Maliau gibbon project

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Structured Observation of Hylobates muelleri in Maliau Basin Conservation Area

Abigail Schoenberg



Basics of primatology

Primatology, as defined by, is the study of non-human primates. According to one paper, the modern concept of primatology emerged from the hope that the simpler structures of primate societies would provide insight into more complex human communities; this is what motivated scientists to begin quests into the field to study primates in their natural habitats (Rees 2007). Now, primatology most often involves long-term studies during which researchers repeatedly immerse themselves in primate habitats, getting to know individuals and groups intimately, and recording behavior in both quantitative and qualitative manners. (Quantitatively, certain behaviors are noted at regular time intervals; qualitatively, personal narratives of what they see are written and published.)

In its earlier stages, primatology was a much more qualitative than quantitative field; the transition occurred as a result of the discovery that interacting with animals to maintain observability might skew data. "Rather than risk disturbing ‘natural’ animal behaviour, primatologists developed non-invasive sampling methods in the interest of objective, reliable data and cross-site comparisons... By the late 1980s, these methods had crystallized into two main techniques: focal animal sampling and instantaneous sampling. Primatology’s progress from descriptive natural history to quantitative, comparative science was attributed to ‘means which are non-manipulative and are therefore less likely to alter or destroy the social system that is being studied’ (Altmann, 1974: 231)" (Rees 2007).

Despite primatology's transition into the more "scientific" realm, it is still the topic of much debate, especially in terms of its use of anthropomorphism. Primatologists "portray themselves not as simple observers of their primate subjects, but active participants in relationships with them" (Rees 2007). Although this sense of interaction is always carefully managed, the fact that primatologists' publications are popular amongst the non-scientific community leads many to believe that anthropomorphism has moved primatology out of the realm of science. Primatologists, however, defend their field by explaining the importance of anthropomorphism in their research. "Schaller (1964:194–95) argues that ‘something vital in our understanding of animals is lost if we fail to interpret their behaviour in human terms, although it must be done cautiously’, quoting Julian Huxley’s assertion that it is ‘both scientifically legitimate and operationally necessary to ascribe mind, in the sense of subjective awareness, to higher animals’" (Rees 2007).

Hylobates muelleri

Information from

This species of gibbon - also known as "Mueller's Gibbon," the "Bornean Gibbon," and the "Gray Gibbon" - is the only species that exists on Borneo outside of the Southwest region of the island. They are usually active eight to ten hours of the day, beginning just before sunrise and ending a few hours before sunset. Most of this time is spent foraging; only 5% of daily activities include social interactions. The fact that this is significantly less than most other primate groups may be due to the fact that Hylobates muelleri lives in smaller groups than other primates. They live in monogamous pairs with up to 3 offspring; males and females are generally social equals. Each group occupies a territory of, on average, 34.2 ha. These territories are defended in a number of ways, including regular morning songs and calling at and chasing intruders. (Gray gibbons rarely resort to physical violence when defending territory.)

Gibbon pairs' territorial songs have been well studied. Paired males will start singing before their sun rises; their female mates will join in sometime after sunrise, and the duet may last, on average, 15 minutes. Lone males sing for much longer; their songs may start before sunrise and last over an hour. This may possibly be to attract mates. Lone females rarely sing.


On July 13-18, I went into the field from 5:30AM to about 8:00AM with a Maliau Basin Guide. We followed the calls that we heard (with the exception of July 13th and July 16th, the days on which rain prevented the gibbons from calling), both on and off trail. (On days or at times when the gibbons were not calling, we merely walked on trails, stopping and looking carefully at every disturbance and every open view of the canopy.) Once a gibbon was in sight, we stood, unmoving (trying to conceal ourselves), watching it for as long as possible, notating every visible and audible behavior. On days that we were testing for reactions to disturbances, we would make our presence known with hooting/shouting. Data was also considered from a spontaneous encounter on a hike on July 9th, as well as from observations at a KK zoo on 6/21. Behavior was recorded in a manner consistent with primatological studies; notations of all behaviors were recorded per-minute.


Group composition

1. How does density of gibbon population vary between the logged and unlogged forest?

In the area of the Maliau Basin Conservation Area near the studies center, there are selections of both logged and unlogged forest. On the near side of the Maliau River, the forest was selectively logged 12 years ago; across the river on the far side, the forest was pristine.

Gibbon population density was estimated using the sounds of group calls. Results were unclear. On the first trial (7/17), it was estimated that there were 5-7 groups within every 1km radius on the logged side of the river, and there were 3-5 groups within every 1km radius on the unlogged side of the river. However, the second trial (7/18) occurred on a day when the gibbons generally seemed to be less vocal (it was slightly foggy out), and we seemed to be hearing more gibbons on the unlogged side of the river than on the logged side; however, unbiased counts could not be made on this day due to the silence on one side of the river.

