Chapter 8
Sketch of the Ecological Position
© 1981 Markus Kappeler
In this chapter an attempt will be made (1) to outline
the basic concept of feeding in the silvery gibbon as opposed
to the concepts of other herbivorous animals which share the
canopy of the tropical rain forest of West Java, and (2) to emphasize
the characteristics of the gibbon discussed in previous chapters
which may allow efficient use of «its» food niche.
8.1. Feeding strategy of the silvery
gibbon
The silvery gibbon feeds mainly on ripe fruit (approx.
2/3 of its food); in addition, it eats young leaves (approx.
1/3 of its food) and - in low quantities - flowers and animal
material (s. chapter 4).
The utilization of these food items which contain
a high amount of nutrients is certainly very profitable.
Ripe fruit are a.o. rich in free sugars
and therefore represent high-grade energy carriers; young leaves
contain many proteins. Moreover both are easily digestible [HLADIK
et al., 1971].
A diet based mainly on the use of fruit is, however,
linked with two main problems:
1. Fruits are a highly attractive food resource for
most herbivorous animals which use the canopy of the tropical
rain forest as feeding space; accordingly there is keen competition
for the supply.
Other larger herbivores which occur
in the Ujung Kulon/Gunung Honje Reserve and whose food spectrum
overlaps, to a certain degree, with that of the silvery gibbon,
are:
1. two langur species (Sunda island
leaf monkey (Presbytis aygula),
silvered leaf monkey (Presbytis cristatus)),
2. crab-eating macaque (Macaca fascicularis),
3. oriental giant squirrel (Ratufa bicolor),
4. red-necked fruit bat (Pteropus vampyrus),
5. Malayan flying lemur (Cynocephalus temminckii),
6. Three hornbill species (southern pied hornbill (Anthracoceros
convexus), rhinoceros hornbill (Buceros rhinoceros), Malayan
wreathed hornbill (Rhyticeros undulatus)).
2. The fruit supply is subject in the course of the
year to severe fluctuations caused by climatic factors (s. figure
2).
In order to coexist with the other frugivores, and
at the same time to ensure a year-round food supply, the gibbon
- like probably all other animal species - presumably adopts
a specific feeding strategy. The above mentioned animal species
(including gibbon) show three basically different concepts of
feeding which will be roughly characterized in the following
sections.
8.1.1. Year-round use of «rich» food
sources
The food supply present in the forest
occurs in different concentrations. Certain tree species have
marked fruiting periods; they bear a large amount of fruit during
a relatively short time-span (1-2 months). (Especially the larger
representatives of these tree species can be termed «rich»
fruit sources.) Others produce only small quantities of fruit
per time-unit, but over a longer time-span (3-5 months; «meagre»
fruit sources; WHITMORE [1975]).
For all frugivores it would certainly
be most economical to consume as much fruit from as few different
trees as possible, i.e. to use primarily rich fruit sources.
Such trees, however, are relatively
rare and occur in only very small numbers, especially when the
food production is generally low [ALDRICH-BLAKE,1978].
Frugivores specialized in using rich
fruit sources are therefore obliged to move considerable distances
between the few trees in fruit at certain seasons.
Judging from the observations, fruit bats and hornbills
seem to be the only ones among the above mentioned, predominantly
frugivorous animal species that primarily use year-round rich
fruit sources. Probably owing to their power of flight, the gain
in terms of energy required in making longer excursions, is worth
the output.
Hornbills - with certain exceptions
for Anthracoceros convexus - were, in general, rarely seen feeding
on small trees. During the season of low food production, they
seemed to fly over wide areas in search of rich fruit sources,
and large flocks gathered on the few trees still in fruit at
that time.
The same holds for fruit bats which
sleep in colonies. In addition, they were observed to switch
their roosting places to different regions in the reserve according
to a certain annual cycle; possibly, in this manner, they avoided
regionally poor food supplies.
8.1.2. Use of alternate food items
The primate species Macaca fascicularis and Presbytis
cristatus which live in groups of up to approximately 40 individuals
were also observed to obtain the major part of their fruit from
rich food sources; unlike fruit bats and hornbills, however,
they are confined to relatively limited home ranges and seem
to be capable of temporarily using other food items when rich
fruit sources are rare or lacking. According to MACKINNON &
MACKINNON [1978], macaques and langurs are «opportunists»
regarding the composition of their diet.
The feeding space of the crab-eating
macaque does not only comprise the canopy but also the forest
floor. If the supply in the tree crowns is insufficient, food
items on the ground resp. in the vegetation near the ground are
used more intensely.
