© 1981 Markus Kappeler
The natural predators of the gibbon in the Ujung Kulon/Gunung Honje Reserve - with respect to body size - are potentially (1) leopards, (2) eagles and owls, and (3) pythons. Venomous snakes (3a) may constitute a danger. A further potential enemy is (4) man.
Cases of predation on gibbons by natural enemies were, however, not observed, nor were any individuals which had obviously been wounded or killed by predators found (observations of this nature could not be gleaned from the local people (guards etc.) either); and the gibbon has not been hunted by man in the reserve for over 10 years.
The behaviour of the gibbon within the framework of predator avoidance in general, and specifically, towards the 4 potentially dangerous species mentioned above is described below. It comprises
1. general predator-avoidance behaviour with a preventive function which is integrated in the daily activity programme,
2. general reactions to disturbing instances (alarm behaviour), and
3. specific reactions to potentially dangerous animals and man.
5.2. General predator-avoidance
The risk of direct confrontation with certain potential predators is reduced a priori by:
1. the exclusivelv arboreal way of life of the gibbon. The gibbon avoids the ground and its proximity, generally remaining at a height of over 10 meters above ground, i.e. at a forest level which is rarely visited by the leopard or python [GRZIMEK, 1973].
2. sleeping and resting in the upper zones of «forest giants» (trees of 50-70 meters in height).
3. the camouflaging colouring of the fur (especially effective in the resting position).
The behaviour patterns which serve general predator avoidance are:
1. General cryptic behaviour.
The way of life of the silvery gibbon is characterized by movements of low intensity and also by acoustic inconspicuousness.
Exceptions are the female song bouts (chapter 6) and territorial border conflicts (chapter 7). In these cases, which involve conspicuous behaviour, the following security measures are taken:
(a) During the song bouts, which are mostly performed by the adult female only, the male remains on a lower level and quietly observes the surroundings. (b) At the end of female song bouts, as well as of territorial conflicts, the animals leave the respective part of the forest extremely quietly. (c) Conspicuous behaviour is immediately interrupted if there is a disturbance.
All daily activities are repeatedly interrupted for short periods of vigilance: The gibbon remains motionless on the spot, scans the surroundings, and is undoubtedly also highly alert towards acoustic stimuli.
5.3. General reactions to disturbing instances
Disturbing instances are:
1. sudden noises or movements which surprise the gibbon,
2. fear/alarm sounds of non-specific animals (uttered by many forest dwellers such as e.g. macaque, wild pig, giant squirrel, mouse deer),
3. motionless snakes, leopards, man, as well as other unknown objects (e.g. fire place, tape recorder).
On being disturbed the gibbon immediately interrupts its activity and reacts with alarm behaviour.
The following behaviour patterns were observed in this context:
1. Immediate vertical fliqht. A qibbon which happened to be relatively near the ground at the time of the disturbance instantly climbed or swang up towards a higher level of the forest.
2. Directed, exploratory surveillance of the area of the disturbance. If the gibbon failed to establish the source of disturbance from its momentary site, it tried to locate it by moving cautiously to different vantage points.
3. Observation of the discovered instance of disturbance. Once the source of the disturbance was discovered, the gibbon observed it attentively and showed readiness for flight.
4. Utterance of hoo notes. Simultaneously with 1. - 3., the gibbon constantly uttered short hoo notes.
The hoo notes of an individual usually caused fellow group members in hearing distance to interrupt their activity, remain on the spot and adopt an attitude of intense vigilance. Several times it was observed that they, too, recognized the source of disturbance and reacted with alarm behaviour; in no case, however, did gibbons immediately show alarm behaviour in response to the hoo notes.
The following behaviour patterns could be observed after an alarm:
1. Quieting down. If the disturbance was obviously of no importance, the gibbon resumed its normal activity. (Examples of such instances are (1) falling dead branches in the near surroundings, (2) sudden swift movements of a group member when catching an insect, (3) the noises of a wild pig foraging.)
2. Remaining quiet and observing, quiet withdrawal, exploring, harassing, flight. These behaviour patterns occurred if the disturbing instance was obviously dangerous (snake, bird of prey, leopard, man; s. section 5.4).
3. Quiet disappearance from the area. If the general disturbance continued, in spite of exploring the affected area (examples: fire place, alarm sounds of non-specific animals), the gibbon as a rule moved to another part of the forest where it resumed the activity in which it had been engaged before the interruption, or began a new one.
5.4. Behaviour during disturbance through non-specifics
The behaviour of gibbons towards snakes was ob served on 4 occasions. One case involved a reticulated python, 3-4 meters in length, on the ground; in the other 3 cases the snakes were smaller unidentified specimens in the branches at a height of 15-25 meters. All four snakes were motionless at the moment of their discovery by the gibbons and did not react to the presence of the animals.
