Usha A. Mathew

The open-zoo concept of the Singapore Zoological Garden allows displays of its exhibits in a naturalistic manner. In contrast to cement floored, steel barred enclosures, island exhibits with rich vegetation are more appropriate as captive animal habitats. A study was conducted on two different species of gibbon displayed on adjacent island exhibits at the zoo. It focused mainly on intergroup and intragroup relationships. The size and shape of the two islands were taken into consideration in order to tabulate the maximum utilisation of the islands. Distinctive behaviour of individual animals as well as social and non-social behaviours of the pairs were observed.

These ‘high flying’ arboreal apes are distributed in the rainforest of South East Asia. They are now found only in ‘pocket-sized’ populations in Indonesia, Indochina, China, Bangladesh, Assam and Malaysia. A field study on gibbon behaviour carried out in Tanjong Triang, Johor, Peninsula Malaysia stated that in 1961, 36,590 sq. miles or about three-quarters of the total Malayan land area was covered by primary equatorial rain forest. Hylobatidae were observed to inhabit three major forest types in the peninsula. That is, the lowland rainforest, from sea level to 2,500 ft; the submontane rainforest at 2,500 to 4,000 ft and the montane forest, above 4,000 ft. This study was conducted at the lowland Malayan rainforest, which covered 60% of the peninsula. This area was composed of 8,000 species of flowering plants and 2,500 of these were trees ¹. Ellefson ¹ describes this luxurious layout as once home to three species of the family Hylobatidae and they are Hylobates lar, Hylobates agilis and Hylobates syndactylus. A single closed social gibbon unit territory averaged approximately 40 hectares.

As an introduction I would like to quote briefly some major findings in Ellefson’s study on wild gibbons’ nuclear units. In his excerpts he stated that in intragroup relationship, wild gibbons’ 24-hr day, on average about 14hrs is spent in a sleeping location either resting and self grooming or sleeping. The remaining 10hrs of waking period are composed of 12% (1hr) social behaviour and 88% (8hr) non-social behaviour. Of, the little more than 1hr of social behaviour, half of these is spent on intergroup conflicts, and territorial behaviour. If intergroup conflicts were primarily the behaviour of adult males, then the actual social behaviour involving most other group members was found to fall below 10% of the waking period ¹.

The following study was set up to investigate the effectiveness of our island exhibits in inducing natural behaviours such as those described by Ellefson.

Behavioural observations of two breeding groups of gibbons, i.e. Hylobates lar and Hylobates m. mulleri were carried out between August to September 1996. The gibbons were housed on island exhibits. These islands are covered with lush vegetation, depicting a miniature form of gibbons’ natural habitat.

Tabulated check sheets with sixty rows, and each row indicating a minute were used. Each gibbon was denoted a code. ‘MM’ and ‘MF’ denoted the male and female Mulleri gibbons and ‘LM’ and ‘LF’ as the male and female Lar gibbons. The actions of each individual animal were observed and marked for every minute. Observations were carried out for sixty days, at three different hours per day, i.e. early morning from 0700hr to 0800hrs, midday from 1200hr to 1300hr and evening from 1800hr to 1900hrs. The number of simultaneous behaviours in each hour was calculated and the average percentage time spent per behaviour per hour was determined. Average time spent on the same type of behaviour by each individual gibbon was compared. A comparison of behavioural activity between wild gibbon colonies and captive zoo colonies was attempted.

The percentage of time individual gibbons spent brachiating, was calculated. ‘MM’, ‘MF’, ‘LM’, ‘LF’, respective readings were 20.04%, 62.14 %, 50% and 21.7% at 0700hrs to 0800hrs. 1200hr to 1300hr observations derived an average time of 44.74%, 8.69 %, 14.65 % and 6.8 % of active movements and the 1800hr to 1900hr observations indicated 15%, 2 %, 4.3% and 2.1 % of brachiation.

