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 Unpublished documents dating from 1925 indicate, however, that sympatry between these two species may also have existed in the region of Sriracha, about 80 km southeast of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.     

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.     

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.     

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.     

Preliminary observations of wild black-crested gibbons (Hylobates concolor concolor) in Yunnan province,people's republic of China

Black-crested gibbons (Hylobates concolor concolor), which inhabit the subtropical forests of southern China and northern Vietnam, have until now never been the subject of any systematic behavioral or ecological field study. This report presents the findings of a three-month field study of wild black-crested gibbons in the Wuliang and Ailao Mountain Game Reserves in Yunnan Province, People's Republic of China. The sites visited consisted of temperate, subtropical, broadleaf evergreen forests, with trees belonging to the families Elaeocarpaceae, Fagaceae, Lauraceae, Magnoliaceae, and Theaceae. Black-crested gibbons were documented from four sites visited and were found to be polygynous, with an average group size of seven to eight animals. The family groups were observed to comprise one adult male, from one to four adult females, and numerous offspring of various ages. Both sexes of this species emit interactive songs, and various features of this loud vocal behavior were analyzed and are discussed here. Since all other gibbon species were found to inhabit only tropical rain forests and live in strictly monogamous family groups, some of the ecological and evolutionary implications of these findings are also presented.     

Coexistence of monogamy and polygyny in black-crested gibbon (Hylobates concolor)

All gibbons are monogamous and territorial with the exception ofHylobates concolor. This paper reports the coexistence of monogamy and polygyny in black-crested gibbons. Based on the fact of two adult females and two offspring of the same age category in one group and other reasons, we suppose that the two adult females have bred in a single group, i.e. a polygynous one. The other main reasons are: (1) a large home range makes it possible for more individuals to live in one group; (2) mutual tolerance among two females; and (3) selection pressure favouring polygyny.     

Vocal Diversity and Taxonomy of Nomascus in Cambodia

It is usually thought that Nomascus gabriellae is the only Nomascus sp. in Cambodia. We studied vocal diversity among different wild populations of Nomascus in Cambodia to assess their taxonomic relationships and to examine whether their vocal patterns correspond to forms previously described for Nomascus leucogenys siki and Nomascus gabriellae. We tape-recorded crested gibbon songs in southern Mondulkiri Province, in central Rattanakiri Province, and in 2 different districts of the Virachey National Park in northern Rattanakiri. We also tape-recorded typical songs of Nomascus leucogenys siki near the type locality of the taxon in the Bach Ma National Park in central Vietnam. We analyzed 40 song bouts from different gibbon groups, including 151 phrases of 33 females and 235 phrases of 39 males. Discriminant analyses revealed marked separation of the most southern songs (Mondulkiri) from those of all other localities. Vocal differences among the 3 gibbon populations in northeast Cambodia (Rattanakiri) are less pronounced; they do not differ more among each other than each of them differs from the northernmost sample from Bach Ma (Vietnam). Vocal characteristics of the study populations revealed no cline, and populations do not differ significantly in vocal variability. We conclude that Cambodian crested gibbons represent 2 distinct taxa: Those from southern Mondulkiri are Nomascus gabriellae, those from northeast Cambodia (Rattanakiri) closely correspond to the sample from Bach Ma (Vietnam) and, together with the latter, represent a different taxon. We provisionally assign them to Nomascus leucogenys siki because of the close geographic distance between Bach Ma and the type locality of the taxon. We postulate that a taxonal boundary exists between southern Mondulkiri and central Rattanakiri and discuss the possible factors that may have acted as distribution barriers.     

Molecular phylogeny of the major hylobatid divisions

We describe DNA sequences for the mitochondrial control region and phenylalanine-tRNA from the four extant gibbon subgenera. In contrast to earlier studies on gibbon phylogeny that used other parts of the mtDNA, the control region depicts the crested gibbons (Nomascus) as the most basal group of the Hylobatidae, followed by Symphalangus, with Bunopithecus and Hylobates as the last to diverge. Our data show that the molecular distances among the four gibbon subgenera are in the same range as those between Homo and Pan, or even higher. As a consequence of these findings, we propose to raise all four gibbon subgenera to genus rank.     

