.

Context and Content.

Context as for genus. There has been a long debate on whether Hylobates moloch encompasses two subspecies rather than being monotypic. The two proposed subspecies H. m. moloch and H. m. pongoalsoni (originally described as H. lar pongoalsoni) are supposed to be living in western and central Java, respectively (Sody 1949). When named by Sody (1949), the geographic ranges of these alleged taxa were not clearly defined. Respective H. m. moloch specimens were derived from Gunung Salak and Purwakarta in the West Javan province, whereas H. m. pongoalsoni vouchers originated from Gunung Slamet and Karang Gondang in Central Java (Sody 1949). Hylobates m. pongoalsoni is supposed to have a lighter colored back and to lack the blackish cap typical for H. m. moloch. However, later studies could not confirm a geographic pattern in fur coloration (Groves 1972; Geissmann et al. 2002). Similarly, genetic data once treated as evidence in support for a subspecific separation (Andayani et al. 2001; Kheng et al. 2018) failed to provide support for this assumption in a recent reevaluation and are now viewed as indicative for an isolation by distance pattern between populations (Nijman et al. 2019; see “Population genetics”). Apart from that, geographic patterns in female song structure were discussed as potential support for a subspecific splitting of H. moloch, again pointing to a separation between individuals in western and central Java (Dallmann and Geissmann 2009). However, song recordings are only available from few sites that do not include the central and eastern regions of the West Javan province and it remains to be clarified to which degree the recovered differences are phylogenetically significant (Dallmann and Geissmann 2009). In consideration of all this, it is commonly argued that the available evidence regarding the intraspecific taxonomy of H. moloch is inconclusive at best and not sufficient to warrant a subspecific differentiation (Geissmann et al. 2002; Dallmann and Geissmann 2009; Nijman et al. 2019). Most contemporary authoritative references list H. moloch as monotypic (e.g., Chivers et al. 2013; Burgin et al. 2020; Nijman 2020).

Nomenclatural Notes.

The genus name Hylobates derives from Ancient Greek (ὑλοβάτης) and translates to forest walker (Illiger 1811). The epithet moloch refers to the mythical juggernaut and stands in the Linnean tradition of naming primates after spiritual entities (Beolens et al. 2009). The complete name may be translated to “demonic forest walker.” Javan gibbon, silvery gibbon, and moloch gibbon are established trivial names (Groves 1972; Beolens et al. 2009). In historic accounts, the names Wouwou and Wau-wau (which since the 1820s have been applied to H. agilis [agile gibbon] as well), and to a lesser extent also Moloch and white gibbon (as opposed to H. lar, the “black gibbon”—Burnett 1828) might appear in reference to H. moloch.

The oldest available scientific name of the Javan gibbon is actually Simia Nanodes Lichtenstein, 1791 (Caspar 2020). However, since this name vanished from the literature in the first half of the 19th century, it has been declared a “forgotten name” (nomen oblitum) following the guidelines of the International Code for Zoological Nomenclature. The junior synonym Simia Moloch Audebert, 1797 was instead accepted as valid (nomen protectum) as it has been commonly referenced as such since its introduction to the literature (Caspar 2020).

The close resemblance between Hylobates moloch and H. abbotti from western Borneo has repeatedly confused taxonomists and led to a suit of names that served to either unite or distinguish the gibbons of Java and Borneo at the species level. Up to the middle of the 20^th century, the names H. cinereus, H. leuciscus, and H. moloch were used ambiguously to describe gibbons of both respective populations, making species assignments in numerous classic works, including the influential treatises on small apes by Adolph H. Schultz, hard to interpret (e.g., Schultz, 1933; reviewed by Groves 1971). For this reason, respective references are not discussed herein. It has even been suggested that the earliest scientific illustrations of silvery-furred hylobatids usually attributed to the Javan gibbon (Audebert’s (1797),Simia Moloch and Schreber’s (1799),Simia Leucisca; see Caspar 2020 for discussion of the specimens) actually refer to Hylobates from western Borneo, rendering both H. moloch and H. leuciscus unavailable names for the Javan species (Matschie 1893). In response, the name H. javanicus was suggested (Matschie 1893). However, the fur color traits of the illustrated gibbons are not sufficient to argue that the specimens originated from Borneo (Groves 1972). Although doubts about its validity could theoretically still be upheld since genetic assessments of the types of Audebert are missing, H. moloch has been firmly established as the Javan gibbon’s binomen for almost a century (Caspar 2020). After H. moloch had been frequently listed as a subspecies of either Bornean Hylobates species or H. lar over the course of the 20th century (Pocock 1927; Sody 1949; Groves 1971), its species-level distinctiveness became gradually more accepted and is now undisputed (Marshall and Sugardjito 1986; Geissmann 1995; Chan et al. 2010; Thinh et al. 2010; Chivers et al. 2013; Roos 2016).

DIAGNOSIS

Hylobates moloch is foremost characterized by long, silvery-gray pelage in both sexes (Fig. 1). The head is typically ornamented by a dark cap and a weakly expressed white face ring which extends into a forward-projecting beard in the chin region. The great-call sequence of the female song is peculiar and diagnostic for the species (e.g., Geissmann 1995). On morphological grounds, H. moloch might well be confused with H. abbotti (Abbott’s gray gibbon) from western Borneo (Geissmann 1995). Pelage color in H. abbotti is described as medium gray or mouse gray to pale brown and duller than in H. moloch and it often, although not consistently, lacks a dark cap (Marshall and Sugardjito 1986; Groves 2001). The fur of H. abbotti is shorter in general but longer over the ears and it does not exhibit the white pointy beard of its Javan congener (Groves 1972; Geissmann 1995; Mootnick 2006). Additionally, the two notably differ in dental morphology, which is more derived in Bornean Hylobates (Frisch 1973). Assignment of infants and young juveniles to either species might be especially challenging. The identification of H. moloch in the wild is unequivocal as it is the only hylobatid inhabiting Java.

GENERAL CHARACTERS

Hylobates moloch exhibits only a single, sexually monochromatic color phase (Groves 1972). Sexual dimorphism in appearance, body mass, and dentition is slight but noticeable (see below). Adults display a long and dense, silvery-gray pelage (Marshall and Sugardjito 1986). The fur may show a wooly texture and is mostly plain-colored. However, it is black in the genital area and surrounding the anus (Mootnick 2006). No genital tuft is developed (Kloss 1929). Females may show a dark gray chest patch that can taper to the abdomen (Geissmann 1995; Mootnick 2006). A dermal gland field is present on the chest but not in the axillary or inguinal areas (Geissmann and Hulftegger 1994). Palms of hands and soles of feet as well as ischial callosities are naked (Groves 1972), as is the face, except for approximately 60 short vibrissae surrounding the mouth region (quantified in other Hylobates species by Muchlinski 2010). All visible portions of the skin are uniformly black. The iris of the eye is a dark amber, whereas the sclera appears dark brown (Caspar et al. 2021).

Hair length varies dependent on body region. It is longest on the upper arm, over the ears and occiput, and between the shoulders, reaching lengths of up to 7 cm (Groves 1972). In older adults, this can result in a prominent semicircular gray wreath of elongated peripheral scalp hair. It anteriorly frames the face and rests posteriorly on the neck. Crown hair on the scalp is typically dark gray to black and grows fanwise from the front, thus creating a more or less well demarcated cap (Marshall and Sugardjito 1986). The expression of the cap is variable among individuals and appears not to correlate with geographic provinces (Groves 1972), as has once been suggested (Sody 1949). It is only faintly present in some individuals, making them resemble H. abbotti (Geissmann 1995). Reportedly, caps tend to be darker in females (Geissmann 1995). The face is framed by a weakly expressed light gray to white face ring. Quite frequently, only a sharply tapering brow streak that might be lighter than the rest of the facial ring and the portion surrounding the mouth is present. The latter forms a forward-pointing beard on the chin in both sexes (“goatee”—Marshall and Sugardjito 1986). Hylobates moloch experiences a slight fur color change during ontogeny. Infants are born with light skin and initially develop a pelage that is paler than that of adults. Their fur is buffy gray to cream-colored and their cap is less conspicuous than in older juveniles and adults (Groves 1972; Marshall and Sugardjito 1986; see “Ontogeny and Reproduction”).

