1984 hatte ich die Gelegenheit, aufbauend auf meiner 1981 abgeschlossenen Dissertation drei Kapitel im Buch «The Lesser Apes» zu veröffentlichen. Das über 700 Seiten starke Werk war im Anschluss an die internationale Konferenz «The Lesser Apes» entstanden, welche im Juli 1980 auf Schloss Reisensburg bei Ulm durchgeführt worden war und an welcher ein Grossteil der damals aktiven Gibbonforscher teilgenommen hatte. Hier das Kapitel «Diet and Feeding Behaviour of the Moloch Gibbon» im Wortlaut:
© 1984 Markus Kappeler
Diet and Feeding Behaviour of the Moloch Gibbon
(erschienen in: H. Preuschoft et al. «The Lesser Apes. Evolutionary and Behavioural Biology», Edinburgh University Press, 1984)
The moloch gibbon, like the other gibbon taxa (Ellefson, 1967; Chivers, 1974; Gittins, 1979) feeds on (a) more or less ripe fruit, (b) young leaves and leaf buds, (c) flowers and flower buds and, occasionally, (d) small animals and animal products. Usually food is sought at a height of more than 1O metres above ground.
In this chapter diet and feeding behaviour in the gibbon population of the Ujung Kulon/Gunung Honje Nature Reserve (West Java, Indonesia) will be considered under the following topics:
1) feeding techniques,
2) feeding strategy (including the place of feeding in the daily cycle and changes during the course of the year),
3) food plants and food preferences, and
4) the quantitative composition of food measured according to food categories (fruit, leaves, flowers, animals).
While feeding, the gibbon remains in one place. It assumes various postures, sitting, standing and hanging, usually leaving one or two limbs free for gathering food. This is accomplished in two ways:
1) The food-bearing branch is grasped and pulled closer to the body, and the food is directly bitten off.
2) The food is plucked with one hand and brought to the mouth; occasionally, the branch is first grasped and brought closer.
In both cases the food may be examined (by smell or taste) before it is ingested.
Fruits are eaten whole and chewed (small ones), or certain parts are bitten off (large ones). In the first case, the seeds are probably swallowed unchewed; in the latter case, pericarp and/or fruit pulp are eaten, but not the seeds. In many cases, only parts of a fruit or bunch of fruit are eaten, and the rest is left hanging or dropped. Eight fruit types which are consumed by the gibbon are shown in Fig. 21.1; on each photograph, whole fruit and fruit with feeding marks can be seen.
Leaf buds and young leaves are mostly plucked with the mouth and eaten whole; usually only the lamina of older leaves and feathered leaves is consumed, being stripped off the central vein (between thumb and fingers) or bitten off directly.
Flower buds are usually eaten whole; older flowers however are chewed, and the remaining fibrous mass is spat out.
Living small animals (e.g. stick-insects, caterpillars, termites) are seized with one hand, squashed and then eaten.
Honey on deserted honeycombs is removed with an upward stroke of the hand and then licked off the fingers.
These feeding techniques are part of two main feeding categories - foraging and stationary feeding bouts - which typically alternate with each other and with bouts of other activities (especially resting and territorial behaviour).
Foraging can be defined as «feeding on the way» to a destination; such destinations are mainly areas bordering neighbouring territories, fruiting trees and sleeping/resting trees.
The group moves without haste and in loose formation (individuals being usually 20-100 metres (horizontally and/or vertically) apart in a certain direction. This direction is kept, but each group member chooses its path individually, so as to encounter food every now and then. Attention is paid equally to fruit, leaves, flowers and animals/animal products. Feeding or examination of objects, brief stops, observing the surroundings, resting and moving on alternate irregularly, so that on the average, during foraging, approximately 40 per cent of the time is actually spent feeding; around 40 per cent is spent travelling and approximately 20 percent on other activities (n = 20 hours).
Movement takes the group from one tree crown to the next in a zone between approximately 10 metres above ground and the top of the canopy.
Foraging is usually extremely quiet. The group moves in an inconspicuous manner, both visually and acoustically, so that its presence is normally detected only by moving branches or occasionally falling parts of vegetation.
Feeding bouts take place in the crowns of certain tree species which are visited while they carry fruit. The whole group meets there. The animals eat almost exclusively of this fruit and only exceptionally food of another category.