‘LM’, the male Lar gibbon displayed stereotypic movements at seven separate observation times. He did this on a 1.5m long horizontally placed branch that was found at the western edge of the island. He could manage only 2 short brachiatory movements due to the limited length of the branch. During each minute he performed around 5 crossover brachiations and the whole episode varied form 9 to 15 minutes. Such abnormal behaviour was observed on ‘LM’ during the morning 0700hrs to 0800hrs studies.

The 4-month-old Mulleri gibbon, ‘MY’ was allowed to move freely, but under close surveillance of his mother, on 26 separate observations. On 8 of these incidences ‘MY’ brachiated very high on the tree branches (about 7 meters high), performing short pendulums to steer off a brachiation.

Self and social grooming were most frequent during the afternoon observations. For ‘MF’, ‘MM’, ‘LM’ and ‘LF’, the average self-grooming time spent was noted at 2.8%, 0%, 0% and 0.36% respectively during the 0700hr to 0800hr studies. Social grooming stayed at 6.923% with the Mulleri gibbon pair and 11.54 % in the Lar gibbon pair during the afternoon observations. 5.6% mother-infant grooming was observed during the late evening observation and 3.1 % self-grooming were also being observed during at the same time.

Resting was marked when the animals reclined in the same position for more than 1 minute without engaging in other activities. During the observations taken during the dawn ‘MF’ had an average resting bout of 25.71%, ‘MM’ reclined the longest during this period at 69.28%, ‘LM’ rested 31.07% and ‘LF’ rested 67.86%. During the midday, after the morning feeding and foraging bouts, the gibbons rested for a longer average percentage, i.e. 54.24%, 73.57%, 75.95% and 83.43%. In the evening they spent 90 to 100% of their time quiescent.

During the 0700hr to 0800hr observations, morning calls took place in 95% of the 60 observations. 80% of the calls were initiated by the female Mulleri gibbon ‘MF’ and 15% were initiated by ‘LF’. 10% of the observation showed both the females beginning the ‘great call’ simultaneously. 1% of the time the male Mulleri gibbon ‘MM’ began to emit soft hoos concurrently with the females. ‘MM’ never initiated a call but responded to the females in 90% of the observation time with a series of hoots. ‘LM’, joined the duet only on two occasions. ‘MF’ emitted a series of great calls every two minutes in 90% of the observations and ‘LF’ emitted a great call only on 20% of the occurrences.

The afternoon tabulations showed ‘MF’ emitted calls in 2.9%, ‘L2’ in 2.7%, ‘MM’ in 0.25% and ‘LM’ in 0% of the total 60 observations. No calls were tabulated during the late evening readings.

‘MF’ presented amorously to ‘MM’ on 26 incidences during the 60 observations. ‘MM’ withdrew from ‘MF’s offers on 6 separate events by moving away from her. On 6 other incidences ‘MF’ showed aggression towards ‘MM’ by chasing him around the exhibit. On the other hand the female Lar gibbon ‘LF’ did not make any sexual inclinations but maintained a much closer relationship to the male with relation to the neighbouring duo. Both the male gibbons cannot be pictured as being greatly subordinate to the females but they seemed to be maintaining a degree of appeasement. As far as intergroup conflicts were observed neither aggressive expressions nor indications of any kind of threatening utterances were exchanged between the males in the adjacent islands.

During the 0700hr to 0800hr observation periods ‘MF’ spend 26% on average on the highest extremity of the tree; ‘MM’ 30%; ‘LM’ 11%; and ‘LF’ 47%. Middle level of the island is the centre of the highest point of the tree and the ground. 39%, 36%, 73%, and 38% of the time were spent in the middle level respectively. The ground was utilised at 26%; 20%; 2% and 1% of the time by the four different animals respectively.

During the afternoon the highest level was utilised at 15%; 10%; 5%; and 7% respectively. The middle level was utilised at 38%; 36%; 53% and 59%. The ground was utilised at 39%; 45%; 15% and 16% by the gibbons respectively.

1800hr to 1900hr showed 100% occupancies at the highest level of the trees. ‘MF’ and baby ‘MY’ usually occupied the same spot on the top of the tree. Particular sites for reclining were selected on the different evenings by the different gibbons.

