Human predation and Kloss's gibbon (Hylobates klossii) sleeping trees in Siberut Island,Indonesia

Kloss's gibbons (Hylobates klossii) and Mentawai langurs (Presbytis potenziani) on Siberut Island, Indonesia, both sleep in emergent trees 34–55m tall, situated on crests and upper slopes of hills. They differ in that 91% of gibbon sleeping trees examined were free of lianas, whereas 89% of langur sleeping trees were draped with thick, woody lianas hanging from the branches to the ground. Because indigenous hunters climb lianas to shoot primates in treetops, gibbons are less susceptible than langurs to nocturnal human predation. Hence, the gibbons' preference for—and apparent control of—the limited supply of vineless trees gives them an advantage over langurs.     

Interspecific territoriality in gibbons (Hylobates lar and H. pileatus) and its effects on the dynamics of interspecies contact zones

We investigated the ecology and interspecific interactions of the two gibbon species (Hylobates lar and H. pileatus) that overlap in distribution within a narrow zone of contact in the headwaters of the Takhong River at Khao Yai National Park in central Thailand. The zone is about 10-km wide, with phenotypic hybrids comprising 6.5% of the adult population. We compared species with respect to diet, territory size, intra- and interspecific group encounters, and territory quality. The two gibbon species exploited the same types of resources within their territories despite variation in the relative abundance of food-plant species between territories. The gibbons were interspecifically territorial, and males of both species displayed aggressive behaviors at both intraspecific and interspecific territorial boundaries. There was no marked difference in the amount of overlap between territories of conspecific and heterospecific pairs of groups. Although the habitat was not homogeneous, territory quality did not vary significantly between species. The species have not diverged in habitat preference or in diet. Neither species dominated in interspecific encounters, and both were reproducing well in the contact zone. We analyzed the potential consequences of several types of interspecific interactions on individual dispersal options and on the structure of the contact zone. Interference competition through interspecific territoriality affects the dispersal of individuals into the range of the other species. In general, territorial competition coupled with limited hybridization leads to predictions of a narrow contact zone or parapatry between species; thus, behavioral and ecological interactions between species need to be considered as potential factors in explaining range borders of primate species.     

Group harmony in gibbons: Comparison between white-handed gibbon (Hylobates lar) and siamang (H. syndactylus)

The siamang (Hylobates syndactylus) is exceptional among gibbons in that its area of distribution almost completely overlaps those of other gibbons, namely the white-handed gibbon (H. lar) and the agile gibbon (H. agilis) of the lar group. The siamang has almost twice the body weight of the gibbons of the lar group (ca. 11 kg vs. 5–6 kg), and it has been suggested that distinct ecological and behavioural differences exist between the siamang and its two sympatric species. The siamang has been claimed to differ from the white-handed gibbon “in the closer integration and greater harmony of group life” (Chivers, 1976, p. 132). However, few quantitative data exist to support this hypothesis. In the present study, intra-group interactions in captive family groups of white-handed gibbons and siamangs (two groups of each species) were recorded by focal-animal sampling. These data failed to show a consistent association between species and most of the behavioural patterns recorded, such as frequency of aggression, percentage of successful food transfer, frequency of social grooming bouts, and duration of social grooming/animal/hr. A significant difference was found for only two of the variables: Individual siamangs in this study showed longer grooming bout durations, and made fewer food transfer attempts than lar individuals. Only the first of these two differences is consistent with the hypothesis mentioned above, whereas the lower frequency of food transfer attempts in siamangs is the opposite of what should be expected under the hypothesis. On the other hand, two of these behavioural patterns showed a significant correlation with the parameters group size and individual age: Both individuals in larger groups and younger individuals tended to show shorter grooming bouts and a smaller proportion of successful food transfers. Our findings indicate that social cohesion within these gibbon groups may be much more flexible according to and depending on social or ecological influences and less rigidly linked to specific gibbon taxa than previously assumed. A considerably larger number of gibbon groups would have to be compared to provide reliable evidence for or against species-specific differences in group cohesion. Another finding of this study—a positive correlation between the frequency of aggression and grooming—is discussed in the light of the functional interpretations commonly attributed to allogrooming behaviour in primates.     

