Chapter 2
Methods
© 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.
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