Mulleri Gibbon Pair

Summing up the activities in a day on Island1, ‘MM’ stayed in a resting mood 81% of the day compared to ‘MF’ who brachiated about 41% of the time and rested only 25%.

Here we notice a distinct contrast in the activities of a male and female pair on the same island, i.e. an ‘overly excited’ female and docile male. On 5 incidences ‘MF’ chased ‘MM’ around the exhibit in a fearless display of aggression. In natural situations gibbons are generally known to be antagonistic toward conspecific adults. In most cases there is balanced dominance between mates. Indications of mutual repulsion of females suggested that females as well as males defended territories, although they take on less chasing then in males ¹. Here we may attempt to rationalise that the meekness of the male could be an act of appeasement. Appeasement displays, just as threat, acts as a signal between the animals. The appeaser is said to be vulnerable and yet the dominant animal rarely attacks further. It is assumed that selection favours the inhibition of aggression when appeasement is offered ². We observe here that behaviours displayed by this pair are acceptably normal as compared to the social gibbon units in their natural habitats.

The mothering behaviour of ‘MF’ was amusing. The young would suckle, sleep soundly and even try on ‘one-hand-grip’ (on mother’s waist), while mother did her impulsive spurts across the island. ‘MY’s other intermittent ‘growing-up’ training sessions included unregimented movements on the island, when its mother was resting. These movements included short runs along the grass, brachiating on the low branches and at times attempting daring sky-high stunts. Also included were play sessions with the mother and brief encounters of ‘touch and run’ game with the father. The longest period of independent movements recorded was 28 minutes on the ground. An average time of period of uninterrupted brachiation on the high branches was 18 minutes. Studies had indicated that an infant’s initial ties with mother are relatively stronger then with the father’s or other playmates.¹

On 8.3 % of the time, ‘MF’ displayed a series of non-stop brachiations through an entire hour, with only brief pauses, which lasted 1 to 2 minutes. This was an impressive performance by a mother gibbon carrying a baby, weighing about one-tenth her body weight.

Lar Gibbon Pair

During the course of the study no dominance display was observed between the pair of Lar gibbons. ‘LM’ the male maintained a more unruffled temperament. The duo extended balanced respect towards each other. ‘LF’s movements around the island were comparatively more meticulous and this could be justified considering her state of pregnancy.

Referring to the stereotypic behaviour of ‘LM’, such behavioural abnormality was mainly observed among captive animals that were housed in a fatuous environment. Stereotyped behaviour, basically results from anxiety associated with a desire to escape, fear or even an anticipation of a scheduled event, for example the feeding time.⁴ Providing an enriched environment in which investigative behaviour can be rewarded will prevent development of stereotyped patterns.¹ Comparatively ‘LM’s’ environmental enrichment requirements on island2 have been adequately provided for. The fact that he has mated with the female assures us that successful pairing had occurred. Ruling out the above causes resulting in his mental instability, we may now focus on the presence of the other male being too close to his domain. More about male-male intergroup behaviour are discussed below.

The zoo has recorded a few incidences when the male ’MM’ had escaped from his island and chased the male Lar gibbon ‘LM’ on the adjacent island. Though both the males had maintained a tranquil sort of relationship, across the islands, during the course of my study, apparently there seemed to be present a hidden antagonism in ‘MM’. Things do not always run smoothly in the animal world and, as we shall see, natural selection has produced a number of ways to deal with conflict situations. In general, conflicts in nature are of rather short duration, but it is easy to produce a chronic conflict situation in laboratory animals². A single male-male confrontational episode as studied¹ is briefly related to compare with the zoo incident. Both males reach a common border and sit adjacent to each other, while not touching. They look towards each other while other members of both groups sit quietly, several yards behind their respective alpha males, inside their respective territories. During the ensuing 1hr, the adult males of the opposing groups sit and stare at each other, emit conflict hoos, swing around in acrobatic displays, and chase each other once or twice. There is no physical contact, no apparent victory and no apparent gain. The members of the 2 groups tire of this ceremony after the first few minutes and move off into their respective territories to forage and feed. The males themselves tire of this intergroup conflict behaviour and drifts apart. This being the norm of the major activity for the day¹.