Previous studies were consulted in an attempt to resolve the conflict. One study on Hylobates agilis revealed that in the Sabangau forest - which is selectively logged - held "a potential gibbon population size of 19,000 individuals" which "represents one of the largest remaining continuous populations of Bornean agile gibbons" (Buckley 2006). This suggests that gibbons, in fact, have a history of surviving and thriving in logged areas. This led me to believe that the probability of either higher counts on the logged side of the river or nearly equal numbers of gibbon groups on both sides is high.

2. What are the patterns of group size and spatial inter- and intra-group distribution?

It was determined that group size ranged from 1 to 5.

On any given day, the group nearest to the one we were observing was (from a sound estimate) 500m away; different groups never came closer to each other than that.

Members within a group were never more than 10m apart. However, they were also rarely closer than 3m apart, with the exception of the juveniles, who, on several occasions, were spotted following directly behind their parents.


3. What are the patterns (spatial and temporal) of a group's day-to-day movement within a territory?

Groups were always found in the same general area as they had been in previous days. At the same time in the morning the next day, they never seemed to be more than 500m from the spot in which they were encountered the morning before.

This led me to believe that groups are not “migratory” within their territories, at least not over short, 5-day periods. Rather, it seems that they have a chosen “sleeping area,” and while they move around their territories during the day, they return to that area at night, in time for us to find them calling in that same spot the next morning.

Research supports the idea that gibbons may not travel very far within their territories when they are not feeding on fruit (ie, feeding rather on insects and foliage). One study on the black-crested gibbon of China revealed that, while groups of that species have territories of over 130 ha, 69.7% of their activity occurred within 29 ha, and that quadrant use was significantly correlated with the distribution of important food patches. One possibility is that these Muellerian gibbons behave in a similar fashion, and that I encountered them at a time when fruit was scarce within their territories, and so feeding on other sources and simply weren't moving much.


4. How do pairs react to lone males in their territory?

This was a situation I encountered twice (7/14 and 7/15); because the situation was in the same area on both days, I believe it was the same pair and the same lone male.

After following the call of the lone male (which, as expected, started before sunrise and continued for at least an hour), we encountered both the male and the pair, who were remaining silent. In both cases, the pair would remain silent and relatively still while the lone male sang his song. (Sometimes the paired male would switch from sitting to hanging, etc., but there was no significant tree-to-tree movement.)

Once the lone male finished his song, the paired male would let out a low hoot (in the first case, it was a single "hoot," and in the second case, it was three successional hoots), and then he and the female would swing out of view together. (This led me to draw a conclusion about such vocalizations: see "Vocalizations".)

5. Is stationary hanging a threatening territorial display?

In three different observations, males seemed to hang stationary in situations where his territory had been threatened.

The first case was at the zoo in KK.

This individual (gender unclear) assumed this position and held it for no less than 5 minutes when a large group of humans approached the edge of his semi-enclosed habitat. When the crowd was finished taking pictures of the pose and began to disperse, the gibbon swung away.

The second was the case of a paired male whose territory had been invaded by a lone male (see video). The paired male remained silent and hung stationary for some time, not leaving until the lone male had stopped his song.

The third observation was of a paired male who, unintentionally, became aware of the research presence. In addition to bursting into song (see "Human-gibbon interactions"), the male swung to a very low branch right above our heads and hung stationary there for at least 30 seconds, simply watching us and displaying his own size.

Because territory seemed threatened in all 3 of these situations, I believe that stationary hanging is a threatening territorial display.

6. How do gibbons react to the sound of other mammals (ie, deer) in their territory?

Gibbons seemed to react much differently when threatened by humans than when threatened by other mammals. When interacting with humans, both lone males and pairs reacted with significant alarm (see "Human-gibbon interactions), but when the sound of Samba deer in the territory was heard (both calls and hoofbeats), reactions were very different. Instead of responding with alarm, gibbons seemed to respond with mere suspicion, then carry on with normal proceedings.

In both events where Samba deer invaded gibbon territory, the gibbons (a lone male and a pair) had been singing. When the sound of the invasion aired, the gibbons would cease their calling, remain still and listen, let out a low hoot or two, and then resume their singing and movement. The whole ordeal never lasted more than 20 seconds.

This led me to believe that the gibbons of the Maliau Basin Conservation Area are no more accustomed to humans than other gibbons. They seem to react with more alarm to human presence than to the presence of other mammals.


7. What is the general distribution of vocalizations throughout the day?

After 5 consecutive days of early-morning observation, this is the distribution I found:

We begin to hear lone males around 5:15am.