Only few exact data on adult body mass in H. moloch are available. Except for a single female and male each (Geissmann 1993) all refer to captive specimens. In total, data on two adult females with a mean body mass of 5,925 g (5,600–6,250 g; ± SD 365 g) and four adult males with a mean body mass of 6,693 g (5,800–7,270 g; ± SD 575.6 g) are available (Geissmann 1993; Michilsens et al. 2009; Zihlman et al. 2011). Despite the small sample size given, the subtle male-biased sexual size dimorphism that can be derived from these data (113%) corresponds well to that found in other Hylobates species (e.g., 108% in the lar gibbon, H. lar—Schultz 1944). Published body mass values tentatively place H. moloch as one of the largest-bodied species of its genus (Geissmann 1993; Zihlman et al. 2011; but see Michilsens et al. 2009).

Body proportions in H. moloch are typical of its genus (Groves 1972). Hylobates species display slightly more massive hind limbs than other gibbons in relation to their body size and lower intermembral indices than crested gibbons (Nomascus) and siamangs (Symphalangus—Zihlman et al. 2011). The following segmental indices have been reported for H. moloch (SD presented in parentheses): intermembral index, 128.3 (2.6) and brachial index, 112.3 (2.6), derived from museum specimens (n = 5—Groves 1972; apparently not discriminated from measurements of H. abbotti); intermembral index, 119.3 (3.4) and humerofemoral index, 107.3 (4.0), derived from carcasses of captive specimens (n = 3; Zihlman 2011). The mean vertebral formula of H. moloch is 7 C, 13.1 T, 4.7 L, 4.9 S, 2.7 Ca (Groves 1972; apparently not discriminated from measurements of H. abbotti).

Skulls of Hylobates species (except for those of the morphologically derived Kloss’s gibbon, H. klossii) are so similar to each other that even experts on hylobatids struggle to diagnose them (Pocock 1927; Marshall and Sugardjito 1986). They exhibit very prominent brow ridges and a pronounced angle where nasals and frontals meet, as well as comparatively weak jaws with small teeth (Fig. 2; Pocock 1927). There is broad interspecific overlap in morphometric variance of skulls within Hylobates (Creel and Preuschoft 1976) and cranial traits are not phylogenetically informative (see “Molecular genetics”). Craniometrically, H. moloch (n = 18) is particularly hard to distinguish from the gibbon species of Borneo (Müller’s gibbon, H. muelleri sensu lato, n = 101) and the pileated gibbon of southern Indochina (H. pileatus, n = 7; Creel and Preuschoft 1976). Sexual dimorphism in the skull morphology of H. moloch is slight. Females (n = 8), when compared to males (n = 10), tend to show a shorter neurocranium with more pronounced curvature of the cranial vault along the medial parietals, a more protruding glabellar region, narrower orbits, shallower jaws, and more nimble zygomatic arches (Creel and Preuschoft 1976). Cranial capacity was 113.3 and 98.3 in one captive male and female, respectively (Zihlman et al. 2011). The slender but compact hyoid apparatus of H. moloch differs notably from that of congeneric species (H. lar and H. pileatus) studied so far (Zihlman and Underwood 2019).

The dental characters of H. moloch are remarkably primitive and thus more distinctive and diagnostic than the bones of the skull. Conforming to the catarrhine ground pattern, each jaw quadrant houses two incisors, a canine, two premolars, and three molars (Fig. 2). Only very rarely, third molars (wisdom teeth) in either the upper or lower jaw are congenitally absent (Frisch 1973). Furthermore, the third molars seldom show signs of morphological reduction, meaning diminution in size and numbers of cusps (Frisch 1973), a trait that separates H. moloch from all its congeners except H. pileatus. Only one upper and lower third molar, respectively, was found to show notable signs of morphological reduction in a sample of 33 such teeth (Frisch 1973). Unique within the genus Hylobates, the molar grooves consistently conform to the ancestral hominoid condition, the Dryopithecus pattern (Frisch 1973). Invariably, a well-developed lingual cingulum is present on the upper molars, whereas it is reduced to varying degrees in congeneric species. Supernumerary cusps commonly occur on the lower but not on the upper molars (Frisch 1973). Both sexes exhibit dagger-like elongated upper canines that are constantly sharpened by grinding along the specialized lower first premolar (Groves 1972).

The following basic skull and dental measurements (mm, SD in parentheses) were derived from a mixed-sex sample of nine H. moloch specimens (Groves 1972): cranial length; 79.2 (1.45); total skull length, 100.2 (1.29); facial height, 30.9 (2.43); M² breadth, 5.93 (0.17); M³ breadth, 6.47 (0.16). A data set of three-dimensional cranial landmarks of H. moloch is available for eight female and 10 male specimens (Creel and Preuschoft 1976).

DISTRIBUTION

Hylobates moloch exclusively inhabits the Indonesian island of Java. It occurs in evergreen rainforests of the Banten, West, and Central provinces (Fig. 3—Kappeler 1984a; Nijman 1995). Although temperatures in these habitats are stable throughout the year, rainfall shows seasonal variation (S. Kim et al. 2012). For instance, at Gunung Halimun-Salak National Park in West Java, a short dry season between June and September (<200 mm precipitation/month) and a wet season from October to December (≥402 mm/month) is typical (monthly precipitation average: 316 ± 175 mm, range = 63–775—S. Kim et al. 2012). Java is one of the most populated areas in the world and has lost more than 90% of its original forests, with remaining forested areas being severely fragmented (Whitten et al. 1996; S. Kim et al. 2011; Malone et al. 2014).

Accordingly, H. moloch occurs in a number of nonconnected rainforest patches, including prominent protected areas, but also in nonprotected secondary forests, especially in Central Java (Fig. 3; see “Conservation”). In western Java (Banten and West Java provinces) it inhabits the Ujung Kulon National Park, the Gunung Halimun-Salak National Park, the Gunung Gede-Pangrango National Park, Telaga Warna, Gunung Papandayan, Gunung Buangrang, Gunung Tilu, Gunung Simpang, Gunung Wayang, and Sangga Buana. In Central Java the range includes the Dieng mountains and Gunung Slamet (Kappeler 1981; Asquith et al. 1995; Nijman 2004; Setiawan et al. 2012; Iskandar et al. 2018).

Apart from human activity, climatic factors and elevation limit the range of H. moloch in Java. It is restricted to high-canopy lowland and submontane rainforests and does not occur in mangroves (Kappeler 1984a). The species is only rarely found at elevations above 1,600 m (Kappeler 1984a; Nijman 2020) but might occasionally venture into areas of up to 2,400 m (e.g., on Gunung Pangrango—Nijman 2004). At this elevation level, vegetation changes to low-canopy high montane rainforest, which cannot sustain H. moloch (Kappeler 1984a). Regions of the Central and East Javan provinces east to 110° longitude exhibit a significantly drier and more seasonal climate than the western portions of the island and present no suitable habitats for the species (Kappeler 1984a). Accordingly, the restriction of H. moloch to western and Central Java primarily derives from climatic factors rather than from anthropogenic disturbance.