Interactions between lar gibbons and pig-tailed macaques at fruit sources

Interactions are reported between white-handed gibbons (Hylobates lar) and pig-tailed macaques (Macaca nemestrina) in Khao Yai National Park, Thailand, in which gibbons selected ripe fruit from sources before macaques arrived on 4 occasions during June and July of 1989. The macaques foraged near gibbons or from shared fruit sources during 13% of gibbon observation time. Gibbons made their presence known in fruit trees when macaques approached on 3 occasions and macaques did not enter fruit trees occupied by gibbons. An aggressive interaction is reported in which a single white-handed gibbon vigorously excluded 22–28 macaques from a rare and valuable fruit tree (Sandoricum koetjape). The observations suggest that exploitative and interference competition may exist between these species.     

Phylogeny and distribution of crested gibbons (genus Nomascus) based on mitochondrial cytochrome b gene sequence data

Crested gibbons, genus Nomascus, are endemic to the Indochinese bioregion and occur only in Vietnam, Laos, Cambodia, and southern China. However, knowledge about the number of species to be recognized and their exact geographical distributions is still limited. To further elucidate the evolutionary history of crested gibbon species and to settle their distribution ranges, we analyzed the complete mitochondrial cytochrome b gene from 79 crested gibbon individuals from known locations. Based on our findings, crested gibbons should be classified into seven species. Within N. concolor, we recognize two subspecies, N. concolor concolor and N. concolor lu. Phylogenetic reconstructions indicate that the northernmost species, N. hainanus, N. nasutus, and N. concolor branched off first, suggesting that the genus originated in the north and successively migrated to the south. The most recent splits within Nomascus occurred between N. leucogenys and N. siki, and between Nomascus sp. and N. gabriellae. Based on our data, the currently postulated distributions of the latter four species have to be revised. Our study shows that molecular methods are a useful tool to elucidate phylogenetic relationships among crested gibbons and to determine species boundaries. Am. J. Primatol. 72:1047–1054, 2010. © 2010 Wiley‐Liss, Inc.     

The yellow-cheeked gibbon (Hylobates gabriellae) in nam bai cat tien (southern vietnam) revisited

Data concerning the status, habitat, and vocalizations of yellow-cheeked crested gibbons (Hylobates gabriellae) were collected during a short field trip to the Nam Bai Cat Tien National Park (southern Vietnam). Nam Bai Cat Tien may be the southernmost locality where crested gibbons (i.e. theHylobates concolor group) still survive. Fewer songs were heard at Nam Bai Cat Tien National Park than at other crested gibbon sites visited by the author. At least two gibbon groups appear to have been greatly reduced in number since previous surveys in the park. There is some evidence that both the gibbon population and the gibbon habitat in Nam Bai Cat Tien are disturbed. The first case of a great call solo song in wild gibbons of theconcolor group is reported. Great calls ofH. gabriellae are described and documented with sonagrams for the first time. They differ from those previously described forH. leucogenys.     

Seasonal variation of diet and time budget of Eastern hoolock gibbons (Hoolock leuconedys) living in a northern montane forest