Volume escalates as the sun rises.

Duets peak between 6:30 and 7:30am.

Everything in the surrounding area seems to quite down around 8am.

Calls in the distance seem to stop by 10am.

This is consistent with the records of vocalization distributions found in literature.

8. How and why is that schedule affected by rain during the night?

On both raining days, gibbons did not call at all; this contrasted with my expectations that calls would merely start later in the morning. In fact, they did not call at any point throughout the morning, afternoon, or evening.

Though there does not appear to be any scientific explanation (yet) as to why rain throughout the night will prevent gibbons from calling the following morning, there is a history of this phenomenon frustrating scientific studies. In one study on population size of white-cheeked gibbons in Vietnam, "the effects of weather on the singing behaviour of gibbons was clearly shown during the survey, e.g. in at least 3 listening posts, the gibbons did not sing until the third day of the survey due to raining and heavy fog. Conversely, the gibbons sang all morning when it was sunny (at least 3 posts had gibbons sing the entire three morning of surveying)." This is believed to have influenced the results of the survey, as "bad weather and continuous raining during this survey may have limited the number of gibbon groups recorded" (Ha 2007).

9. What are the various types of vocalization and what are their functions?

I identified four different types of vocalizations over the course of my observations (with the help of

  • Pair Song: The well-studied duets of paired gibbons (including male and female solos) include patterned, rhythmic, repetitive calls, which increase in tone and speed, over and over again. This was found to be both inter-group communication and intra-group communication. The duet included alternating vocalizations between the male and the female as the male roamed around the area, potentially to let each other know of their whereabouts. However, the stereotypical hooting song seemed to be listened to by surrounding groups, and when groups in the distance performed them, the observed group would stop vocalizations and listen.
  • Lone Male Song: The calls of lone males attempting to attract mates is much less patterned and rhythmic. Though the tones seem to be the same as those of the paired males, rhythms are not repeated. Inter-group communication.
  • Low Hooting: This appears to be a warning call from one member of a pair to another (intra-group communication), coding information about the subsequent behavior of departure from the area.
  • Guttural Sounds: This appears to be a noise of alarm, most often recorded in threatened lone males.

Human-gibbon interactions

10. How does a lone male's reaction to a researcher's presence differ from a group's reaction?

A group's reaction to researcher presence was much more aggressive than a lone male's. Lone males would often stop their songs, let out many guttural sounds, and leave the area. Contrarily, when our presence was unintentionally revealed to a pair, an eruption of aggressive movements and vocalizations took place and did not stop for an entire hour, and the male actually came closer to us for intimidation purposes, as opposed to leaving the area. Potential explanations include the idea that lone males do not really have their own territories to defend; evidence is that one of the two lone males I saw was twice intruding on another group's territory.

Lone males also have less to lose in an encounter with an intruder because they have no mate or offspring. It makes more sense for them to air on the side of caution and protect their lives by fleeing, as opposed to facing the risks of getting closer, vocalizing (and thereby revealing their whereabouts), and attempting to intimidate.

Additionally, groups have backup from their family members (even males, who have the socially equal females to depend on), whereas lone males do not.

Therefore, it makes sense that groups' reactions to researcher presence would be more aggressive than lone males'.


There is a relatively large amount of risk in placing total confidence in these observations. Compared to typical primatological studies, this one was remarkable short. Five days - especially when no prior knowledge of the species in question existed, and no prior primatological experience was involved - is a dangerously short time span in which to draw definitive conclusions.

Additionally, as in all primatological studies, the dangers of projecting my own ideas about what I saw into my data are high. While anthropomorphizing my subjects may be considered acceptable among the world of primatologists, the chances of it becoming an interference is probably unusually high in this case, because I had no prior primatological experience.

However, structuring my observations of these animals allowed me to make the most of my short amount of research time. Though future studies should be longer and more thorough, this process may be considered a valuable preliminary study of an understudied primate.


Buckley, Cara. 2006. "TITLE UNKOWN" Primates. 47:4:327-335.

Fan, Peng-Fei, et al. 2008. "Effects of food and topography on ranging behavior of black crested gibbon (Nomascus concolor jingdongensis) in Wuliang Mountain, Yunnan, China." American Journal of Primatology. 70:9:871-878.

Ha, Nguyen Manh. 2007. "Survey for southern white-cheeked gibbons (Nomascus leucogenys siki) in Dak Rong Nature Reserve, Quang Tri Province, Vietnam." Vietnamese Journal of Primatology. 1:61-66.

Rees, Amanda. 2007. "PersonhoodReflections on the Field: Primatology, Popular Science and the Politics of Personhood." Social Studies of Science. 37:881-907.

An Extra Piece of Fabulousness

Everybody do the funky gibbon!

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