Chapter 2


© 1981 Markus Kappeler

2.1. Preliminary investigation

During one month gibbons were observed as often and as long as possible in Turalak. The aim of this preliminary investigation was to establish a procedure which was suitable for conducting a long-term study of the silvery gibbon.

2.1.1. Difficulties in observing gibbons in the wild

The first visits to the study area revealed the main difficulties in studying gibbons in the wild.

In contrast to ground-dwelling mammals, gibbons leave practically no visible traces from which information about their way of life may be derived. The only traces that can be evaluated are remains of food and excrements. The study of the silvery gibbons therefore had to rely mainly on direct visual observations of the animals, complemented by acoustic records.

Direct observation of the gibbons posed the following problems:

1. Localization of the gibbons. The animals could rarely be located during the day, because they (1) do not leave any typical traces, (2) rarely move in a visually or acoustically conspicuous manner, and (3) mostly remain hidden in the canopy - and, moreover, above the lowest forest stratum which severely impedes visibility. For these reasons, the gibbons could, as a rule, only be located during the female song bouts (s. chapter 6).

2. Maintaining contact with the gibbons. In spite of the fact that the gibbons around Turalak had not been hunted for over 10 years, they were very shy and tried to avoid any contact with man. Mostly they had already perceived the observer before he noticed them. Their attention was attracted no less by acoustic than by visual stimuli (for reactions s. section 5.4.), making a continuous observation of undisturbed gibbons impossible.

2.1.2. Social organization

A second important finding during the preliminary investigation was the fact that silvery gibbons are sedentary; as in the other gibbon taxa [s. e.g. ELLEFSON, 1967; CHIVERS, 1974; TENAZA, 1975; GITTINS, 1979], monogamous family groups inhabit stable home ranges which may overlap at the periphery with home ranges of neighbouring groups, but are for the most part territorial units (s. chapter 7).

The size of groups varied between 2 and 5 individuals, consisting of the adult pair and up to 3 young.

This maximum number of 3 young per group, normal for all gibbon taxa, results from the two facts (1) that the reproductive cycle is approximately 2 years in gibbons, and (2) that subadults split off from the family at the age of 5-7 years [NAPIER & NAPIER, 1967].

2.1.3. Choice of observation method

Because of the above mentioned difficulties in observing gibbons in the wild (absence of typical traces, secretive way of life, poor visibility, avoidance of man), it was impossible - for a long-term study - to consider any observation method that relied on recording the activities of individuals seen by chance («censusing» or «scanning»; ALTMANN [1974], CLUTTON-BROcK [1977]); observations had rather to be confined to one single group, where - in a phase preparatory to the main study - the basis for a continuous observation of its activities was laid («focal animal observation»; ALTMANN [1974], CLUTTON-BROCK [1977]).

This preparatory work can basically be done in two different ways:

1. The selected gibbon group is habituated to the presence of the observer so that the animals do not avoid him any more, but allow themselves to be observed openly. This has been achieved by ELLEFSON [1967], CHIVERS [1974] and GITTINS [1979] with «their» groups.

Since the process of habituation takes from several months up to one year, it could not be considered for the present study.

2. The places preferred by the selected study group are located. At these places and along routes used by the group, blinds, paths, etc. are made from which to study the animals when they are unaware of the observer's presence.

The latter method was chosen for this study since - especially because of the gibbon's sedentary way of life - it can be applied with relative ease.


2.2. Preparatory phase

2.2.1. Choice of a study group

As a study group, an established gibbon pair without any young, living in the centre of the study area, was chosen (group D).

The choice of this group seemed to be especially advantageous since the observer's risk of being discovered is definitely smaller in a small group than in a large one, and the probability of continued observation proportionally greater.

The disadvantages of this choice were that possible variations of behaviour typical for the different age classes and intragroup interactions - apart from intrapair interactions - could not be observed.

The reason why the pair had no offspring is not known; presumably, it was a young and recently formed pair, for - when revisiting the reserve in summer 1978 - the female was carrying an approximately 1-year old infant.

2.2.2. Establishing the conditions for a continuous observation

During a two-months period, group D was observed as often and as long as possible to get to know the extent of the forest area used by the animals. Their ranging routes and also fleeing routes when discovering the observer gave valuable information on the boundaries of the home range, since the animals - even when fleeing - were always intent upon not leaving the forest area with which they were familiar (s. 5.4.4.).

At the end of this period, the size of the group home range, the places favoured at certain times of the day and the routes used were roughly known, so that the animals could be located daily.

To establish conditions for a continuous observation of the group, a network of trails - adjusted to the animals' preferred places and main routes - was made, along which swift and quiet foot travel was possible. In this manner, direct encounters with the group could be avoided.

The vegetation above the trails was partially cut away to improve visibility upwards, and marks were made for the observer's orientation.