In some cases the animals obviously know where the fruit-bearing trees are and move towards them directly (especially after bouts of inactivity) or indirectly. Occasionally an individual discovers such a tree during foraging and utters soft hoo notes, whereupon the whole group gathers there.
During a feeding bout the animals do not pass through the crown of the particular tree but move around inside it, according to the supply of fruit.
Feeding activity does not seem to increase conspicuously during a feeding bout as compared with foraging: each single fruit is eaten «deliberately», after which the gibbon usually pauses, looks around, maybe grooms itself, basks and otherwise rests, so that on the average, during feeding bouts, around 50 per cent of the time is actually spent feeding, while another 50 per cent is spent on other activities (n=20 hours). Thus engaged, a group may spend from five to seventy minutes in the same tree.
Usually (i.e. in an undisturbed situation) a feeding bout ends by one group member leaving the tree to look for food elsewhere or to indulge in some other activity. The other members follow at short intervals. In this way feeding bouts are gradually replaced by foraging, resting, etc.
Feeding in the Daily Programme.
The gibbon group is active every day from approximately 0530 hours until about 1630 hours, i.e. about 11 hours (n = 18 days distributed over the whole year). The time period from leaving to entering a sleeping tree is about ten hours. By far the greatest part of the active period, i.e. on average 7.4 (5.4-8.7) hours (70 per cent), is spent feeding (foraging and feeding bouts taken together).
In general, feeding occurs in four or five periods distributed over the day. A single period may last up to 3 hours, and may include more than one foraging and/or feeding bout. Basically feeding periods alternate with periods during which the whole group rests.
Feeding and resting are occasionally interrupted by the group for territorial disputes or reactions to predators; individually, many interruptions - from a few seconds to several minutes - take place during which the animal is vigilant, urinates or defecates, grooms, and so forth.
The daily time spent foraging, on an annual average, is 5.0 (2.5-6.3) hours; the time spent on feeding bouts is 2.4 (1.6-4.3) hours (n = 18 days). These times are subject, on the one hand, to variations in the course of the year (see section below) and, on the other, to daily variations caused by disturbances (weather, territorial disputes, man, etc.).
On most days feeding begins and ends with a feeding bout. This is because the gibbon group usually moves into its sleeping tree directly after a feeding bout (in the neighbourhood of the feeding tree visited) and visits the same feeding tree again early in the morning.
Feeding bouts tend to accumulate in the first half of the day. Since more fruits are eaten per unit of time during feeding bouts than during foraging, the consumption of fruit is greater in the morning than in the afternoon. A possible causal explanation of this fact is the need to appease early morning hunger, i.e. to raise the blood sugar level (which has sunk over night) by free sugar in fruits.
Feeding in the Course of the Year.
The total daily time spent feeding as well as the daily number of feeding bouts and their duration are subject to slight changes in the course of the year, which are correlated with the seasonally varying supply of food (Figure 21.2).
In the season when many tree species carry fruit (June-August), 7.1 hours, on average, are spent feeding; many relatively short feeding bouts occur (on average sixteen bouts of 11 minutes; n = 6 days).
In the season when few tree species carry fruit (February-April), 7.9 hours, on average, are spent feeding; only few, but relatively long, feeding bouts occur (on the average six bouts of 21 minutes; n = 6 days).
Food Plants and Food Preferences
Food plant species were classified as such if gibbons were observed feeding on a representative of the particular species. Feeding marks of gibbons on plant parts cannot be distinguished from those of other primate species, so that no certain proof of food plants could be obtained by this means.
Food plants were marked on observation days and later samples of these plants were obtained. The species were identified by the Herbarium Bogoriense (Bogor).
The observations showed that the spectrum of food plants used by the gibbon population in the Ujung Kulon/Gunung Honje Reserve comprises at least 125 different species of plants (in 43 families).
Categorization of food plants according to the growth type shows that most of them are trees (108 species; 86 per cent); in addition, climbers (14 species), palm trees (2 species) and epiphytes (1 species) were visited for feeding.
All species used are shown in Table 21.1, with the growth type to which they belong and the parts of the plant which were eaten.
Obviously not all plant species are used by the gibbons; and those which are used contribute in different ways to their nutrition. One gibbon group was studied more closely (group D; Turalak study area) to answer the following questions:
1) What proportion of the total number of plant species present in their home range is used by gibbons for food?