FOSSIL RECORD

The fossil record of small apes in general is extraordinarily sparse with the vast majority of fossils being isolated dental remains (Harrison 2016). The oldest known hylobatids are the 12.5–13.8 million year old (middle Miocene) Kapi ramnagarensis from Jammu and Kashmir, India (Gilbert et al. 2020) and Yuanmoupithecus xiaoyuan from the late Miocene of Yunnan, estimated to be about 7–9 million years old (Harrison 2016). All other hylobatid fossils are noticeably younger and date to the Pleistocene or Holocene. Most fossil sites are located in continental South Asia and yield finds that are largely attributable to the extant gibbon genera (Harrison 2016).

The oldest gibbon fossils on the island of Java date back to the early Pleistocene. With an estimated age of 0.8 million years, a partial femur from the famous Javan Trinil site, East Java (precisely the Trinil H.K. bone bed), represents the oldest hylobatid fossil known from the Sunda Islands but its generic affiliation is uncertain (Ingicco et al. 2014). Fossils of definitive Javan Hylobates are extremely rare, as only two isolated teeth are known (Hooijer 1960). They were found at the Sangiran and Punung A sites in Central and Eastern Java, respectively, in deposits that also bear teeth of the once abundant siamangs (Symphalangus syndactylus) which eventually went extinct on Java (Hooijer 1960). An age of approximately 120,000 years is assumed for fossils of the Punung fauna, whereas 800,000 years is estimated for the Kedung Brubus fauna at Sangiran, which thus includes the oldest unambiguous occurrence of Hylobates on the island (Ingicco et al. 2014). Although the few dental remains of Javan Hylobates are at times assigned to the extant H. moloch, their species identity remains unclear (Hooijer 1960; Harrison 2016).

FORM AND FUNCTION

The postcranial morphology of Hylobates moloch is highly specialized for forelimb-powered suspensory locomotion in the forest canopy. As characteristic for hylobatids, it employs a combination of brachiation (arm-swinging—72%), leaping (25.6%), climbing (2%), and bipedalism (0.4%) to move through the tree tops (percentages based on observations on wild individuals by YY). Two kinds of brachiation are differentiated, the continuous and the ricochetal type. The latter is specific to the Hylobatidae and characterized by an aerial phase when swinging from one handhold to the next (Reichard et al. 2016). This aerial phase is missing during continuous brachiation. The anatomical adaptations to support their unique style of locomotion, as described below, are uniformly shared among all small apes, but have been repeatedly described in detail for H. moloch specimens (e.g., Kohlbrügge 1890; Chapman 1900; Donisch 1973; Michilsens et al. 2009; Zihlman and Underwood 2019).

Hylobatids have strongly elongated arms and hands, which assist in brachiation (Reichard et al. 2016). Compliant to the specific demands of brachiation, the shoulder mobility of these animals exceeds that of all other primates, including humans (Chan 2008). The sockets of the scapulae show a permanent cranial orientation in small apes, omitting extensive rotation of the shoulder blades to raise the arms above the head (Donisch 1973). Besides that, the hylobatid trapezius and rhomboid muscles are specially conformed to stabilize the shoulder blades against the steady gravitational pull experienced during brachiation (Donisch 1973). A greater mobility of the upper body relative to the abdomen is realized by a caudally shortened M. latissimus dorsi, which does not insert at the pelvis, as it is the case in humans and great apes (Donisch 1973). The humerus exhibits a 120° torsion along its axis as well as a broad trochlea, and the olecranon fossa is markedly deep, occasionally even perforated, to reinforce the elbow joint (Groves 1972). Forelimb muscles controlling arm extension are reduced. The anconeus muscle is absent, making the M. triceps brachii, which attaches to a flat olecranon, the only elbow extensor (Michilsens et al. 2009). On the other hand, the arm-flexing M. biceps brachii shows a unique attachment pattern in hylobatids and is specifically adapted to enhance rapid ricochetal brachiation (Jungers and Stern 1980). Peculiarities of the hylobatid wrist were discussed by Lewis (1969).

The individual phalangeal bones of the fingers are elongated and curved to facilitate grip during swinging and climbing (Susman et al. 1982). The well-developed thumb is highly mobile and separated from the palm of the hand by a deep cleft reaching to the base of the first metacarpal (Zihlman and Underwood 2019). Given their elongated hands and fingers, small apes execute precision grips preferably by pressing the thumb against the radial side of the index finger (Christel 1993). The M. accessorius interosseus, an apomorphic muscle of hylobatids, increases stability of the index finger during this type of grasping (Susman et al. 1982). All this enables surprisingly meticulous manipulations, even of small objects such as seeds (Christel 1993). The feet are well adapted for grasping as well and mirror the morphology of the hands in displaying curved toes and a deep cleft between the abducted opposable hallux and the remaining toes (Vereecke and Aerts 2008). They can also be employed for object manipulation (Torigoe 1985).

When moving on the ground or on large branches, H. moloch preferably walks bipedally. Similar to humans, small apes display a long achilles tendon that allows storage of elastic energy (Vereecke and Aerts 2008). Nevertheless, different from humans, their feet exhibit planar soles and a flexible midfoot region which facilitate pedal grasping (Vereecke and Aerts 2008). When walking upright, their posture is characterized by flexed knee and hip joints (Vereecke and Aerts 2008).

There are no conspicuous specializations in regard to food processing and digestion in H. moloch. The gastrointestinal system conforms to the hominoid ground pattern and exhibits a clearly demarcated cecal appendix (Groves 1972).

Few studies are available on the sensory biology of hylobatids and none is exclusively devoted to H. moloch. The derived morphology of the hylobatid vestibular organ facilitates their fast and agile locomotion and has been studied in detail (Urciuoli et al. 2020). All small apes have acute trichromatic color vision. The wave length tuning of their retinal opsins resembles that of other catarrhine primates, including humans (Hiwatashi et al. 2011). The sensitivity of other sensory channels in H. moloch and its relatives, such as hearing and olfaction, remains understudied but can be expected to largely correspond to that of great apes and humans. Despite the presence of persisting nasopalatine ducts, hylobatids lack a functional vomeronasal organ (Maier 1997).

ONTOGENY AND REPRODUCTION

Ontogeny

The neonatal mass of Hylobates moloch is 376 g (± SD 39.8; n = 3), falling well into the range of other Hylobates species (Geissmann and Orgeldinger 1995). Wild infant H. moloch are weaned at an age of about 22 months (Yi 2020).

Hylobates moloch typically undergoes a noticeable fur color change during ontogeny. Infants are born with light skin and initially develop a paler fur than adults. They appear buffy gray to cream-colored and their cap is usually less conspicuous than that of older juveniles or adults (Groves 1972; Marshall and Sugardjito 1986). Females may develop a dark pectoral patch (see “General Characters”), earliest around the age of 5 years, which initially forms at the center of the chest (Mootnick 2006). From a behavioral perspective, around the age of 1 year, H. moloch infants acquire locomotor independence from their mothers, spending about 70% of their time farther than 1 meter away from them (Burns 2015; Yi 2020). At Citalahab, Gunung Halimun-Salak National Park, offspring survival rate was found to be 100% until 4 years of age (0–1 years: n = 12, 1–2 years: n = 9, 2–4 years: n = 7), and 66% between 4 and 6 years of age (n = 6), presumably due to predation (Lappan et al. in press). Immatures show a peak of solo and social play during adolescence at about 5–7 years of age (Burns et al. 2010). When developing into subadults, they start to peripheralize themselves from the rest of the family, especially from same-sex parents (Burns et al. 2010; also observed by YY in the wild). At this stage, they are also least groomed by other individuals among family members (Burns 2015). Eventually, subadults disperse from their natal group and either establish territories or replace same-sex residents within established pairs. At Citalahab, Gunung Halimun-Salak National Park, a H. moloch subadult dispersed at an age of 8.8 years and three subadults of unknown age dispersed when they were assumed to be older than 9 years (Lappan et al. in press). In captivity, H. moloch females give birth to their first infant at an average age of 8.8 years (± SD 0.7; n = 11—Hodgkiss et al. 2010).