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.     

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.     

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.     

Intragroup and Intergroup Social Interactions in White-Handed Gibbons

One of the least studied aspects of the Hylobatidae is their social behavior. In the past, researchers have attributed this to a general lack of affiliative social interactions due to small group size and the hostile nature of intergroup encounters. Data collected on white-handed gibbons (Hylobates lar) in Khao Yai National Park, Thailand, indicate that gibbons engage in higher rates of social behavior than had previously been appreciated. In addition to relatively high rates of intragroup social contact, gibbons engaged in social behavior, including play and grooming, with members of neighboring groups. These findings obligate a renewed consideration of the nature and function of territoriality in primates.     

Hypertrichosis in a gibbon (Hylobates muelleri)

A case of facial hypertrichosis in a museum specimen of a Bornean gibbon (Hylobates muelleri) is described. Apparently, hypertrichosis has not previously been reported to occur in nonhuman primates. Similar pathological conditions are known to occur in humans, but it is unknown whether any of the numerous forms of human hypertrichosis is equivalent to this case in gibbons.     

Longitudinal patterns of reproduction in wild female siamang (Hylobates syndactylus) and white-handed gibbons (Hylobates lar)

I present the 6- year reproductive histories of three wild female siamang (Hylobates syndactylus)and four white-handed gibbons (Hylobates lar)at the Ketambe Research Station (Sumatra, Indonesia). Reproductive output varied considerably among females. Two females failed to gestate: both were nulliparous young adult H. lar,one of which remained unpaired for 4 years after dispersing from her group, while the other lost her recently acquired mate to another female. Only one- (a white-handed gibbon)- gave birth more than once, yielding interbirth intervals of 22 and 31 months. Pair bond stability or reduced interspecific feeding competition or both factors may have contributed to the brevity of these intervals. The other females- one H. lar,and three H. syndactylus-each gave birth once, suggesting minimum interbirth intervals exceeding 4–5 years (H. lar)and 3 years (H. syndactylus)in these individuals. Even given the pronounced variation observed among H. lar,these data suggest that interbirth intervals may often exceed the 2- to 3- year interval commonly attributed to these two species. Sources of reproductive failure were 1) maternal abandonment of the neonate due to impaired ability to provide maternal care (H. syndactylus,),(2) premature or stillbirth (H. syndactylus,),and (3) pregnancy termination (H. lar).These data and a review of information on longevity and age at menarche suggest that the actual lifetime reproductive output of a siamang or white-handed gibbon female may often fall far short of the 10 offspring/lifetime originally proposed for these species. Indeed, females may rear as few as five offspring to weaning in a lifetime, which is a figure reminiscent of the reproductive potential of some pongids. Finally, variance in female reproductive success is higher than expected in these monogamous species, which suggests that females (and males) are under strong selective pressure to exert mate choice, possibly through acquisition of (new) mates and extrapair copulations. Future research must clarify the availability of opportunities for paired adults to engage in these sociosexual behaviors.     

Gibbon foraging decisions and the marginal value model

We use data from an observational field study of frugivory in two sympatric gibbons, lar (Hylobates lar) and siamang (H. syndactylus), to test assumptions and predictions of the marginal value model (MVM). A key prediction of the MVM is that marginal gain rates at the time of leaving the patch are equal across patch types. We found that this is not the case for gibbons: rates of energy intake at the end of feeding sessions were significantly different for different types of fruit, and we could not attribute this to temporal variation in fruit availability. Initial and final caloric intake rates were highly correlated. This suggests that gibbons do not adjust the time spent in patches in order to maximize the average rate of energy intake. Similar results were obtained for all other currencies considered. Gibbon foraging appears to satisfy several, but not all, assumptions of the MVM. As required by the model, fruit patches occur as discrete units, patches are encountered sequentially, travel time between patches exceeds search time between items within a patch, search for and search within patches are incompatible activities, and intake rates decline over time spent in a patch. However, the declining rates we detected may be an effect of satiation instead of patch depletion, patches probably are not encountered at random, and group members may not forage independently. Thus, our results suggest that the MVM is not an adequate model of gibbon foraging behavior, but they do not invalidate the MVM per se.     