2) What is the relationship between the availability of different plant species and the extent to which each is used? (What food preferences exist?)
Only tree species are considered.
Studies on Gibbon Group D.
To obtain some idea of the gibbon's potential food sources all trees (within the home range of group D), which reached a height of at least 20 metres (smaller trees were practically never visited by the animals for feeding) and which were accessible to the gibbons via the canopy, were recorded and identified.
The 975 trees (of 77 species) that were located in the home range of group D and could basically be considered as food suppliers (according to the above criteria) are listed in Table 21.2.
The gibbons used 61 of these tree species, i.e. 80 per cent of the total number of species present. The species used were represented by 889 trees, i.e. 90 per cent of the total number of trees; however, it is not certain whether they were all visited by the gibbons for feeding; the observations in this respect are incomplete.
The question of food preferences cannot be answered precisely for the following reasons:
It was not possible to examine the intestinal contents nor to collect and analyse faeces regularly; therefore the feeding times for each food species were compared instead of the quantities of food eaten. This method assumes that the time invested in eating a certain quantity is the same for each food species (see section on feeding bouts).
It is difficult to say what the supply of fruit, young leaves and flowers of the various food plant species is at a certain moment. It is impossible to measure these quantities, even approximately. It is certain, however, that the quantity of a certain food species is positively correlated with the total volume of the tree crowns that offer this food. To obtain a base for comparison, the crown volumes of the various tree species were compared.
Basically, it can be said that the supply of fruit, young leaves and flowers of each species is subject to considerable variation in the course of a year. Certain tree species have a marked flowering, fruiting and budding season; in others, the seasonatity is not clearly marked; or the individuals of the same species do no flower and fruit synchronously. Thus (1) food preference for each species can only be determined during certain phases of the annual cycle, i.e. temporarily, and (2) number and composition of the preferred food species change constantly during the year.
Use (feeding times).
The feeding times for each tree were recorded on eighteen days distributed over the whole year.
Fifty-three of the total of sixty-one tree species in the home range of group D were visited by the animals for feeding on these days so that, for each of them, a total feeding time could be determined. Eight species were not visited; their contribution to the total amount of food consumed by the gibbons is apparently very small.
The specific feeding time on a species as a percentage of the total time spent feeding is in five species more than 5 per cent on each (together about 40 per cent), in twenty-five species 1-5 per cent (together about 50 per cent), in twenty-three species 0-1 per cent on each (together about 10 per cent), and in eight species unknown (probably very small percentages).
The ten most commonly used tree species (see Table 21.2), on which the gibbons spent approximately half the total feeding time, were:
1. species no. 2: Dracontomelon mangiferum
2. species no. 46: Artocarpus elastica
3. species no. 11: Dillenia excelsa
4. species no. 35: Garcinia dioica
5. species no. 38: Planchonia valida
6. species no. 49: Ficus callosa
7. species no. 3: Spondias pinnata
8. species no. 50: Ficus variegata
9. species no. 42: Lagerstroemia speciosa
10. species no. 6: Saccopetalum horsfieldii.
Supply (crown volume).
The crown volume, based on the width of the crown and assuming a spherical crown, was estimated for each tree in the home range of group D.
The total crown volume of the sixty-one different food tree species varied between about 100 and 150.000 m3 per tree species; in twelve species it lay between 20.000 and 150.000 m3, in eleven species between 10.000 and 19.500 m3, in 20 species between 1000 and 9500 m3, and in 18 species between 100 and 900 m3.
The ten species with the largest crown volume, which together made up approximately 75 per cent of the total crown volume of the area, were:
1. species no. 42: Lagerstroemia speciosa
2. species no. 46: Artocarpus elastica
3. species no. 74: Vitex heterophylla
4. species no. 68: Pterospermum javanicum
5. species no. 75: Vitex pubescens
6. species no. 2: Dracontomelon mangiferum
7. species no. 73: Pentace polyantha
8. species no. 38: Planchonia valida
9. species no. 9: Terminalia microcarpa
10. species no. 3: Spondias pinnata
Preference (relation between use and supply).
The degree of preference for each food tree species results from the ratio t:v, where t = specific feeding time as percentage of total time spent feeding (relative use), and v = specific crown volume as percentage of total crown volume in the area (relative supply).