The maximum recorded longevity for H. moloch is approximately 47 years, as was reported from a wild-caught female kept at Assiniboine Park Zoo in Winnipeg, Canada (Thompson 2018). Little is known about the life spans of wild H. moloch. Some individuals at Citalahab in the Gunung Halimun-Salak National Park are around 30 years old and are still breeding (Lappan et al. in press).

Reproduction

Hylobates moloch forms stable breeding pairs with so far no reported cases of extrapair matings (see “Reproductive behavior”). Reproductive anatomy and physiology conform to that of other Hylobates species and have been studied in particular to inform captive-breeding efforts (Astuti et al. 2004; Thompson 2018; Notosoediro et al. 2019). No true scrotum is developed in males, instead the testes are situated in parapenial pouches above the base of the penis (Groves 1972). Males have a small baculum (4.5 mm; n = 1) while females lack a baubellum (Groves 1972). As all haplorrhine primates, H. moloch is a spontaneous ovulator, cycling in an approximately monthly rhythm and experiencing menstruation. The average ovarian cycle calculated as days between events of menstrual bleeding is 25.6 days (± SD 1.6; n = 7) or 27.3 (± SD 3.1) days, if determined based on the recurring presence of the vulval sexual swelling (n = 11—Hodgkiss et al. 2010). Captive H. moloch experience menarche at 6.2 years and start showing monthly sexual swellings at 6.5 years of age (± SD 1.0 and 0.7, respectively; n = 11—Hodgkiss et al. 2010). Even though sexual swellings in hylobatids are far less pronounced than in primates with naked sexual skin, such as macaques (Macaca) or chimpanzees (Pan—Palombit 1995), they are conspicuous enough to be visually detected (compare H. lar—Dahl and Nadler 1992). During swelling of the vulva, the urethral eminence, the lobes of the labia minora, and portions of vaginal epithelium become exposed as tumescent pink structures, covering an area of approximately 10 cm² (data from H. lar—Dahl and Nadler 1992). Sexual swelling can be observed in H. moloch females throughout the entire gestation period (n = 7 pregnancies—Hodgkiss et al. 2010), which is about 210 days (Thompson 2018). The mean interbirth interval of captive H. moloch is 2.3 years when infants survive, and 1 year when infants die during or shortly after birth (± SD 0.4 and 0.3, respectively; n = 11—Hodgkiss et al. 2010). Wild H. moloch show an average interbirth interval of 3.65 years and no seasonal clustering of births (± SD 0.7; n = 11; Lappan et al. in press; Yi et al. 2020a). Considering the age at dispersal, interbirth intervals and an assumed longevity of 35 years, a wild female H. moloch may have around seven offspring in her lifetime. This number may be lower in reality, considering that reproductive failures have been frequently reported in other wild Hylobates species (Palombit 1995).

ECOLOGY

Population characteristics

Population size estimates for Hylobates moloch have been calculated several times over the last two decades (Nijman 2004; Iskandar et al. 2009; Supriatna et al. 2010; Iskandar et al. 2018; Kheng et al. 2018). Range mapping, line transects and fixed-point counts are variably used to determine population size (Nijman 2004), and the most recent estimates suggest that between 2,640 and 4,178 individuals of H. moloch remain in the wild (Kheng et al. 2018). Gunung Halimun-Salak National Park is home to the largest population of about 900–1,221 individuals (Kool 1992; Asquith et al. 1995; Sugardjito and Sinaga 1999). The Dieng mountains (492–881 individuals), Gunung Gede-Pangrango (347–447 individuals), and Ujung Kulon National Park (300–560 individuals) also hold great numbers of individuals (Nijman 2004, 2006; Iskandar et al. 2009; Supriatna et al. 2010, Setiawan et al. 2012). The current total population size is decreasing (Andayani et al. 2008, see “Conservation”). Most population density surveys have so far relied on acoustic monitoring, but respective results might have been affected by a considerable variation in singing frequencies between subpopulations (see “Communication”).

Median group density for all populations is 2.7 groups/km² in lowland forest (<500 m elevation), 2.6 groups/km² in hill forest (500–1,000 m), and 0.6 groups/km² in lower montane forest (1,000–1,750 m—Nijman 2004). Average group size of H. moloch may vary between populations. Kappeler (1981) reported the average group size to be 3.3 individuals. When taking a regional perspective, the lowest average group size reported is 2.2 individuals/group in Leuweung Sancang Nature Reserve at 20–108 m elevation and in Gunung Simpang Nature Reserve at 1,150–1,384 m (Iskandar et al. 2018), both in southern West Java, and the highest is 4.7 individuals/group in Ujung Kulon National Park at 15–30 m elevation (Rinaldi 1999).

Space use

Hylobates moloch families are territorial and often have conflicts with other groups at the border of their home ranges (Yi et al. 2020a). Home range sizes vary from 9.8 to 38.7 ha between populations (Kappeler 1981; Malone and Oktavinalis 2006; Yi et al. 2020b). These variations in home range size might be related to differences in elevation, general tree density (Citalahab; 288 ± 107 trees/ha, Nature Reserve Leuweung Sancang; 465 ± 42 trees/ha), and food tree density (Citalahab; 139 trees/ha, Nature Reserve Leuweung Sancang; 241 trees/ha; Malone 2007; S. Kim et al. 2011). A H. moloch family group shares considerable portions of its home range with other groups, averaging 3.3 ha (± SD 1.1) when approximated by minimum convex polygons (9% of average home range; n = 3—S. Kim et al. 2011) or 6 ha when estimated by kernel density (16% of average home range; n = 2—Yi et al. 2020b).

Hylobates moloch travels distances of about 800–1,400 m a day (Kappeler 1981; Malone 2007; Ham et al. 2017). In general, Hylobates family groups have a smaller home range size than expected based on their daily path length, which makes them highly defendable. Indeed, H. moloch may traverse its home range several times within a single day (Yi et al. 2020b). However, daily path lengths do not vary according to home range sizes and there are no seasonal differences in daily path length (S. Kim et al. 2011).

In hylobatids, both sexes disperse when they reach sexual maturation and dispersed individuals are often found to occupy a vacant area close to their natal home range or replace a breeding individual in a neighboring group (Tilson 1981; Brockelman et al. 1998). In Symphalangus syndactylus (siamang) and Hylobates lar, males disperse shorter distances than females (Lappan 2007; Matsudaira et al. 2018), but dispersal distances and potential effects of sex have not yet been quantified in H. moloch.

Wild hylobatids sleep in a sitting position on bare branches. Hylobates moloch changes sleeping trees each night and favors tall trees, a behavior that might be related to predator avoidance (Ario et al. 2018). Furthermore, H. moloch prefers not to sleep in proximity to points of recent aggressive intergroup encounters, suggesting that it specifically selects sleeping trees to evade such events (Yi et al. 2020b).