Most gibbons dwell in the tropical forests of Southeastern Asia, but eastern hoolock gibbons (Hoolock leuconedys) survive in high montane forest ranging from 1,600 to 2,700 m a.s.l. in Gaoligongshan (>24°30′N), Yunnan, China. To assess the behavioral adaptations of hoolock gibbons to the montane forest, we related temperature and food availability within the habitat to the seasonal behavioral patterns of a family group and a solitary female between August 2010 and September 2011 in Nankang, Gaoligongshan National Nature Reserve. The maximum temperature was 29.2 °C and the minimum temperature was ?0.3 °C during the period. The monthly mean temperature was <10 °C between December and February, making Nankang the coldest gibbon habitat reported so far. Nonfig fruit and fig availability declined to nearly zero in cold months. The family group increased resting and decreased travel and social behaviors when the monthly mean temperature was low. Compared with other gibbon populations, the hoolock gibbons spent proportionally less time feeding on figs and other fruit than other gibbon populations except Nomascus concolor and Symphalangus syndactylus. Only 36 species of plants provided nonfig fruit or figs, which is less than the number of fruit species consumed by any other gibbon population observed during a similar period of time (about 1 year). Hoolock gibbons shifted their diet to leaves and increased feeding time when fruit was not available. We conclude that diet flexibility and an energy-conserving strategy during the cold season when fruit is scarce have enabled the hoolock gibbons to survive in a northern montane forest.     

Modeling the Ecological and Phenological Predictors of Fruit Consumption by Gibbons (Hylobates albibarbis)

Understanding the ecological interactions between plant reproductive strategies and frugivore feeding behavior can offer insight into the maintenance of tropical forest biodiversity. We examined the role of plant ecological and phenological characteristics in influencing fruit consumption by the White‐bearded gibbon (Hylobates albibarbis) in Gunung Palung National Park, Indonesian Borneo. Gibbons are widespread across Borneo, highly frugivorous and perform important seed dispersal services. We compare multiple models using information criteria to identify the ecological and phenological predictors that most strongly influence gibbon fruit use of 154 plant genera. The most important predictors of resource use were the overall abundance of a genus and the consistency of fruit availability. Plant genera can maintain constant fruit availability as a result of (1) individual stems fruiting often or (2) stems fruiting out of synchrony with each other (asynchrony). Our results demonstrate that gibbons prefer to feed on plant genera that provide consistent fruit availability due to fruiting asynchrony. Because gibbons feed more often on genera that fruit asynchronously, gibbons are more likely to disperse seeds of plant genera with this reproductive strategy. Research on other frugivorous species is needed to determine whether the results for gibbons are generalizable more broadly. Finally, these results suggest that asynchronously fruiting plant genera may be particularly important for habitat restoration in tropical forests designed for frugivore conservation.     

Demography of agile gibbons (Hylobates agilis)

Demographic processes and the structure of a population of agile gibbons (Hylobates agilis) were investigated over 6 years in the Gunung Palung Reserve, Indonesia. Estimates of population size, density, and biomass revealed a population whose groups were stable in size and composition. Demographic processes place gibbons at risk, however, to short-term changes in their environment. Patterns of survival, fecundity, mortality, and dispersal combined to produce negative rates of growth. In addition, a top-heavy age-class distribution, with adults forming a large fraction of animals, makes it unlikely that this population could recover rapidly from a decline in numbers. Two behavioral factors, territoriality and monogamy, account for the size and stability of gibbon groups. Monogamy imposes limits on group size, while mating patterns and territoriality decrease the impact of sources of high mortality common in other primate species. These relationships underscore the fundamental importance of behavioral influence on demographic processes and social structure.     

高黎贡山赧亢白眉长臂猿食性及日取食量

通过确定白眉长臂猿的取食特征及日取食量,探讨不同个体间和春、秋季间日取食量是否存在差异,分析影响日取食量和取食特征的因素,为白眉长臂猿日能量需求及其营养容纳量研究奠定基础。2007—2008年春、秋季,在高黎贡山赧亢采用焦点动物取样法观察和记录数据。结果发现,白眉长臂猿取食36种食物的40个部位,其果实的食物单元、取食速度、单个鲜重值等取食特征不同于嫩叶。春、秋季白眉长臂猿每日取食果实量所占百分比最高、嫩叶所占百分比次之、茎和花所占百分比最低。春、秋季家群中雌猿的日取食量均高于雄猿。与家群的雌猿相比,独猿(♀)春季的日取食量稍高。在春季,独猿取食果实持续时间约为家群的2倍,而取食嫩叶的则与家群几乎相等。相同个体秋季的日取食量高于春季的,且春、秋季日取食各部位的量所占百分比也不同。分析结果表明,食性及日取食部位的量反映了白眉长臂猿选择食物的最基本策略。雌雄个体大小差异、能量消耗、食物特征、动物生理需求等因素影响白眉长臂猿的日取食量。白眉长臂猿取食持续时间与觅食树的食物资源量相关。     

