Chapter 1


© 1981 Markus Kappeler


1.1. Set of questions

The Javan silvery gibbon (Hylobates lar moloch) is almost exclusively restricted to primary tropical rain forest below approximately 1500 meters [KAPPELER, 1981].

This vegetation type, which shows a singularly large biomass and is exceedingly high in species diversity, is inhabited by a large number of animal forms of life; according to WHITMORE [1975] the tropical lowland rain forest of South-East Asia contains the most luxuriant and most complex ecosystems on earth.

The ecological niches are extremely manifold. Some animal species are ecological specialists and, as such, are equipped with niche-specific physiological, morphological and ethological characteristics; others are generalists. Each species is limited to certain factors of space and time and, accordingly, shows particularities in the use of resources.

In this paper the silvery gibbon is investigated as a component of the tropical rain forest of West Java. The following subjects are treated:

- technique and style of locomotion,
- diet and feeding,
- avoidance of predators,
- female song bout,
- home range and territory.

Studies of intragroup social relations could not be conducted because of the difficulties involved in observing the animals in the wild.

H.l.moloch is the sixth member of the Hylobatidae on whose way of life a study - based on long-term field observation - is presented.

Detailed ecological and ethological studies have already been carried out on H.l.carpenteri in North Thailand [CARPENTER, 1940], H.l.lar in Malaysia [ELLEFSON,1967], H.syndactylus in Malaysia [CHIVERS,1974], H.l.agilis in Malaysia [GITTINS, 1979] and H.klossii in Indonesia [WHITTEN,1980].

Until now, H.l.moloch has not been the subject of any investigation.


1.2. Study area

The data for this paper were collected from August 1975 to October 1976 in Ujung Kulon/Gunung Honje Nature Reserve in West Java (Indonesia).

1.2.1. Nature Reserve Ujunq Kulon/Gunung Honje

Detailed descriptions of the Nature Reserve Ujung Kulon/Gunung Honje have been given by SATMOKO [1961], SCHENKEL & SCHENKEL-HuLLIGER [1969], HOOGERWERF [1970], HALDER [1976] and BLOWER & VAN DER ZON [1977].

The reserve harbours Java's most westerly gibbon population which is estimated at 400-1400 individuals [KAPPELER, 1981a].

The population inhabits the climax rain forest of the Gunung Honje Range and its south-western spur extending into the depression between the Javan main land and Ujung Kulon peninsula (figure 1). The extent of high rain forest is approximately 100 In the Gunung Honje area it is delimited by man-made clearings; in the area of the flat isthmus, which is practically at sea-level, the forest gives way to vegetation flourishing in brackish water.

Some few individuals - isolated from the rest of the population - inhabit a small patch of high rain forest in the south-east of the Ujung Kulon peninsula (Tereleng, 'E'; figure 1); the rest of the peninsula is not inhabited by gibbons [for reasons s. KAPPELER, 1981a].

1.2.2. Turalak

Turalak ('T'; figure 1), within the Ujung Kulon/Gunung Honje Nature Reserve, was chosen as the main study area. It lies on the gentle slope of the Gunung Honje spur running south-west, approximately 50 meters above sea-level.

The temperatures, measured at Turalak in the shade at 25 meters above ground varied during the study year between a minimum 21-26°C (night) and a maximum 25-30°C (day).

Turalak was especially suitable for a long-term study for the following reasons:

1. Vegetation. Turalak is covered with exceptionally homogeneous primary tropical lowland rain forest and therefore provides an optimum gibbon habitat [s. KAPPELER, 1981a].

A further advantage is that, in large areas of this part of the forest, the lowest stratum consists of Arenga obtusifolia. This palm largely inhibits the development of thick undergrowth [s. SCHENKEL et al., 1978], so that walking in the forest is quite easy.

2. Gibbon population. Corresponding to the good quality of the forest, the density of the local gibbon population is especially high.

The evaluation of «transects» [WILSON & WILSON, 1975] of 5 km length, which were performed in a north south direction through forest bordering the study area, yielded an average gibbon density of 9 individuals/ (n = 7). In the study area itself, 18 individuals lived in an area of 0.8, corresponding to a local density of 22 individuals/

3. Topography. The terrain is very flat: the study area is drained by a multitude of small rivulets and ditches, separated from each other only by slight elevations; steep slopes are not found. These conditions highly facilitate movement on foot, allowing the gibbons to be followed and observed with comparative ease.

4. Accomodation. Turalak is only a few minutes walk from the guard post Kalejetan (figure 1); it was therefore not necessary to build a special camp.