Diet

Hylobates moloch is predominantly frugivorous, with fruits comprising about 65% of its diet (S. Kim et al. 2011). Figs represent an especially important food source, encompassing about one-half of the fruits consumed (S. Kim et al. 2011). Although figs are considered to be fallback food rather than a preferred source of nutrients in many hylobatid species (Leighton and Leighton 1983; Harrison and Marshall 2011), H. moloch consumes more figs than would be expected based on their availability in the forest (S. Kim et al. 2012). Besides fruit, the diet of H. moloch also includes leaves, flowers, and arthropods (S. Kim et al. 2011). Young leaves are consumed whole, whereas only the lamina of older leaves are eaten after being stripped off from the central vein (Kappeler 1984b). Young flowers are eaten whole as well. Older flowers, however, might be chewed on for a while but will be spat out eventually (Kappeler 1984b). Occasionally, H. moloch consumes honey from honeycombs (Kappeler 1984b), ruddy soil (Yi et al. 2020c), and bird eggs (Choi A., personal communication, Ewha Womans University, Seoul, Republic of Korea, 10 May 2020). Data on the nutrient composition and water content of the different plant organs consumed are available for the Citalahab site (Oktaviani et al. 2018). Foraging typically takes place in the canopy, at least 10 m above the forest floor, and encompasses approximately 70% of the apes’ waking hours (Kappeler 1984b). Hylobates moloch drinks water by putting its hands in tree holes and licking up the dripping water afterwards (Yi et al. 2020c), and some immatures may put their head directly into a tree hole to drink (YY, personal observation). Otherwise, no free water is consumed in the wild.

The diet composition of H. moloch varies between subpopulations (Malone 2007). Fruit consumption is relatively similar between populations (61%, Ujung Kulon—Kappeler 1984b; 63%, Gunung Halimun-Salak National Park—S. Kim et al. 2011; 67%, Gunung Gede-Pangrango National Park—Rahma 2011), while leaf (38%, 24%, 14%: same order) and flower consumption (1%, 12%, 19%: same order) are more variable. Hylobates moloch in Ujung Kulon feeds on fruits from 40 different plant families, whereas those in Citalahab in Gunung Halimun-Salak National Park include 25 families in their diet (Kappeler 1984b; S. Kim et al. 2012). All species and the respective plant parts consumed at the two mentioned localities are listed by Kappeler (1984b) and S. Kim et al. (2012).

At Citalahab in Gunung Halimun-Salak National Park, Ficus sinuata, F. punctata, F. recurva (Moraceae), Callicarpa pentandra (Lamiaceae), and Sandoricum koetjape (Meliaceae) are important food sources (each comprising >5% of feeding time) and all of them are consumed more than expected based on their availability in the forests (S. Kim et al. 2012). However, most important food plants (each listed engrossing >5% of feeding time) are different in Ujung Kulon, which again points to substantial local variations in diet: Dracontomelon mangiferum (Anacardiaceae), Artocarpus elasticus (Moraceae), Dillenia excelsa (Dilleniaceae), Garcinia dioica (Clusiaceae), and Planchonia valida (Lecythidaceae—Kappeler 1984b).

Hylobates moloch families consume more fruits and less young leaves during the wet season than during the dry season (S. Kim et al. 2011). Moreover, they change their daily path lengths and dietary breadth according to seasonal fruit and flower availability (S. Kim et al. 2012). They rely on spatial memory to forage and when an individual tree is fruiting, they increase the frequency of visits to other trees of the same species, as has been shown at Citalahab (Jang et al. 2021). However, this behavior occurs regardless of the synchrony levels of the plant species, possibly because the preferred fruit trees in this area, figs, do not exhibit fruiting synchrony (Jang et al. 2021).

Diseases and parasites

Comparatively few data are available on pathogens in Hylobates moloch but reasonable extrapolations from studies on congeneric species can be made. Important viral infections are caused by the gibbon hepatitis B virus (GiHBV) and by members of the herpes virus group as well as of the pox virus group (Keeling and McClure 1972; Payne et al. 2003; Starkman et al. 2003). Hepatitis B is an especially common but consistently asymptomatic infection in H. moloch (Payne et al. 2003; Thompson 2018). Historically, the oncogenic gibbon-ape leukemia virus (GaLV) was considered an important viral pathogen in small apes in general but it is no longer found in captive hylobatids and is probably not prevalent in wild populations (Siegal-Willott et al. 2015; Brown and Tarlinton 2017). Important bacterial diseases in Hylobates species include yersiniosis, tuberculosis, shigellosis, salmonellosis, and infection with Chromobacterium violaceum (Keeling and McClure 1972; Cheyne et al. 2012). They can also be hosts for a number of different groups of protozoan (notably Plasmodium but also Balantidium and Entamoeba—Yazthi and Handajani 2010) and metazoan parasites. The latter prominently include nematodes (e.g., Ascaris, Dirofilaria, Enterobius, Oesophagostomum, Physaloptera, Strongiloides, Trichuris), as well as to a lesser degree cestodes (e.g., Bertiella, Cysticercus, Hymenolepis) and acanthocephalans, but typically not trematodes (Hayama and Nigi 1963; Keeling and McClure 1972). Important noninfectious diseases found in captive H. moloch encompass idiopathic and traumatic epilepsy as well as hemochromatosis (Cheyne et al. 2012).

Interspecific interactions

Hylobates moloch does not share its range with other gibbons but with three other diurnal primate species, the grizzled leaf monkey (Presbytis comata), the Javan lutung (Trachypithecus auratus), and the long-tailed macaque (Macaca fascicularis—Kool 1992; Nijman and van Balen 1998; Iskandar et al. 2018). Presbytis comata and T. auratus are foli-frugivorous and highly arboreal species, potentially competing for food resources. Adult H. moloch have generally neutral but occasionally aggressive interactions with T. auratus (i.e., chasing them away from fruiting trees), while immature H. moloch at times play with both P. comata and T. auratus (i.e., play chasing or tail pulling—Choi A., personal communication, Ewha Womans University, Seoul, Republic of Korea, 10 May 2020).

The predation risk for H. moloch is low, given its vigilance, relatively large body size, and swift locomotion in the forest canopy. Nocturnal predators include leopards (Panthera pardus melas) and pythons (Malayopython reticulatus) while diurnal predators encompass several species of raptors (Santiapillai and Ramono 1992; Reisland 2013). Hylobates moloch relies largely on crypsis to avoid predation (Nijman 2001; Reisland 2013). Once detected and disturbed by a potential predator, it typically vocalizes and flees (Reisland 2013). During a predation attempt on an infant H. moloch by an unidentified bird of prey, all group members produced alarm calls and moved away from the area (YY, personal observation).

HUSBANDRY

Compared to most other hylobatid species, Hylobates moloch is well represented in zoological gardens. In December 2018, the international Java gibbon studbook, which is based in Perth (Australia) and is overseen by the World Association of Zoos and Aquaria, recognized 89 individuals in 14 institutions worldwide (Thompson 2018). However, additional animals are present in local Indonesian institutions not covered by the studbook. An extensive survey of gibbons in Indonesian rescue centers and zoos conducted between 2003 and 2008 recovered 86 captive H. moloch individuals in the country, all kept in facilities on either Java or Bali (Nijman et al. 2009). Outside of Indonesia, H. moloch is most commonly kept in the United Kingdom, where five zoos together house more than 30 individuals (Thompson 2018). The earliest reports of captive H. moloch in Europe data back to the mid-18th century (Caspar 2020) and animals were sporadically exported from Java throughout the 19th and early 20th century (e.g., Sclater 1896). However, the genetic roots of the current captive population outside of Indonesia do not trace back beyond presumably 17 founder individuals caught in the 1960s to 1980s (Campbell and Cocks 2008). Since 2007, there have been no exports of H. moloch individuals from Java to international zoos (Thompson 2018). Hylobates moloch has never been established in laboratories but it was at least temporarily kept at the Japan Monkey Center in Inuyama (Hayama and Nigi 1963; Torigoe 1985).

