Siberut Island Research Articles

The ecology of singing in Kloss gibbons (Hylobates klossii) on Siberut Island, Indonesia

Data are presented from a study of habituated Kloss gibbons on Siberut Island, Indonesia. Male Kloss gibbons can sing at any time from 0100 to 1300 hr, but the majority of songs is concentrated in the hour before dawn. Female Kloss gibbons sing only after dawn and the song bout includes a dramatic visual display. Neither countersinging nor coordinated chorusing has been proved in either sex. Males sing before dawn as often as possible but are inhibited by wet nights and by minimum temperatures below 21.5°C; postdawn songs of both sexes are inhibited by rain. The occurrence of any particular type of song bout is independent of the occurrence of the other types. Song trees used by males and those used by females do not differ in height. Song trees emerged from the neighboring canopy more than other available trees of similar height in the gibbons’ home range. Female song trees were most abundant on the slopes and where the trees were tallest. Almost all the male’s night trees could have been used for singing from had the weather been suitable. There was a greater likelihood of the male’s traveling a long way to the day’s first fruit source on mornings when he sang before dawn than on mornings when he did not. Considerations of sound transmission through tropical rain forest reveal that the times and frequencies used for singing by Kloss gibbons are optimal for communicating with neighboring groups.

A Numerical Analysis of Tropical Rain Forest, Using Floristic and Structural Data, and Its Application to an Analysis of Gibbon Ranging Behaviour

(1) As part of a study of the manner in which Kloss gibbons use the forests available to them, data were collected in two study areas of Lowland Evergreen Rain forest on Siberut Island, Indonesia, from 2459 trees of at least 15-cm dbh along transects of quadrats the total area of which was 11.3 ha. The identity of each tree, its height and climber load, the abundance of Myrmecodia epiphytes, and the position of its crown within the neighbouring canopy were recorded. (2) These data were analysed using classification and ordination techniques and as a result nine forest types were derived. These could be interpreted ecologically. One was on dry level ground, one on wet level ground, one on peatswamp, two on major ridges, and four on minor ridges. These types were generally recognizable by indicator species, and that gibbons recognized them too was reflected in their behaviour. (3) Seven of the forest types, in twenty-two blocks, were in the 31-ha home range of the main study group of gibbons. The number of times that gibbons were seen feeding in a forest type was very highly correlated with the density of trees that were potential fruit sources for gibbons. (4) Female gibbons sang from those forest types with the trees that were tallest and above the break of slope. The gibbon group slept in those forest types in which the trees had a low frequency of the myrmecophilous epiphyte Myrmecodia tuberosa, presumably to avoid being bitten by ants. (5) Few meaningful correlations were found between forest type and specific activities. However, forest types such as peatswamp were under-used consistently, while others such as those on minor ridges were used disproportionately often for certain activities. This information allowed the conventional two-dimensional view of a home range to be extended to a four-dimensional view by including differential canopy use and changes in behaviour through the day.

Home range use by Kloss gibbons ( Hylobates klossii) on Siberut Island, Indonesia

A home range is a reservoir of resources which are distributed throughout its area and joined by pathways. These resources are used for different activities and at different times of day and while ‘space-time systems’ have been described for a few terrestrial mammals, no previous study has attempted a similar analysis for an arboreal mammal. Kloss gibbons were studied for two years on Siberut Island, Indonesia, and one group was habituated to an observer. The ranging patterns of this group are analysed in two, three and four dimensions with respect to the types of forest in which the animals lived. Finally, differences between the results of this and previous studies of Kloss gibbons are discussed and it is suggested that a home range can be more permanent than the animals within it.

Interspecific spacing between gibbons (Hylobates klossii) and langurs (Presbytis potenziani) on Siberut Island, Indonesia.

Spatial arrangement and social interactions of two sympatric and ecologically similar primate species, Hylobates klossii and Presbytis potenzianai, are described from field observations made between July 1972 and October 1974 on Siberut Island, Indonesia. Gibbon territories and langur home ranges overlap extensively. Because gibbons have the ability to supplant langurs at shared food sources, langurs are at a competitive disadvantage. To avoid or decrease the frequency of hostile interactions with gibbons, langurs locate their core areas on boundaries between adjacent gibbon territories, which permits langurs to retreat across these barriers in response to gibbon movements. Langurs further enhance segregration by leaving their sleeping trees earlier than gibbons, gaining additional feeding time on contested food sources. This form of interspecific spatial organization between gibbons and langurs resembles certain predator-prey spacing systems, where territorial boundaries between adjacent predators serve as sanctuaries for prey populations.