The behaviour of the gibbon on discovering the snake was, in all four cases, the same: It immediately interrupted its activity, uttered hoo notes and, remaining where it was, closely observed the snake (alarm behaviour).
Then it approached to within 5-10 meters of the snake and watched it attentively. It changed its location several times, always however keeping higher than the snake. After some (5-10) minutes, it finally moved away and unhurriedly left the tree crown. In no case did the gibbon resume its normal activity in the same crown, respectively, crown group.
In two of the cases a second group member also remarked the snake whereupon it behaved in the same way as the individual which had noted the snake first.
5.4.2. Birds of prey and owls
Of the 15 species of birds of prey which occur in the reserve [HOOGERWERF, 1970], the following may be regarded as potential enemies, especially of younger gibbons: 1. Black eagle (Ictinaetus malayensis), 2. White-bellied sea eagle (Haliaeetus leucogaster), 3. Changeable hawk eagle (Spizaetus cirrhatus), 4. Crested serpent eagle (Spilornis cheela).
Larger species of owls which occur in the reserve are: 1. Buffy fish owl (Ketupa ketupa), 2. Spotted wood owl (Strix seloputo). They are wholly resp. partially diurnal and kill a.o. smaller mammals as e.g. mouse deer which may reach a weight of 2 kg (personal observation). They may surely therefore also be considered as predators of younger gibbons.
Observations of encounters between gibbons and owls are not available so that nothing can be said about the reactions of the gibbon to these birds.
However, on several occasions gibbons reacted to birds of prey flying past. Three levels of intensity could be discerned:
1. The gibbon remains where it is, following the bird of prey with its eyes for a short time and possibly uttering single hoo notes. It then resumes its previous activity, often before the bird of prey has disappeared from sight. Gibbons would usually react in this way to especially high flying eagles, smaller and medium-sized birds of prey (regardless of their flying height) and, occasionally, also to passing hornbills.
2. The gibbon crouches ready to flee, utters hoo notes and observes the bird of prey until it has disappeared from sight. After a short phase of vigilance, it resumes its normal activity.
This reaction was typical when eagles were passing by or circling at medium height.
3. The gibbon flees a few meters, stops and observes the bird of prey while uttering hoo notes. Even after the predator has disappeared, it still intently watches the sky for a while before engaging in some normal activity again.
This reaction to low flying, and therefore suddenly appearing, eagles was shown when the gibbons were at the periphery of a tree crown. (In two cases this behaviour was also observed after the sudden appearance of a large hornbill.)
On 3 occasions the behaviour of gibbons towards a leopard was observed. The same behaviour occurred on a 4th occasion; that it had been elicited by a leopard could only be established later, on the basis of fresh leopard tracks.
In all interactions the leopard was harassed (in no case did a gibbon flee after discovering a leopard):
According to their position, the members of the gibbon group moved upwards or downwards to gather at a vantage point, some 20-30 meters above ground, from where, apparently, the leopard could be conveniently observed.
After initially uttered hoo notes, all group members (including infants carried by their mother) emitted loud screams at short, irregular intervals and showed bursts of agitated movement in the area above the predator. A coordination of calls or motion within the group did not occur.
While screaming, the individual group member remained temporarily at a vantage point and observed the leopard intently; then vigorous swings in the branches followed again, however without leaving the neighbourhood of the predator.
Occasionally one of the group members (mostly the adult male) swang down to a lower level of the forest (10-15 meters), glanced at the leopard and swang upwards again. These advances were carried out without vocalization.
On all four occasions, the leopard retreated after a short time (on two of them the observer had caused its flight). After 0.5-1 minute, the gibbons stopped their harassing. They were vigilant for another 2-3 minutes with orientation to the ground, however quieted down progressively and finally resumed normal activities.
On the two occasions when the leopard fled from the observer, 2 resp. 3 gibbon groups in the near surroundings, through whose home range the leopard apparently moved, also started harassing.
Harassing as a possible reaction to predators has also been observed for H.syndactylus [CHIVERS, 1974]; in H.syndactylus, too, all animals of the family group uttered single screams uncoordinatedly and over a longer time; however they remained in one place, i.e. were not in motion in the area above the predator.
On the contrary, in H.l.lar and H.l.agilis such a reaction to the presence of a predator does not seem to occur [ELLEFSON, 1967; GITTINS, 1979]; according to ELLEFSON , family groups of H.l.lar possibly reacted in such cases with female song bouts (s. chapter 6).
In the observed encounters gibbon/snake on the one hand, and gibbon/leopard on the other, the reaction of the gibbons largely showed a constant pattern, and the behaviour was typical for each of the two cases.
The reactions to passing birds of prey showed different levels of intensity.
In the reactions to man, a pronounced variability was observed. 4 main types of reaction - and certain combinations of these - occurred:
1. Remaining quietly in the branches and observing,
2. Cryptic withdrawal,
3. Swift conspicuous flight through the tree crowns,
4. harassing of man.