This trail system with a total length of more than 4 km was surveyed and mapped. To record the structure of this part of the forest, all trees of at least 20 meters in height (concerning this minimum height s. chapter 4) were determined, and the height and width of the crowns estimated; the trees were also mapped (s. figure 7).


2.3. Main study

During the main study an attempt was made to record the way of life of the study group continuously during the cycle of a year.

2.3.1. Observation schedule

On observation days, the study group was located at dawn - before or while leaving its sleeping tree - and then observed, if possible, until it moved into its sleeping tree in the evening. If the observer lost contact with the group, an attempt was made to reestablish it. However, since this was not always possible, observations were more frequent during the first hours of the day.

This resulted in a different number of observation hours per hour of the day. Later, when determining frequency distributions of activities dependent on the time of day (s. chapters 4 and 6), this was accounted for by averaging the data for each hour of the day.

No systematic observations were made at night since the preliminary investigation had shown that the animals were not active then.

2.3.2. Recorded data

Records were made of

1. the behaviour of the observed individuals, including intraspecific contacts (partners, neighbours, trespassers) and interspecific contacts (food competitors, predators, man),

2. the behaviour of conspecifics and animals of other species interacting with the observed individuals,

3. the resources used,

4. the location (horizontal and vertical) of the observed individuals,

5. the seasonal changes of the different plant species, and

6. the weather conditions.

2.3.3. Methods of documentation

A written protocol in the form of an activity change protocol was kept; the location of the animals was noted in detail on the map to a scale of 1:1000.

This form of protocol made it possible

1. to describe the different kinds of behaviour - including complete scenes of interaction - and to determine the influence of weather and food abundance,

2. to establish the temporal and spatial occurrence of the different kinds of behaviour and, thus, the activity programme with regard to time and space.

2.3.4. Material

To aid visual observation of the animals, KERN 10/50 binoculars were used.

The vocalizations were recorded with an UHER REPORT 4200 tape recorder and a NIVICO IVC directional microphone, and later reproduced graphically on a KAY ELECTRICS 6061 sonagraph. The behaviour was documented by sketches and photographs; for this a MINOLTA SRT 101 camera with a NOFLEXAR 400/5.6 objective was used.


2.4. Comparative observations on other groups

The observation method assumed that from the behaviour of a single group - studied over a limited time span - conclusions could be drawn about the be haviour of the subspecies.

To support this inference - and possibly to complete the observations made on the study group - comparative observations were made (1) occasionally on neighbouring groups of group D in Turalak, and (2) during two 2-week periods on gibbon groups in other parts of the reserve (Tereleng ('E') and Gunung Honje ('H'); s. figure 1).


2.5. Classification and identification of individuals

2.5.1. Age classification

Observed individuals were divided into the age classes infants (up to approx. 2 years), juveniles (approx. 2-4 years), subadults (approx. 4-6 years) and adults.

On the basis of external appearance alone, only infants which were less than 1 year old could be classified as such; their coat is creamy-white. All older individuals are uniformly silvery grey in colour (except for the crown hair which is dark-grey to black; comp. GROVES [1972]).

Age classification had therefore to be established on the basis of body size and certain characteristics of the behaviour, and was, accordingly, only practicable if observations could be made over a longer period or repeatedly.

The animals were classified as follows:

1. infants: small individuals which are permanently or occasionally carried by the mother (including creamy-white individuals),

2. juveniles: definitely not fully grown individuals which are, however, no longer carried by the mother,

3. subadults: nearly or entirely fully grown individuals which live in the company of an adult pair, but are - in contrast to the latter - not, or rarely, engaged in territorial activities and show a somewhat isolated way of life,

4. adults: fully grown individuals which live solitary or in a pair and show territorial activity (including individuals which carry an infant or possess enlarged nipples from suckling infants).

2.5.2. Sex determination

The external appearance of the silvery gibbon does not show any marked sexual dimorphism. Sex determination on the basis of shape was therefore rarely possible in the forest; exceptionally, adult females could be classified as such on the basis of enlarged nipples.

Generally, the sex of an individual had to be determined on the basis of its sex-specific role in song bouts (chapter 6), territorial conflicts (chapter 7) or infant care.

However, since it is primarily the adult gibbons that are engaged in these social activities, an accurate appraisal of sex could, as a rule, only be made in this age category. Moreover, during most short encounters with individuals no sexually dimorphic behavioural characteristic could be ascertained, so that sex classification was often not possible at all.

2.5.3. Identification of individuals

It was quite impossible to identify gibbons on the basis of individual external characteristics.

On the other hand, group home range and composition allowed discrimination of the different family units and subsequently - based on age/sex characteristics - of the different individuals since, per group, each of the five mentioned categories (adult male, adult female, subadult, juvenile, infant) is represented, at most, by 1 individual.