Variations in group composition and population density of the two sympatric Mentawaian leaf-monkeys

The Mentawai snub-nosed langur (Simias concolor) and the Mentawai langur (Presbytis potenziani) were observed on Siberut Island, off the west coast of Sumatra, for about 20 months during 1974–1978. The Mentawai snub-nosed langur was organized into monogamous groups in the major study area, but formed larger polygamous groups in some limited areas. Where larger polygamous groups were observed, the snub-nosed langur was found at an extremely high density. On the other hand, in the area where monogamous groups were observed, they occurred at a low density. Such differences in social organization are discussed in relation to excessive hunting by the natives. Mentawai langurs were found in small monogamous groups throughout the island, a condition never observed in any other known Old World monkeys.

Ecological separation of three diurnal squirrels in tropical rainforest on Siberut Island, Indonesia

Reduced species diversity on islands is well documented. The present paper examines the ecological separation of diurnal squirrels on Siberut Island, Indonesia, and contrasts the results with those of studies on the neighbouring mainland.

Family formation strategies of Kloss's gibbons.

This study describes how adults of Kloss's gibbons (Hylobates klossii) inhabiting the central rain forest of Siberut Island, Indonesia, help their offspring achieve reproductive success. Adults of these monogamous apes exclude maturing offspring from the family and territory through intrasexual aggression. Adults may then assist the offspring to establish a territory contiguous to theirs. Aid may take the form of accompanying offspring into suitable areas not yet occupied or forcefully annexing part of an adjacent territory. Replacement of an adult by its offspring may also occur. A conclusion is that the probability of a young gibbon gaining a territory and mate is greater in the proximity of its parental group than in areas farther afield.

Solution for Siberut?

The authors describe the conservation plan they prepared at WWF's request while they were engaged on a two-year study of the Indonesian island of Siberut in 1976–78. Their plan is designed to conserve as much as possible of both the remarkable wildlife, which includes many endemic species, and the people's traditional way of life. Edited and coordinated by Jeff McNeely, whose article describing Siberut is on page 159, this plan is now being considered by the Indonesian Government.

Social Organization of Simakobu Monkeys (Nasalis concolor) in Siberut Island, Indonesia

Simakobu monkeys ( Nasalis concolor ) are restricted to the four Mentawai Islands off the western coast of Sumatra, Indonesia. Data were collected during 348 days between September 1972 and October 1974 at a 200 ha field site on the largest island of Siberut. Social structure among families of two to five animals (mean = 3.5) was determined for groups living in the central primary rain forest. Families occupy a home range of ca . 25–30 ha partially overlapping the space of adjacent families. The rare intergroup conflicts are male to male interactions dominated by loud vocalizations. Simkobu evade human predation by minimizing conspicuous movements and vocalizations, by concealment in the canopy, or by rapid terrestrial flight when detected. This cryptic behavior associated with the monogamous social group, unique among Asian Cercopithecidae, may be a behavioral adaptation to avoid human predation. The dominance of primates in the depauperate Mentawai Island's mammalian fauna and absence of other relevant predators are considered responsible for Simakobu's specialized defenses against man.

Songs, Choruses and Countersinging of Kloss' Gibbons (Hylobates klossii) in Siberut Island, Indonesia

This study evaluates the social spacing mechanism of song as it occurs in Kloss' gibbons. The study population included individuals in 13 family groups whose composition and territories were known (TENZA 1975) plus a number of others. Sonagrams illustrate individual and sexual differences in singing. Sex differences in chorusing, countersinging and other behavior related to song are described. Variations in singing or chorusing or both are related to season, time of day, sex, age, spatial factors and social factors. The adaptive functions of singing, countersinging and chorusing are discussed. It is concluded that: (1) Song is mainly for interterritorial communication between members of the same sex, (2) male-song probably also functions in mate attraction and (3) chorusing is primarily an adaptation reducing predation risk to singing gibbons.

Territory and monogamy among Kloss' gibbons (Hylobates klossii) in Siberut Island, Indonesia.

Behavior of Kloss' gibbons was studied from July 1 to October 7, 1972 in Siberut Island, off the coast of western Sumatra, Indonesia. Reproductive groups are monogamous families with a mean family size of 3.4 individuals (n = 11 families). Such families occupy territories averaging 6.7 ha. Adults defend their territories only against members of the same sex. This intrasexual defense of territory maintains the monogamous mating system. Other social units were unmated resident females, unmated resident males, floating males, and a courting pair. Males establish territories before mating, perhaps with help from their parents. Males guard their families against predators. Females lead progression through the territory. Subadults remain peripheral to their families but other family members tend to remain within 10 m of one another.