A detailed description of these behavioural patterns is given below. Also the conditions which cause the different categories of reaction (aspects of the actual situation) and the function of the different types of reaction are discussed.
The discussion is based on observations of encounters (1) between gibbons and the observer himself, and (2) between gibbons and other humans (mostly guards).
a. Remaining quiet
The gibbon remained in its place, motionless and silent, and observed the human intruder.
This behaviour occurred when (1) the gibbon felt undetected, and (2) the vegetation offered concealment as protection.
If the gibbon succeeded in remaining undetected, it would wait until the intruder had moved out of the area, and then usually resume its previously interrupted activity.
If the human directed his attention to the hiding gibbon, or protracted his stay in the surroundings, the gibbon would react immediately resp. after several (3-10) minutes with flight or harassing.
b. Cryptic withdrawal
In certain cases the gibbon moved away inconspicuously either (1) to another place where it then remained quietly and observed the surroundings (and otherwise behaved as described above), or (2) into an other part of its home range where it resumed - mostly without delay - its normal activity.
Such inconspicuous withdrawal occurred when there was an appreciable distance between the gibbon and human, and the former was in a relatively exposed place.
If, subsequently, the human detected the gibbon and directed his attention towards the animal, it would react with flight or harassing.
Basically, therefore, the gibbon shows a tendency to avoid direct confrontation with man by remaining quiet resp. cryptic withdrawal. The reactions of harassing and flight which are more costly in terms of energy only occur if the gibbon sees that it is being observed and possibly pursued, i.e. if it feels itself directly threatened.
c. Swift conspicuous flight
Swift flight in the presence of man always took place at a medium level of the forest (25-30 meters height) where the conditions of the substrate allowed fast brachiation (s. chapter 3). The fleeing animals generally followed routes which they knew would allow them efficient progress. Since established gibbon groups possess such escape routes in every part of their home range, they mostly fled in single file.
While the withdrawing gibbon usually moved to some other part of its home range, where another activity was soon resumed, the route of the fleeing gibbon led as a rule to resp. through central parts of its home range; after flight, the gibbon remained quiet for several more minutes in an attitude of heightened vigilance.
The flight behaviour of an individual was not necessarily contagious; other group members reacted first of all with alarm behaviour and fled only if they themselves had also discovered the man.
Flight was the alternative reaction of gibbons to harassing (s. below); in no case, during one and the same encounter, did both types of reaction occur.
Flight was the typical reaction of non-established gibbons, but it frequently also occurred in established, territorial animals.
The harassing of man showed basically the same characteristics as the harassing of leopards: all individuals of the gibbon group were involved; no call coordination within the group could be noted; the animals were almost constantly in motion in the area above the intruder; single short advances of one individual to a lower level occurred.
The end phase in the harassing of man was different however from that in the harassing of leopards (Corresponding to the different behaviour of the two types of enemy; in all observed cases man reacted contrary to the leopard - not by leaving the area but by staying and observing the display of the gibbons.):
Occasionally - after several minutes of harassing - one of the group members quietly left the place. The remaining animals continued to harass, but also soon withdrew, one after the other. They often took different directions and only gathered again in a remoter part of the forest to form a loose group. There they resumed their normal activities.
From the withdrawal of the first to that of the last group member, several minutes might pass. The average duration of harassing of man was 16 minutes (7-24; n = 8).
In one case a female was observed, for the duration of harassing, to «deposit» her infant in a high tree some 50 meters away. There it remained quietly until - during the withdrawal of the group - it was picked up by its mother again and carried away.
Only few gibbon groups reacted to the presence of man with harassing, and they were invariably established, territory-owning groups.
This points to the existence of a correlation between the ownership of a territory and the readiness to react with harassing: Apparently, harassing requires a certain measure of «self-assurance» which only owners of territories possess.
Territory-owning groups also perform screaming behaviour, in which all members participate, during border conflicts with neighbouring gibbon groups. Here it serves as display behaviour and - in connection with the directed aggressive behaviour of the adult males - to reinforce territorial boundaries (s. 7.5.1.).
Analogous to the border-conflict screaming, harassing of predators may also represent display behaviour with the aim of intimidating the predator and possibly driving him out of the territory. (There is also a certain similarity between the directed aggressive behaviour of the adult males during the border conflict screaming and the exploratory advances - shown mainly by the adult males - when harassing predators.)
In Ujung Kulon/Gunung Honje Reserve it was noted that also those gibbon groups, which had reacted with harassing during the first 2-3 encounters with the observer, fled immediately during subsequent encounters.
Apparently the reaction of man (remaining in the area, observing) provides - in contrast to that observed in the leopard (leaving the area) - a negative feedback and leads finally to a change in the type of reaction of the gibbon.
From this it may be deduced why various established gibbon groups - already in their first encounters with the observer - reacted with flight: Presumably, in local populations, former experiences with man are passed on and later determine their reaction towards him.