Ranging behavior of eastern hoolock gibbon (Hoolock leuconedys) in a northern montane forest in Gaoligongshan,Yunnan, China

Generally, food abundance and distribution exert important influence on primate ranging behavior. Hoolock gibbons (genus Hoolock) live in lowland and montane forests in India, Bangladesh, Myanmar and China. All information about hoolock gibbons comes from studies on western hoolock gibbons (Hoolock hoolock) living in lowland forest. Between August 2010 and September 2011, we studied the ranging behavior of one habituated group of eastern hoolock gibbon (H. leuconedys) living in a seasonal montane forest in Gaoligongshan, Yunnan, China. Results show that the study group did not increase foraging effort, calculated in this study as the daily path length, when fruit was less available. Instead, the gibbons fed more on leaves and decreased traveling to conserve energy. They relied heavily on a single food species in most study months which was patchily distributed within their total (14-month) home range, and during most months they used only a small portion of their total home range. In order to find enough food, the group shifted its monthly home range according to the seasonal availability of food species. To satisfy their annual food requirements, they occupied a total home range of 93 ha. The absence of neighboring groups of gibbons and the presence of tsaoko cardamom (Amomum tsaoko) plantations may also have influenced the ranging behavior of the group. Further long-term studies of neighboring groups living in intact forests are required to assess these effects.     

Reassessment of age of sexual maturity in gibbons (hylobates spp.)

From studies of both wild and captive animals, gibbons are thought to reach sexual maturity at about 6 to 8 years of age, and the siamang (Hylobates syndactylus) at about 8 to 9 years. However, a review of the literature reveals that in most cases the exact age of the maturing animals was not known and had to be estimated. This study presents seven case reports on captive gibbons of known age. Captive males of the white-cheeked crested gibbon (H. leucogenys leucogenys) and of the siamang (H. syndactylus) can breed at the age of 4 and 4.3 years, respectively. Similarly, hybrid females (H. lar × H. moloch) and siamang females can breed at 5.1 and 5.2 years, respectively. This finding may help to improve the breeding success of captive gibbon populations. It is not clear whether gibbons reach sexual maturity earlier in captivity or whether sexual maturity is also reached by 5 years of age in the wild. Possible implications for the interpretation of group size regulation and of reproductive strategies of wild gibbons are discussed.     

Predation on giant flying squirrels (Petaurista philippensis) by black crested gibbons (Nomascus concolor jingdongensis) at Mt. Wuliang, Yunnan, China

Predation on vertebrates is infrequent in gibbons. In a 14-month field study of the central Yunnan black crested gibbon (Nomascus concolor jingdongensis) at Mt. Wuliang, Yunnan, China, we observed gibbons attacking, killing and eating giant flying squirrels (Petaurista philippensis). During 845 h of observation on one study group, the gibbons attacked giant flying squirrels 11 times, and succeeded in 4 cases. Although all members of the group attempted to attack the squirrels, all four successful attacks were made by the same adult female. The victims were infants in three cases and a juvenile or sub-adult in one case. Black crested gibbons also attacked adult giant flying squirrels by grabbing their long tails and throwing them from the canopy, but they failed to catch or kill the prey in three cases observed. Passive meat sharing occurred in three out of four successful cases. Besides hunting giant flying squirrels, the black crested gibbons also ate eggs or chicks in two birds’ nests and one lizard.     