Mirroring the situation in the wild, H. moloch is generally kept in small family groups consisting of a breeding pair and up to four offspring. It reproduces well in captivity. Because infants are rarely rejected by their mothers, hand raising, although typically successful, is uncommon (Thompson 2018). Isolation of offspring from the natal group is recommended to be undertaken between an age of 6 to 10 years (Thompson 2018). Of special veterinary concern is the high prevalence of GiHBV in captive H. moloch, for which a vaccine is available (Payne et al. 2003). However, although more than one-half of the captive population might carry the virus, infections appear to be consistently asymptomatic and no incidents of transmission to humans are known (Thompson 2018; see “Diseases and parasites”). Hylobates moloch responds well to different environmental enrichment strategies in captivity, especially foraging enrichment (Wells and Irwin 2009; Gronqvist et al. 2013). General housing requirements and maintenance guidelines for the species align with those for other hylobatids and have been summarized by Campbell and Cocks (2008). Hylobates moloch is noteworthy for being one of few primate species of which zoo-bred individuals have been repeatedly reintroduced into native habitats by means of international conservation efforts (see “Conservation”).

BEHAVIOR

Grouping behavior

Hylobates moloch almost exclusively lives in nuclear families grouped around a mated pair (Fig. 4). In contrast to that, many other hylobatid species may also form polyandrous or polygynous groups in varying frequency (e.g., Lappan 2007; Malone and Fuentes 2009).

It is extremely rare to encounter H. moloch groups with more than two adult animals. One case of a family encompassing two adult females and one adult male was reported from Leuweung Sanchang (Malone and Oktavinalis 2006). Additionally, two H. moloch groups with six and seven individuals, and thus likely containing more than two fully mature individuals, have been observed in Sokokembang forest in Central Java (Setiawan A., personal communication, Swaraowa, Yogyakarta, Indonesia, 15 May 2020). Hylobates moloch can therefore clearly be described as a socially monogamous species (see “Reproductive behavior” for comments on sexual monogamy). Data on long-term stability of pair bonds in H. moloch are available from Gunung Halimun-Salak National Park. No changes in mated pair composition were noted in three habituated and five unhabituated groups over 13 years at this locality (Yi et al. 2020a; Lappan et al. in press).

Social interactions between group members are mostly affiliative and only very rarely aggressive (Yi 2020). Hylobates moloch pairs maintain a close spatial relationship and males show greater social investment into their partner than vice versa (Yi 2020). Wild individuals spend 2.4 (± SD 1.2) % of their time grooming, representing the most frequent social behavior observed in the species (Yi 2020). When undisturbed by recent intergroup encounters (see below), pairs groom each other on average 0.8 (± SD 2.0) times per hour (Yi et al. 2020a). Wild H. moloch males groom and request grooming from females more than vice versa and do so most frequently when their partner is cycling (Yi 2020; Yi et al. 2020a). Grooming in captive pairs does not seem to follow a similar pattern (Burns 2015).

Intergroup encounters in H. moloch mainly involve chasing between males and singing by females, different from other Hylobates species which are often duetting during intergroup encounters (Kappeler 1984a; Malone 2007; Yi et al. 2020a). Groups encounter each other exclusively in the overlapping area of their home range, on average 0.49 (± SD 0.63) times a day and intergroup encounters are mostly aggressive (73%), with chases occurring on average 3.9 (± SD 4.7) times per encounter (Yi et al. 2020a, 2020b; all data derive from Gunung Halimun-Salak National Park). In most cases, males alone are involved in aggressive interactions (87%: male–male chasing—Yi et al. 2020a). Chasing sometimes leads to individuals falling from a tree; however, physical attacks (i.e., hitting) seem very rare (5% of encounters) and no lethal injuries have been reported to date (Yi et al. 2020a). Females sing in 32% of encounters for 12 (± SD 7) min on average out of 80 (± SD 52) min of encounter duration. No affiliative interactions (i.e., grooming or copulation) between adult H. moloch from different groups have been reported so far, but immatures are sometimes observed to play together (Yi et al. 2020a). Intergroup encounters are more aggressive when cycling females and dependent infants are present (Yi et al. 2020a). Just as male chasing, female singing seems to be related to intrasexual competition (Yi et al. 2020a; see “Communication”). Infanticide has not been reported in H. moloch.

Reproductive behavior

Hylobates moloch reproduces throughout the year without seasonal clustering of births (see “Ontogeny and Reproduction”). Copulation takes place in a posturally flexible dorsoventral position (Amarasinghe and Amarasinghe 2011).

Pair–partner replacement has been repeatedly observed in both sexes of other hylobatid species (Guan et al. 2013; Hu et al. 2018). In H. moloch, however it has not been documented, yet. Pair bonds appear to be more stable in this species and can last for well more than a decade (see “Grouping behavior”). Similarly, extrapair copulations have been frequently reported in congeneric species, such as H. lar (Barelli 2015). However, no studies to determine the frequency of extrapair paternities in H. moloch have been conducted so far. It is therefore not clear, whether it represents a both socially and sexually monogamous species.

Direct parental care in Hylobates species is mostly provided by the mother. Hylobates moloch females carry their offspring for approximately 1 year and also tolerate infants to take food from their hands or mouths in the wild (Yi et al. 2020c). Parent–offspring conflict is not deemed severe in H. moloch given their slow-paced ontogeny (Yi et al. 2020c).

Fathers indirectly care for their offspring through territorial maintenance, playing, and grooming (Burns et al. 2010; Yi 2020). Indeed, wild H. moloch fathers do engage in allogrooming more frequently than do mothers or offspring (Yi 2020). Juvenile H. moloch stay within shorter distance to their fathers than to their mothers after gaining locomotor independence, coforage, and preferably sleep next to their fathers, suggesting indirect paternal care during the juvenile period (Yi 2020).

Communication

All hylobatid species are known to produce stereotypic, elaborate bouts of long-distance calls termed “songs,” especially in the early morning hours (Marshall and Sugardjito 1986). Song structure is species and often sex-specific and it is largely genetically determined (Geissmann 1993). Usually, males and females within a bonded pair synchronously perform their songs as a duet. However, Hylobates moloch, along with its sister species H. klossii, are the only gibbons which are not duetting. Instead, both sexes exclusively sing solo song bouts, albeit males in H. moloch only sing very rarely (Fig. 5; Kappeler 1984a; Geissmann and Nijman 2006; Ham et al. 2016). It remains unknown why these two species abandoned duetting in favor of solo songs.

Most studies on communication of H. moloch focus specifically on the great-call, the climax of the female song, as it is highly distinctive and occurs comparatively frequently (Fig. 5A). A great-call sonogram of H. moloch is divided into three sections: pre-trill phase, trill, and termination phase (Fig. 5A; Dallmann and Geissmann 2009). Female great-calls are typically accompanied by vigorous swinging and jumping displays at the climax. During that, dead branches are frequently broken off, which has been hypothesized to be an intentional act to further amplify the display (Kappeler 1984a). Interindividual variation is higher than intraindividual variation in great-call variables (Dallmann and Geissmann 2001), and some populations can be distinguished from each other by their great-call acoustic structure (Dallmann and Geissmann 2009).

