Contributions to Zoology, 69 (3) (2000)Vincent Nijman: Geographic distribution of ebony leaf monkey Trachypithecus auratus (E. Geoffroy Saint-Hilaire, 1812) (Mammalia: Primates: Cercopithecidae)

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Being located in the extreme east of the Sundaic subregion, Java and Bali are the most isolated of the remaining land masses and also furthest from the Asian mainland. Java harbours a slightly impoverished non-human primate fauna compared to the other Sundaic islands. There are 5 species, including one nocturnal prosimian, the slow loris Nycticebus coucang, compared to 13 species on Borneo and 12-13 species on mainland Sumatra; the exact species number depends on the taxonomy followed. However, a relative high proportion of them are endemic, viz. 60% (compared to 38-43% on Borneo, and 8-17% on mainland Sumatra). The endemics comprise one species of Hylobatidae, the Javan or silvery gibbon Hylobates moloch (Audebert, 1799) and two species of Colobinae, the grizzled leaf monkey Presbytis comata (Desmarest, 1822) and the ebony leaf monkey [(1)]) Trachypithecus auratus (E. Geoffroy Saint-Hilaire, 1812). The Javan gibbon and the grizzled leaf monkey are confined to the wettest forest types, which are more common to the western part of the island, and can be found as far east as Mts Dieng and Mt. Lawu, respectively (Nijman and Sözer, 1995; Nijman, 1995, 1997). The ebony leaf monkey’s range encompasses a larger area, and the species can be found in other forest types as well, on Java, Bali and Lombok[(2)]. Despite its distribution encompassing a larger area than the other Javan endemics, its range is still restricted, and its habitat has largely dissappeared. Ebony leaf monkey are listed as Vulnerable according to the IUCN threat criteria (Eudey, 1987; IUCN, 1996). The species is protected by Indonesian law (included in Presbytis cristata) and is listed on Appendix II of the CITES convention.

The ebony leaf monkey has for a long-time been regarded as conspecific with the silvered leaf monkey, T. cristatus (e.g., Pocock, 1935; Napier, 1985; Wolfheim, 1983), but it was given its specific status by Weitzel and Groves (1985). The species specific status of T. auratus is now generally accepted (e.g., Weitzel et al., 1988; IUCN, 1994; Brandon-Jones, 1984, 1995; Corbet and Hill, 1992; Oates et al., 1994; Maryanto et al., 1997). T. cristatus and T. auratus occur allopatrically with the former having a disjunct distribution with populations from southern Burma, southern Thailand, Cambodia and southern Laos and Vietnam, and from the western coast of West Malaysia, Sumatra and Borneo (Corbet and Hill, 1992), while the latter, as stated before, ranges from Java eastwards to Lombok. Both species can be distinguished by skull and dental characters (Weitzel and Groves, 1985; Maryanto et al., 1997). Furthermore, T. cristatus is brown, brownish-grey or blackish brown, while the Javan species is more blackish tingled with brown and grey. In both species some populations are polymorphic in pelage coloration, with melanic and usually a small proportion of erythristic individuals occurring together. This proportion may vary between areas. In the ebony leaf monkey, these populations are restricted to the easternmost part of Java (see Discussion), while in the silvered leaf monkey these populations hitherto only have been recorded from Abai at the mouth of the Kinabatangan River in eastern Sabah, Borneo (Davis, 1962; Payne et al., 1985). Weitzel and Groves (1985) concluded that the type specimen of T. auratus, an erythristic female, must have originated from the easternmost part of Java (see also Brandon-Jones, 1995).