For twenty-eight tree species this ratio is >1; that is, proportionally more food is consumed from these species in relation to their supply. They are considered to be preferred species. For six species the ratio is 1, and for a further nineteen species it is <1; the latter are considered to be less preferred food tree species. For eight species, the ratio could not be calculated, because the data on feeding times were not available; they must be considered as less preferred food tree species.
The ten most preferred food tree species were:
1. species no. 11: Dillenia excelsa
2. species no. 2: Dracontomelon mangiferum
3. species no. 35: Garcinia dioica
4. species no. 49: Ficus callosa
5. species no. 6: Saccopetalum horsfieldii
6. species no. 50: Ficus variegata
7. species no. 56: Eugenia polyantha
8. species no. 32: Flacourtia rukam
9. species no. 24: Bridelia minutiflora
10. species no. 18: Antidesma bunius
A similar result is obtained by the following method:
Food preference can be recognized principally by the behavioural strategy of use: certain tree species are visited for feeding bouts; they are (1) visited by the whole group (simultaneously), (2) used three-dimensionally, and (3) the destinations to which movement is directed. The other tree species are only passed through when the gibbons are looking for food (see section on foraging). As a result there are differences in the time of use of the various tree species. Observation of feeding behaviour therefore allows preferred and less preferred food tree species to be discerned.
In the home range of group D, thirty-two species were regularly visited for feeding bouts (preferred species), while twenty-nine species were only used while foraging (less preferred species). All twenty-eight tree species for which the ratio use : supply was >1 were visited for feeding bouts.
Composition of Food According to Food Categories
Exact measurements of the quantitative composition of the gibbon food according to the categories fruit, leaves, flowers and animal could not be made. The method for determining the amount of food taken per plant species was therefore applied (see above), the different food categories as percentages of the total food taken being determined according to feeding times per food category.
The 7.4 (5.4-8.7) hours that were spent daily for feeding (on an annual average) were distributed among the food categories as follows:
(1) fruit 4.4 (3.2-6.0) hours (61 per cent),
(2)1eaves 2.9 (1.1-5.0) hours (38 percent),
(3)flowers 0.1 (0.0-0.3) hours (1 per cent).
Animal food was consumed during less than one minute daily on the annual average (n = 18 days distributed over the whole year).
If the composition of food - according to the food categories fruit, leaves, flowers, animal - ascertained for the moloch gibbon is compared with that ascertained by other investigators for the lar and agile gibbon, they are found largely to conform:
Fruit : Leaves : Flowers : Animal
Moloch 61 : 38 : 1 : 0
Lar 67 : 33 : 0 : 0 (Ellefson, 1967),
Lar 60 : 30 : 1 : 9 (Chivers et al., 1975),
Lar 60 : 35 : 1 : 4 (MacKinnon and MacKinnon, 1978),
Lar 50 : 29 : 7 : 13 (Raemaekers, I979)
Agile 57 : 39 : 3 : 1 (Gittins, I979).
As in many other aspects (see Raemaekers, 1977; Chivers, 1977b; MacKinnon, 1977), the siamang differs here from the «small» gibbon taxa; leaves constitute nearly half of its diet on an annual average:
Fruit : Leaves : Flowers : Animal
Siamang 41: 48: 5: 6 (Chivers, 1974, 1977a),
Siamang 44: 45: 3: 8 (MacKinnon and MacKinnon, 1978),
Siamang 36: 43: 6 : 15 (Raemaekers, 1979)
The proportion of feeding time on each food category was subject to variations in the course of the year. These variations were connected with the seasonally varying fruit abundance; in periods of low supply (February-April ) the proportions were (in the above sequence) 49: 50: 1 : 0 (n = 6 ), in times of heavy fruiting (June-August) 68: 30: 2 : 0 (n = 6).
Compared with the considerable variations in the abundance of fruit during the period of low supply approximately five times fewer tree species produced fruit than during the period of heavy fruiting - these variations of the proportion of feeding times are quite small; the silvery gibbon's diet always contained a high proportion of fruit.
A more or less stable composition of the food according to food categories, despite the seasonally varying proportion of the supply, has also been ascertained by Chivers (1977a) for the siamang and by MacKinnon and MacKinnon (1978) for the lar gibbon.