Langurs are adapted to fulfill a large
part of their food requirements by feeding on mature leaves:
The exploitation of considerable quantities
of mature leaves requires special anatomic and physiologic features
since the nutrients contained therein are «protected»
by cellulose and often also by toxic secondary components [JANZEN,
1978; HLADIK,1978].
Presbytis species dispose of such
features («ruminant like digestion», BAUCHOP &
MARTUCCI [1968); in the gibbon, however, and also - possibly
with the exception of the flying lemur - in the other herbivores
they are absent [CHIVERS & HLADIK, 1980], i.e. mature leaves
can practically not be exploited by these animals.
Presbytis aygula and Cynocephalus
temminckii live in small groups resp. solitary, and the density
of their populations seems - at least in climax rain forest -
to be very low. They were rarely observed, and their way of feeding
remained largely unknown.
P. aygula was never seen on rich fruit
sources; it might feed mainly on leaves the whole year round;
possibly, however, it pursues a similar feeding strategy to the
gibbon (s. below).
8.1.3. Use of «meagre» fruit sources
The silvery gibbon - like M. fascicularis and P. cristatus
- lives all the year round in a relatively small home range;
in contrast to them, however, the composition of its diet, according
to food categories, remains more or less constant throughout
the year (s. chapter 4). In this respect, therefore, it behaves
rather like fruit bats and hornbills.
In spite of the fact that its foraging excursions
are limited to a relatively small sector of the forest, the gibbon
seems to be able to keep a constantly high percentage of fruit
in its diet by especially exploiting the meagre fruit sources
whose fruit production is less affected by seasonal fluctuations
than that of rich food sources [ALDRICH-BLAKE, 1978].
The gibbon ocasionally also exploits
rich food sources; during 2/3 of the time which it spends gathering
food, however, it is also travelling, i.e. it feeds on many different
trees, each of which - because of its small size and/or little
marked fruiting period - offers only few fruits at any one visit.
The exploitation of these meagre fruit
sources may be related - in respect of both space and time -
to the long distances covered by the gibbon every day; obviously,
to procure a sufficiently large quantity of rare and widely distributed
food items requires a considerable locomotory effort between,
as well as in, the different food sources.
According to the observations made
in the Ujung Kulon/Gunung Honje Reserve, the giant squirrel which
lives singly or in pairs also seems to use preferentially the
smaller fruit sources, but to feed rather on the less ripe fruit
which are often protected by toxic components [HLADIK et al.,
1971] resp. by hard or prickly shells.
8.2. The efficiency of the gibbon
in exploiting «its» food niche
In retrospect it can be seen that most of the characteristics
of the gibbon, which have been discussed in this paper, are correlated
with its efficiency in exploiting meagre fruit sources. They
will be briefly considered once again under this aspect:
1. Brachiation.
Owing to its manifold forms of locomotion, the gibbon, when foraging,
is not confined to certain tree structures as a substrate but
can utilize the nourishment available at either extreme of the
food-supplying tree crown - from the thick trunk to terminal
branches - without major difficulties. Brachiation, which is
lacking in sympatric primates, allows the gibbon, in particular,
to move among the finest twigs along the periphery of the tree
crown; unlike other primates, it does not have to return repeatedly
to more central parts of the crown (where the substrate is firmer;
s. GRAND [1972]). The time and energy expenditure required in
«combing» through many meagre food sources daily
may be relatively low compared with the reward.
2. Broad food-plant spectrum.
The qibbon is not specialized in exploiting certain plant species
but is, in this respect, a generalist, apparently capable of
gathering food from 90 percent of all the trees present in its
home range. The gibbon's ability to make extensive use of the
diversity of plant forms occurring in its habitat may, on the
one hand, enhance foraging organization and, on the other, contribute
to minimising seasonal variations in «its» food supply.
3. Dispersal of the gibbon group on foraging excursions.
Due to a largely individual choice of route by the different
group members while foraging, more food sources are undoubtedly
detected and exploited than if the animals moved together along
a common path.
4. Territoriality and monogamy.
Each qibbon group restricts its excursions to a certain part
of the forest. This may be of advantage for the animals insofar
as an intimate knowledge of the area is an aid to early recognition
of the fruiting periods in various plant species, especially
also where fruiting is inconspicuous as in most plants constituting
the meagre food sources, i.e. it provides continuous and comprehensive
information about the actual and prospective food supply.
Territoriality affords the gibbon group a.o. an exclusive
claim to the food supply in its home range; in conjunction with
monogamy, this implies extensive regulation of exploitation pressure,
resulting in (1) a long-term availability of food for the owners
and (2) - because of constantly improved knowledge of the area
- increasing efficiency of the animals in procuring food.
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