Little is known about the ecology of the species in natural forest areas, since only a limited number of studies have been conducted so far. Most of the comprehensive studies, i.e., those of Brotoisworo and Dirgayusa (Brotoisworo, 1983; Brotoisworo and Dirgayusa, 1991), Kool (1993; Kool and Croft, 1992), and to a lesser extend Megantara (1994) have been conducted in the Pangandaran nature reserve. Pangandaran is a small c. 500 ha. uplifted limestone peninsula at the southeastern corner of West Java. Parts of the area are covered with teak, Tectona grandis and mahogany, Swietenia spp., stands, while the remainder consists of rather dry evergreen forest (Whitten et al., 1996). Data presented by Brotoisworo (1983) suggest densities of c. 185 to 195 individuals km-2. Typical densities in areas where the species has been studied, which are often selected because the species is relatively common, range in the order of 20 to 75 individuals km-2 (Kartikasari, 1986; Supriatna et al., 1989; Bismark and Wiryosoeparto, 1980 in Supriatna et al., 1989; Nijman and van Balen, 1998), with probably the more typical density leaning towards the lower figure (unpubl. data). Pangandaran receives more visitors than any other conservation area in Indonesia, possibly 500,000 annually (Whitten et al., 1996), quite possibly having an effect on the socio-ecology and structure of the ebony leaf monkey population. Hence, findings and conclusions arising from studies conducted on the species in Pangandaran are probably not representative and cannot unhesitatingly be extrapolated to other areas.

As many other species within the genus Trachypithecus, the ebony leaf monkey lives in groups with one adult male and a number of immature males, females, and young (see reviews by Bennett and Davies, 1994; Newton and Dunbar, 1994). These group sizes range from 3 to over 30 individuals (pers. observ.; Supriatna et al., 1989; Brotoisworo, 1983). Group sizes on Java seem to differ between areas with different climatic conditions; median group sizes in areas with a more pronounced dry season, which are mainly found in the eastern half of Java as well as along the island’s northern coast, tend to be larger than those found in areas with a perhumid climate (unpubl. data). Extra-group males either live as solitaries or can team up with other bachelor males in bands or small groups (pers. observ.; Bennett and Davies, 1994; Brotoisworo, 1983).

A number of studies have been performed on the species’ feeding behaviour (Kartikasari, 1986; Supriatna et al., 1989; Brotoisworo and Dirgayusa, 1991; Kool, 1993; Djuwantoko et al., 1994) often partially in teak plantations. Like all colobines, ebony leaf monkeys possess a fore-stomach digestive system, which allows them to break down cellulose (Bauchop and Martucci, 1968; Kay and Davies, 1994). This makes the species able to cope with a substantial amount of foliage, a relative unnutricious food source, in their diet (for an overview of food selection in colobines see e.g., Waterman and Kool, 1994). Indeed, although depending on habitat type and seasonality, a large part of their diet consists of leaves and flowers (55% and 67% Kool, 1993; 56% Kartikasari, 1986; 59% Supriatna et al., 1989; 90% Brotoisworo and Dirgayusa, 1991; 94% Djuwantoko et al., 1994). As in most colobines, ripe fruits is not a favoured food source, and often only the seeds are consumed. When fruit is eaten, e.g., figs Ficus spp, mahogany Swietenia macrophylla, and acacia Acacia leucophloea, it is mostly not ripe (Kool, 1993; Kartikasari, 1986).

The geographic distribution of a species is perhaps the fundamental unit of ecology and biogeography. It affects probability of extinction (Jablonski, 1987) and different range sizes of species ultimately determine the number of species in a given area. The distribution of a species is determined by its geographical range and by the evenness or patchiness of occurrence within its range. Ecological analysis of species distribution patterns depends on the accuracy of the distributional data on which it is based: if a distributional pattern has been incorrectly described any interpretation of that pattern will be erroneous (cf. Wiens, 1992). Brockelman and Ali (1987) stress the importance of publishing reliable and detailed data on the distribution of primates. The need for accurate mapping of species’ ranges, their relevance to biogeographic processes and the structure of local species assemblages, and the potential sources of error in estimating species-range-size distributions have recently been discussed by Jones (1998) and Gaston (1996). Furthermore, for the proper assessment of a species’ conservation status and in order to monitor changes in abundance, range, and status an accurate description of its range of occurrence is essential. Therefore, as part of an ongoing study on the ecology and conservation status of ebony leaf monkey, the present paper attempts to thoroughly assess the geographical distribution of ebony leaf monkeys on Java, Bali and Lombok.