Daily female singing frequency in H. moloch varies and has been determined to vary between 0.25 (Gunung Halimun-Salak National Park—Ham et al. 2016) and 1.2 times a day (Dieng mountains—Geissmann and Nijman 2006). Kappeler (1984a) suggested that song bout frequency might be related to food availability or weather. However, Yi et al. (2020a) reported that at least female singing in the context of intergroup encounters does not correlate with fruit availability.

The female song of H. moloch may function in resource defense (Kappeler 1984a). Wild H. moloch songs have been studied in this context from the perspective of both listener and caller (Ham et al. 2016, 2017). First, playback experiments revealed that female solo songs carry information about the identity (stranger versus neighbor) and status of the caller (paired versus unpaired—Ham et al. 2016). Moreover, H. moloch responds faster when calls are played back from the center of its home range compared to the border, suggesting a function in territorial defense (Ham et al. 2016), similar to other hylobatid species (Mitani 1985; Raemaekers and Raemaekers 1985). There is less evidence to suggest that female H. moloch songs play a role in intergroup avoidance, pair-bond reinforcement, or sexual advertisement (Ham et al. 2017).

Hylobates moloch male solo songs (Fig. 5B) remain understudied, given their rarity. Kappeler (1984a) reported the absence of male solo songs in western Java, while Geissman and Nijman (2006) showed that males sing about once a week before dawn in Central Java. They also noted that male solo songs occur at much earlier daytimes than the songs of females, around 0355–0440 h when it is still dark, whereas female songs start after 0500 h (Geissmann and Nijman 2006). Geissmann et al. (2005) analyzed the structure of male songs and distinguished between expiration notes (oo; 1 type, wa; 4 types, chevron; 7 types, and variable notes; 1 type) and inspiration notes. In general, song bouts of H. moloch males are less stereotypic than those of females in note types and phrase structures and show greater interindividual differences than are known from other gibbon species (Geissmann et al. 2005). Immatures of either sex do not produce distinct songs, but both may join into the great-call sequences of their mothers (Geissmann and Nijman 2006; Yi et al. 2022a). Male immatures, as opposed to females, gradually cease to co-sing with their mothers as they develop into subadults (Yi et al. 2022a).

Besides exclusive song phrases, H. moloch emits different types of loud call bouts, which received less scientific attention (e.g., scream bout, harassing call bout; summarized by Geissmann and Nijman 2006). Short-range intragroup vocalizations of H. moloch (e.g., hoo calls) remain virtually unstudied and little information is available on nonacoustic modalities of communication. However this lack of data indicates gaps in the literature rather than insignificance to the animals. As in other apes, H. moloch uses a range of gestures (Bell 2015) and facial expressions (Florkiewicz et al. 2018) for tactile and visual communication, both of which have so far only been studied in captivity.

Miscellaneous behavior

On average, wild adult Hylobates moloch spend 36 (± SD 18) % of their time feeding, 41 (± SD 18) % resting, and 14 (± SD 11) % traveling; there is no seasonal variation in activity budget (S. Kim et al. 2011). However, the activity budget changes during and right after intergroup encounters (Yi et al. 2020b). Perhaps unsurprisingly, pregnant and lactating females rest more than cycling females in captivity (Burns 2015). A study of a single adult captive male revealed an average nightly sleep duration of 11 h 53 min within an observation period of 52 days (Reyes et al. 2021).

Cognitive traits of H. moloch are severely understudied, as is the case with gibbons in general. Only a few studies experimentally addressed the cognition of H. moloch in captivity, all relying on small sample sizes and none aiming to replicate the results of others. Gaze-following abilities (n = 4—Liebal and Kaminski 2012), problem-solving in a puzzle-box paradigm (n = 1—Holtkötter 1997), understanding of physical causality (n = 1—Cunningham 2006), learning set formation (n = 3—D’Agostino and Cunningham 2015), novel object responses (n = 1—Torigoe 1985; n = 10—Cunningham 2006), utilization of visual cues in competitive situations with a human experimenter (n = 3—Sanchez-Amaro et al. 2020), induced tool use in a raking task (n = 4—Cunningham 2006), as well as the functional understanding of tool properties (n = 4—Taylor 2019) have been studied so far and the corresponding results align with data from congeneric species. Hylobates moloch, just as all other primates except for the great apes and humans, do not recognize their mirror image (Suddendorf and Collier-Baker 2009). In the wild, cognition in H. moloch has so far only been experimentally studied through playback experiments in order to elucidate song functions (Ham et al. 2016; see “Communication”).

GENETICS

Cytogenetics

The diploid chromosome number (2n) of Hylobates moloch is 44, as in all Hylobates species (van Tuinen et al. 1999). Gibbon chromosome structure is comparatively fragile due to a unique class of retrotransposons (LAVA) which integrate into genes regulating spindle and kinetochore assembly, interfering which chromosome segregation (Carbone et al. 2014). Therefore, hylobatids display notable cytogenetic plasticity at both the inter- and intraspecific level. Different types of chromosome inversions involving chromosomes 8 and 9 have been documented in H. moloch (van Tuinen et al. 1999). This can result in heteromorphic chromosomal pairs. A correlation between chromosomal inversion type and vocal characteristics has been tentatively proposed based on observations on a mixed-sex sample of seven H. moloch individuals (van Tuinen et al. 1999) but has received no further study so far.

Molecular genetics

The complete nuclear (compare Veeramah et al. 2015; GenBank GCA_009828535.2) and mitochondrial genomes (Chan et al. 2010; Carbone et al. 2014) of Hylobates moloch have been sequenced. Phylogenetic analyses based on molecular data have so far largely relied on mitochondrial and gonosomal sequences. With some exceptions (Thinh et al. 2010; Chan et al. 2012), respective analyses strongly indicated a sister group relationship between H. moloch and H. klossii from the Mentawai islands (e.g., Takacs et al. 2005; Chan et al. 2010; Israfil et al. 2011). These species bear no obvious morphological resemblance but share a highly derived song pattern (Takacs et al. 2005; see “Communication”). Molecular clock estimates date the divergence between the two species to the late Pliocene to early Pleistocene, between 2 and 3 million years ago (Chan et al. 2010; Israfil et al. 2011). This clade is closely affiliated with the remaining southern Hylobates species from Borneo (H. albibarbis [Bornean white-bearded gibbon], H. muelleri) and Sumatra (H. agilis) from which they split during the course of the late Pliocene (Chan et al. 2010; Israfil et al. 2011; but see Chan et al. 2013 for younger estimates). Genetic differentiation of Hylobates populations from the Sunda islands is remarkably low. When comparing sequence divergence rates between H. moloch and H. muelleri, one study found genetic differentiation comparable to that between chimpanzee subspecies (F_ST = 0.33, 14 autosomal loci—Chan et al. 2013), while another one noted a complete lack of divergence (F_ST = 0.00, 20 autosomal loci—S.K. Kim et al. 2011). These genetic similarities either point to persistent gene flow between respective species, which appears unlikely given their island endemism, or to a conserved genetic profile inherited from a recent common ancestor. Surprisingly, there is evidence for low-rate unidirectional gene flow from H. moloch to H. lar and H. pileatus which indicates ancient admixture between the southern Hylobates populations of the Sunda islands and the more basal mainland species of the north (Chan et al. 2013; Shi and Yang 2018).