High interannual variation in the diet of a tropical forest frugivore (Hylobates lar)

Frugivores must deal with seasonal changes in fruit availability and changes from year to year, as most species of tropical forest fruiting trees have considerable interannual variation in phenology and many are mast fruiters. We quantified seasonal and interannual changes in the fruit diet in a frugivore and important seed disperser, the white‐handed gibbon, Hylobates lar, in Thailand. We used 40‐d following data during April and May replicated in six consecutive years to study interannual variability in the diet and compared it with seasonal changes measured in monthly samples of the same size collected in three successive years. The 40‐d periods of following also allowed us to measure the decline in dietary similarity with time over a finer scale. We measured fruit diet similarity between replicated 5‐d periods using the percentage overlap (Renkonen's) index and Jaccard's similarity index. Seasonally, average dietary overlap between adjacent months was low, and similarity approached zero after four months. Average rate of decline in similarity exceeded 20 percent per 5‐d period. Variation in fruit species in the diet between years was high and was correlated with interannual variation in fruiting phenology. The strongest correlation occurred in the case of Nephelium melliferum, a highly preferred species that dominated the diet in good fruiting years. It is difficult to separate changes in food species preference from changes in availability from year to year. We devised a relative measure of preference that depends on the degree to which the gibbons rely on prior knowledge to find sources.     

Diet and ranging behavior of the endangered Javan gibbon (Hylobates moloch) in a submontane tropical rainforest

Altitude influences forest structure and food abundance and distribution, which in turn affect primate feeding and ranging patterns. Javan gibbons (Hylobates moloch) are endemic to forests spanning a broad range of altitudes on Java, Indonesia. Most information about Javan gibbon behavior comes from studies in lowland forests, while the vast majority of wild gibbons remaining inhabit hill and lower montane forests. We studied the diets, activity patterns, and ranging behavior of three gibbon groups in hill/lower montane (950-1,100?m asl) forest in the Gunung Halimun-Salak National Park (GHSNP) from April 2008 to March 2009. The mean home range size was 37?ha and the mean daily path length was 1,180?m. The study groups spent 36% of time feeding, 41% resting, 15% traveling, 6% engaging in social behavior, and 2% in aggressive interactions. Fruit was the most important food (63% of feeding time) followed by leaves (24%), and flowers (12%). Our results suggest that Javan gibbons in higher elevation habitats have substantially larger home ranges than lowland populations, despite broad similarity in their activity budgets and diets. Conservation managers should consider the effects of altitude and habitat quality on gibbon ranging behavior when developing habitat corridors, selecting sites for translocation or reintroduction projects, and designating and managing protected areas.     

Song Functions in Nonduetting Gibbons: Evidence from Playback Experiments on Javan Gibbons (<Emphasis Type="Italic">Hylobates moloch</Emphasis>)

Territorial, pair-living primates usually perform long-distance calls as duets in which adult males and females coordinate their calls. Previous studies using playback experiments have shown that gibbon duets convey information about the status of the caller (location, familiarity, sex of the caller, and paired status) and gibbons use this information to respond to achieve several nonmutually exclusive functions, including intragroup contact, territorial defense, and pair-bond advertisement and strengthening. However, not all pair-living gibbons duet, and it is unclear whether the same results should be expected in nonduetting species. We conducted song playback experiments (N = 47 trials) to test hypotheses about song functions in nonduetting gibbons on two groups of wild Javan gibbons (Hylobates moloch) in the Gunung Halimun-Salak National Park, Indonesia. Javan gibbons initiated movement toward the speaker more quickly in response to songs broadcast in the center of the territory, stranger songs, and songs of unpaired individuals than to songs at the border, neighbor songs, and songs from paired individuals. These results suggest that Javan gibbons can localize songs, and that Javan gibbon songs transmit information about the identity and paired status of the caller. Our results imply that Javan gibbon solo songs are likely to function for territorial defense and pair-bond advertisement like duets in other primates.     