Preliminary survey on the population and distribution of gibbons in yunnan province

Of the gibbons in China,Hylobates hoolock, H. lar, andH. concolor leucogenys are distributed only in Yunnan Province, whileH. c. concolor occurs limitedly in Yunnan Province and Hainan Island. During the periods from February to November 1984, and March to August 1985, the authors undertook surveys of the population and distribution of gibbons in 56 counties of 10 prefectures in Yunnan Province, and obtained the first reliable population and distribution data on the gibbons of this Province. The findings serve to fill an important blank in gibbon research in Yunnan and should pave the way for further research in the future.     

Ecology and behavior of wild black-crested gibbons (Hylobates concolor) in China with a reconsideration of evidence for polygyny

The black-crested gibbon,Hylobates concolor, is one of the few species of gibbons that has not yet been the subject of a long term field study. Field observations in the Ai Lao and Wu Liang Mountains of Yunnan Province, China indicate that in this area the habitat and ecology of this species differ markedly from those of other gibbons that have been studied to date. These differences are correlated with some behavioral differences. In particular, these gibbons apparently have greater day ranges than other gibbons. It has also been suggested that this species lives in polygynous groups. To demonstrate this requires observation of groups with two or more females with young. Our own observations and those from other recent studies suggest that there are alternative explanations consistent with available data.     

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.     

Demography of Agile Gibbons (<Emphasis Type="Italic">Hylobates agilis</Emphasis>) in a Lowland Tropical Rain Forest of Southern Sumatra,Indonesia: Problems in Paradise

Gibbons are among the best-studied Asian primates, but few studies address their demography and life history strategies. We used annual censuses to study the demography of agile gibbons (Hylobates agilis) between 1998 and 2009 in rain forests of Bukit Barisan Selatan National Park, Indonesia. The population declined from 22 individuals (9 groups) to 14 individuals (5 groups) over the 12 yr of study. Infant survival to the juvenile age class was 33.3%, and 16.7% of infants survived to the subadult age class. The interbirth interval was 3.83 ± 1.15 yr and birth rate was 0.22–0.28 infants female^–1 yr^–1. Two groups colonized the study area but subsequently disappeared. We documented 7 immigrations, 17 disappearances, and ≥10 transients in the population. Compared to lar gibbons (Hylobates lar) and Bornean white-bearded gibbons (Hylobates albibarbis), Way Canguk’s agile gibbon population is characterized by slow reproduction, low survival, and high group turnover. We hypothesize that, although the habitat is high in fruit resources, agile gibbons may be displaced or excluded from the best fruit resources by larger and more numerous competitors, incurring costs of decreased opportunities to forage and increased travel, and leading to higher mortality for young agile gibbons. The reproductive potential of this agile gibbon population is insufficient to compensate for high mortality, and the population is unlikely to persist without immigration from outside the area. Given the agile gibbons’ endangered status and limited capacity to respond demographically to change, it is likely that intensive management interventions will be required to conserve this species.     

Biology and behaviour of reintroduced gibbons

We observed and recorded the behaviours of gibbons undergoing rehabilitation, before and after release, to document the behavioural and social changes of gibbons in the rehabilitation program and develop criteria for determining the suitability of a pair of gibbons for release. Hylobates albibarbis were observed at the Kalaweit Gibbon Rehabilitation Project in Kalimantan, Indonesia. Data were collected on animals both pre- and post-release and on wild gibbons for comparison. Data presented here show that reintroduced gibbons are capable of surviving without human intervention. In addition, their behaviour is similar to that of wild gibbons in terms of activity budgets, position in the canopy, body posture, pair association (PA) and diet. Prior to this study, no attempt has been made to quantify the rehabilitation process for gibbons, and rehabilitation project personnel require data reporting all aspects of a release so that improvements can be made. It is important to report these data for the benefit of future releases. Criteria, based on the behaviour of wild gibbons, are proposed to assist rehabilitation centers in assessing the suitability of gibbon pairs for release.