Population genetics

Patterns of intraspecific genetic differentiation and dispersal in Hylobates moloch remain debated (see “Nomenclatural notes”). Based on mitochondrial genetics, a reproductive barrier situated between the West Javan stratovolcano complexes Gunung Gede-Panrango and Gunung Masigit–Simpang–Tilu was proposed, which would give rise to two intraspecific clades (Kheng et al. 2018). However, the respective study has been criticized for insufficient reporting of the geographic origin of samples (Nijman et al. 2019). A reevaluation of the data with updated information on sample providence found no support for the two clades originally proposed (Nijman et al. 2019). Nevertheless, H. moloch shows genetic variation in mtDNA and nDNA comparable to other hylobatids, displaying high rates of heterozygosity (Kheng et al. 2018; Smith et al. 2018; Nijman et al. 2019). In line with this, genomic data suggest that before large-scale anthropogenic habitat destruction set in, the effective population size of H. moloch was notably large, exceeding that reported for other small ape species (Carbone et al. 2014).

CONSERVATION

Hylobates moloch is listed as “Endangered” (EN); criteria A4cd on the IUCN Red List of Threatened Species and in Appendix I of the CITES convention (Nijman 2020), and is protected under Indonesian law since 1924 (Peraturan Perlindungan Binatang Liar 1931 No. 266, Undang-Undang No. 5/1990, Surat Keputusan Mentri Kehutanan No. 301/Kpts-II/1991 and No. 882/Kpts-II/1992). Most recent estimates suggest that between 2,640 and 4,178 individuals of H. moloch remain in the wild (Kheng et al. 2018, see “Population characteristics” for details). In 1996, H. moloch was temporarily designated as “Critically Endangered” (Supriatna et al. 1994; Andayani et al. 2001; Nijman 2004), but further investigations demonstrated that undocumented populations in both western and Central Java had survived (Nijman and van Balen 1998; Asquith 2001), changing the status to Endangered. This, however, does not mean that the main threats for H. moloch, particularly deforestation and the illegal pet trade (see below), have decreased. Mostly due to these factors, the total population might decline dramatically in the near future. By 2045, numbers of wild H. moloch are expected to shrink down to about 50% of the population size that was present at the beginning of this millennium (Nijman 2020). Hunting for food and sport pose further threats to the species (Kappeler 1984a; Nijman 2020).

Even though the trading of H. moloch is strictly prohibited and penalized, considerable numbers of individuals are regularly confiscated by the local authorities, particularly immatures (Nijman et al. 2009, 2021). To obtain infants, their mothers are killed and the mortality of trafficked juvenile gibbons is likely substantial as well (Nijman 2009). Besides physical markets, hylobatids are illegally traded via the internet, with online platforms becoming increasingly relevant over the last years. A recent study on the illegal online trade of small apes on social media platforms in Indonesia found 106 individuals for sale, with the largest species fraction being represented by H. moloch (38 individuals—Nijman et al. 2021). Accordingly, the implementation of existing laws that prohibit wildlife trade online needs to be better enforced to effectively protect gibbons, including H. moloch (Nijman et al. 2021).

Suitable habitats for H. moloch have become exceedingly rare and segregated due to deforestation. The total wild population is scattered over about 30 forest fragments (see “Distribution”), 10 of which harbor more than 100 individuals (Nijman 2020). Several studies assessed habitat suitability for H. moloch at various Javan localities, including Gunung Halimun-Salak National Park, Gunung Tilu Nature Reserve, Gunung Gede-Pangrango National Park, and the Dieng mountains (Dewi et al. 2007; Nursal 2007; Berliana 2009; Suheri et al. 2014; Widyastuti et al. 2020). Hylobates moloch prefers forests with closed canopy and tall trees (Nijman and van Balen 1998) and generally avoids areas disturbed by human activity (Reisland and Lambert 2016). Noninsulated electric power lines transversing forest patches pose a notable lethal electrocution risk for this species (Yi et al. 2022b).

There is a high risk that the largest current H. moloch populations, which are found in Ujung Kulon National Park, Gunung Halimun-Salak National Park, and the Dieng mountains, will go extinct within 100 years if current threats remain (Smith et al. 2018). The forests inhabited by H. moloch are mostly situated within protected areas but some, particularly in Central Java (i.e., Dieng mountains, Gunung Slamet) and around Gunung Wayang in West Java, remain vulnerable (Nijman 2020). Based on surveys conducted between 1994 and 2002, it was estimated that about a one-third of the total population of free-living H. moloch occupies forests that are not legally protected (Nijman 2006). Thus, safeguarding and managing those habitats as well as the current protected areas, especially their boundaries, will be necessary for the species’ effective conservation. Additionally, restoring vegetation to create corridors between fragmented habitats is urgently needed to warrant its long-term survival (Suheri et al. 2014).

Apart from such measures, reintroduction programs could support local populations of H. moloch. Since 2014, reintroductions have occurred at Gunung Tilu in West Java, in a project coordinated by the British Aspinall Foundation (Wedana et al. 2021). At this site, both confiscated H. moloch and those originating from British zoos (Howlett’s and Port Lympne Wild Animal Parks) are rehabilitated and released into the wild. By the end of 2019, 40 individuals had been reintroduced, seven of which were former zoo animals, and four postrelease births were reported. However, postrelease monitoring of these gibbons is typically restricted to a few months, with only 13 individuals from the project being tracked in the wild over a period of at least 1 year (Wedana et al. 2021). Hence, it is difficult to evaluate the long-term survival of the released animals. The Gunung Tilu reintroduction project, including notes on pre- and postrelease management, was recently summarized by Wedana et al. (2021). Further reintroduction efforts are realized in the Javan Gibbon Centre at Gunung Gede-Pangrango National Park, where former pet gibbons are rehabilitated and released (Amarasinghe and Amarasinghe 2011; Ario et al. 2018).

REMARKS

Hylobates moloch is a prominent animal for rural communities in Java and is subject to numerous myths and local folklore. These include tales of the gibbons being descendents of humans that were once banished to live in the forest (Ujung Kulon National Park—Permana et al. 2020) as well as of an assumed ability of gibbons and monkeys to shape-shift into leopards (West Bandung district, West Java—Permana et al. 2019). In the vicinity of Ujung Kulon National Park, gibbon songs are believed to forebode various weather events (Permana et al. 2020). In general, H. moloch is perceived as a benign and noble animal, whose presence in the forests is appreciated (Permana et al. 2019, 2020). Harming it is tabooed in some communities, while others hunt gibbons for food and sport (Kappeler 1984a; Permana et al. 2020). Throughout the range of H. moloch, juveniles are illegally kept as pets (Kappeler 1984a). Reports of this practice date back to at least the late 18th century, when the species became known to the Western world in the wake of the Dutch colonization of Java (van Iperen and Schouwman 1780; Caspar 2020). To the present day, the pet trade continues to pose an important threat to gibbon populations on Java (Nijman 2020).

Synonymies completed 1 May 2022

DOI: 10.1093/mspecies/seac006

Version of Record, first published online October 5, 2022, with fixed content and layout in compliance with Art. 8.1.3.2 ICZN.

Nomenclatural statement.—A life science identifier (LSID) number was obtained for this publication: urn:lsid:zoobank.org:pub: 9070870D-C6F6-4B1A-ACF3-80EAFADEDF02

Acknowledgments

We would like to thank Holly Thompson for providing insight into the international studbook of the Javan gibbon, and Ahyun Choi as well as Arif Setiawan for sharing personal observations. We further want to acknowledge Thomas Geissmann for allowing us to use his recordings of Javan gibbon songs and Tony King for sharing information on reintroduction projects for the species. Vincent Nijman and one anonymous reviewer provided insightful comments which significantly improved the manuscript. KRC was supported by a Ph.D. fellowship of the German National Academic Foundation (Studienstiftung des deutschen Volkes). YY was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2021R1A6A3A03039709) and the Foreign Youth Talent Program (QN2021014010L) from the Ministry of Science and Technology of the People’s Republic of China.

LITERATURE CITED