Gibbons (Nomascus gabriellae) provide key seed dispersal for the Pacific walnut (Dracontomelon dao), in Asia's lowland tropical forest

Understanding the mutualisms between frugivores and plants is essential for developing successful forest management and conservation strategies, especially in tropical rainforests where the majority of plants are dispersed by animals. Gibbons are among the most effective seed dispersers in South East Asia's tropical forests, but are also one of the highly threatened arboreal mammals in the region. Here we studied the seed dispersal of the Pacific walnut (Dracontomelon dao), a canopy tree which produces fruit that are common in the diet of the endangered southern yellow-cheeked crested gibbon (Nomascus gabriellae). We found that gibbons were the most effective disperser for this species; they consumed approximately 45% of the fruit crop, which was four times more than that consumed by macaques – the only other legitimate disperser. Gibbons tracked the temporal (but not spatial) abundance of ripe fruits, indicating this fruit was a preferred species for the gibbon. Both gibbons and macaques dispersed the majority (>90%) of the seeds at least 20 m away from parent crowns, with mean dispersal distances by gibbons measuring 179.3 ± 98.0 m (range: 4–425 m). Seeds defecated by gibbons germinated quicker and at greater rates than seeds spat by macaques, or in undispersed fruits. Gibbon-dispersed seeds were also more likely to be removed by unknown seed predators or unknown secondary dispersers. Overall, gibbons play a key role in the regeneration of the Pacific walnut. Our findings have significant implications both for the management of the Pacific walnut tree dominating tropical rainforest as well as the reintroduction program of the Southern yellow-cheeked crested gibbon.     

Combining species distribution modeling and distance sampling to assess wildlife population size: A case study with the northern yellow-cheeked gibbon (Nomascus annamensis)

Population size and distribution data for wildlife species play an important role in conservation and management, especially for endangered species. However, scientists seriously lack data on the population status of many species. The northern yellow-cheeked gibbon (Nomascus annamensis) is found in southern Lao PDR, central Vietnam, and northeastern Cambodia. The population of the species has significantly declined due to hunting, habitat loss, and the wildlife trade. To examine the population size and distribution of N. annamensis, we conducted a field survey in Song Thanh Nature Reserve, Quang Nam Province, central Vietnam from February to April 2019 using the audio point count method. We combined Distance Sampling and Ecological Niche Modeling to estimate the population of the gibbons. Results showed that the total suitable area for the gibbons was about 302.32 km², with the two most important variables of the habitat model being the distance-to-villages and forest type. We detected 36 gibbon groups through field surveys and estimated 443 (95% CI, 278–707) gibbon groups in Song Thanh Nature Reserve. Our results indicate that the gibbon population in Song Thanh Nature Reserve is the largest known population of N. annamensis in Vietnam. In addition, our study was the first to combine species distribution modeling with distance sampling to estimate gibbon density and population size. This approach might be useful in surveying and monitoring gibbon populations because it takes imperfect detection probability into account in estimating gibbon population density while estimating the area of potential habitat using environmental variables.     

Sound Spectrum Characteristics of Songs of Hainan Gibbon (Nomascus hainanus)

Gibbons are characterized by their species-specific calls, or songs. There are few studies of songs of Hainan gibbon (Nomascus hainanus). To study the sound spectrum characteristics and test for intergroup differences in Hainan gibbon song, we studied the singing behavior of Hainan gibbons in Bawangling National Nature Reserve, Hainan Province, China, intermittently from August 2002 to February 2013, collecting 184 recordings. Our results show that: 1) Hainan gibbon song bouts occur mainly 0–4 h after dawn. 2) The songs of adult males living in groups are composed mainly of one to three short notes and one to five long notes, while solitary adult male songs consist only of long frequency modulated notes and no short or single notes. 3) The song chorus is dominated by adult males, while females add a great call. Males do not have a great call, unlike those in other gibbon species. There are no female solos. 4) The sound spectrum frequency is similar in adult males living in two different groups, but the duration of the first long note differed significantly between the groups. The sonic frequencies of male and female songs are lower than those of other gibbons: no more than 2 kHz. Hainan gibbon sound structure is simple, although females participate in the chorus, reflecting their primitive status among gibbon species.     

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.