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 ).
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 , 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:
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 ). 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.