Sympatry between white-handed gibbons (Hylobates lar) and pileated gibbons (H. pileatus) in Southeastern Thailand

White-handed gibbons (Hylobates lar) are not known to occur to the east or southeast of Bangkok. The reliably documented localities ofH. lar nearest to this area are about 120 km northeast of Bangkok. There, in the Kao Yai National Park, is the only known zone of contact betweenH. lar and the pileated gibbon (H. pileatus), another species of the so-calledlar group. Unpublished documents dating from 1925 indicate, however, that sympatry between these two species may also have existed in the region of Sriracha, about 80 km southeast of Bangkok. Therefore, a large zone of overlap in the distribution of the two species may originally have existed. In most parts of this hypothetical zone, gibbon habitat appears to have been destroyed, with the Khao Yai Park possibly representing the last remnant of the once large contact zone.     

Territorial behavior in the Agile Gibbon

A group of agile gibbons (Hylobates agilis)was studied for 2 years in the Malay Peninsula. The behavior of the gibbons during territorial disputes is described, the relationships with neighbors are investigated, and the frequency and duration of disputes are analyzed. The nature of the territorial boundary and the type of territorial behavior exhibited by the gibbons are discussed. An attempt is made to identify the immediate precursors of disputes, and some suggestions are given to explain why disputes occur so frequently and are of such long duration.     

Abundance and Distribution of Sympatric Gibbons in a Threatened Sumatran Rain Forest

Agile gibbons (Hylobates agilis) and siamangs (Symphalangus syndactylus) are sympatric small apes inhabiting threatened forests of Sumatra, Indonesia. We censused both species in the 3,568-km² Bukit Barisan Selatan National Park, at the southern limit of their ranges, over a 7-mo period in 2001. First, we monitored daily calling rates from known populations to develop probabilities of calling during a specified number of days and used the probability of calling at 1 time during 3 days to convert calling rates to abundance. Next, we used 3-day calibrated call count censuses (n=31) stratified by distance from forest edge and across a range of elevations to estimate species-specific group densities. We used group size from the known populations as well as data collected ad libitum during the census to convert group density to individual density. Agile gibbon group density averaged 0.67 km^–2 (SE = 0.082) and group size averaged 2.6 (SE = 0.73) for a population estimate of 4,479 (SE = 1,331) individuals. Siamang group density averaged 2.23 km^–2 (SE = 0.245), and group size averaged 3.9 (SE = 1.09) for a population estimate of 22,390 (SE = 8,138). Agile gibbon and siamang densities are negatively correlated, with agile gibbons more abundant in mid-elevation forests and siamangs most abundant in lowland and submontane forests. The small group sizes of agile gibbons indicate potential survival problems in infant and juvenile size classes. Although neither species is presently threatened by direct human disturbance, continued deforestation will jeopardize the long-term viability of both species in Bukit Barsian Selatan National Park and on Sumatra.     

Mechanisms of force and power production in unsteady ricochetal brachiation

Brachiators travel by swinging beneath handholds, and it is not obvious how these animals manage to accelerate and decelerate in a horizontal direction, especially when moving rapidly. Most previous analyses focused on brachiation in highly constrained laboratory conditions that induced steady-state locomotion. Emerging understanding of brachiation suggests that much of gibbon locomotory behavior and morphology must be considered within the context of the complexities of the natural environment: the forest canopy is three-dimensional, with high variation in handhold availability and properties. The goal of this paper is to quantify the active mechanisms by which gibbons can dynamically control their velocity.Force production and kinematics were analyzed from a white-handed gibbon Hylabates lar during ricochetal brachiation. Both the mechanisms of force production and power input may be inferred for accelerating and decelerating brachiation by combining force data with kinematics. Examples of steady-state, accelerating, and decelerating ricochetal brachiation are highlighted.Gibbons are able to produce net horizontal impulses by releasing early (resulting in a loss of potential energy, but an accelerating horizontal impulse) or delaying release (associated with an increase in potential energy, and a decelerating horizontal impulse).Torque about the shoulder, leg-lifting (or dropping), and elbow flexing (or straightening) are discussed as potential mechanisms for controlling energy within the brachiating system. Of these possibilities, leg-lifting and arm-flexing were observed as mechanisms of adding mechanical energy. Net energy loss, and substantial torques about the shoulder, were not observed.     

A New Generic Name for the Hoolock Gibbon (Hylobatidae)

Contrary to usual practice, the generic nomen Bunopithecus is not applicable to hoolock gibbons. We recount the history of its application and explain why it is spurious. We supply a new generic name, list the characters and content of the genus, and compare it to the other 3 genera of the Hylobatidae.     

Hainan Black-crested Gibbon Is Headed For Extinction

Although Hainan black-crested gibbons have been on the list of the most endangered primate species in the world for many years, their environment is still deteriorating, especially on Hainan Island. Our findings indicate that the species is unlikely to survive the next decades unless efficient conservation policies and strategies are put in place immediately. Census data show that populations of the species used to occur across the whole island, but in 2003 only 13 individuals could be found, confined to a small region, the Bawangling Natural Reserve (19 02¹–19 08¹N and 109 02¹–10913¹ E), in the western part of the island, covering only 14–16 km². In other words, ca. 99% of the habitat has vanished in the past 300 years. Such dramatic change has pushed the species to the edge of extinction; only 2 groups and 2 solitary adult males, remained in 2003. Two adult females, 2 juveniles and one infant comprise Group A, in Donger, the core area of the western part of the reserve; and 1 adult male, 2 adult females, 1 juvenile and 1 infant formed another group (B), confined to another core area (Nanchahe) in the northern part of the reserve. The dramatic decline in the gibbon population has occurred due to vegetation reduction, ecological deterioration and extensive human impact. The forest cover was reduced from 95.5% 2000 years ago to just 4% in 1999; and the human population in 2003 was 330% larger than in 1950.