Inter-domain conflicts between our two males on island1 and island2 are inevitable, fundamentally because they are undesirably too close to each other. In order to elude such typical intergroup conflicts, I would suggest that similar social groups be located far away from each other.

In comparison to the distinct pattern of vocalisation between the male and female it was stated¹ that the male continues to call in between a female’s great calls and the females hoots too, but when a female begins another great call, the male stops calling and waits for her to complete her call¹. In my observations both females initiated a call and it was responded by ‘MM’ in 90% of the time. And during a great call ‘MM’ pauses for the females to complete the call. ‘LM’s least active participation to the females’ great calls would be indicative that he is subordinate to ‘MM’. He would probably be defeated in a territorial conflict. Apparently in such cases, where ‘LF’s’ great calls are not responded by a male’s call, a male from an adjoining territory would attempt to meet the calling female. We could state this as a cause of ‘MM’s’ occasional abrupt visits to island2. If a social group is in a no-man’s land and gives the calls, another group sometimes comes over and begins an intergroup conflict; the vocalisations also enables a lost group member to relocate its group and probably serve to bring unmated adults together ¹.

The gibbons on both islands occupied the uppermost part of the trees, for retiring for the night. Each selected its own site to sleep. ‘MF’ spend the rest of the time grooming or nursing young ‘MY. ‘MM’ usually selected a fork of a tree and most of the time reclined in a position similar to ‘a man sitting on a beach chair’. ‘LM’ and ‘LF’ also select the topmost sleeping spots. Most of the rest of the evenings are uneventful except for ‘MF’ who gets disturbed when someone she likes or dislikes passes-by the island. She would immediately grab ‘MY’ and with a few swift-flying stunts she tries to reach the branches closest to the subject that aroused her.

During the early hours of the morning, the gibbons were still found lazing on the upper forks of trees, or actively brachiating along the horizontal ropes at the middle level. They seldom wandered to the ground level, as the grass was basically moist with dew during this time.

The afternoons were mostly spent at the middle and at ground level of the islands. This being the hottest part of the day, often they would recline in their respective huts, under the shady undergrowth, or just sunbathed on the cool grass.

In concluding I would like to state that the two important behaviours affecting factors of survival of a gibbon in the wild have been seen here. Firstly, provision of natural tall trees on this island environment has enabled the gibbons to master the skill of brachiating through the trees.

Secondly, conquering the apprehension of height, is an acquired survival trait, that the gibbons on the island have well mastered. Such gibbons become ideal subjects for future reintroduction to their natural wild habitats. Comparatively the gibbons ‘down under’, i.e. the handraised and the gibbons held in small sized enclosures are quite destined in such unnatural conditions for their lifetime.

The Mulleri gibbons ‘MF’, ‘MM’ and baby ‘MY’, and the White handed gibbons ‘LM’ and ‘LF’ have never failed to amaze the zoo visitors. Such displays are more appealing and educational as they allow the public to observe the natural behaviours of the animals.

I would like to thank Mr. Subash Chandran, Senior Curator (Zoology), for consenting to a behavioural observation study of the gibbons on the island exhibits in the zoo. He has been a great encouragement to me in completing the observations and has diligently proof read my paper. I would also like to thank Dolly and Alice for proof reading and Dr. Sasha for assisting to assimilate pieces of straying paragraphs together.

1. Ellefson, O.J (1974) A Natural History of White Handed Gibbons in Malay Peninsula

2. Aubrey, Manning (1979) an introduction to Animal Behaviour

3. Wallach, Joel (1978) Zoo and Wild Animal Medicine

4. Fox, M.W. (1971) Psychopathology in Man and Lower Animals. J.Am.Vet. Med. Assoc.