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 Unpublished documents dating from 1925 indicate, however, that sympatry between these two species may also have existed in the region of Sriracha, about 80 km southeast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

Songs of hybrid gibbons (Hylobates lar × H. muelleri)

Two hybrid offspring, one male and one female, were produced by a Hylobates muelleri female mated to a Hylobates lar male at Micke Grove Zoo, Lodi, California. Songs of the hybrids were studied at adulthood and compared to the parental-type songs. The hybrid female song is uniquely different from either parental type. The hybrid male song resembles the male song of H. lar, but contains an element that may be unique to H. muelleri. The study demonstrates the utility of sound-spectographic analysis of hybrid vocalizations to further understanding of primate behavioral inheritance.     

Food sharing among captive gibbons (Hylobates lar)

A captive family group of gibbons engages in food sharing during consistently patterned sequences of behaviors in which begging gestures are employed. The predominant occurrence of the behavior involves the juvenile female begging from her older, adult sister who acted as her “surrogate mother”. An examination of the variables potentially affecting the behavior, such as hunger, the availability and accessibility of preferred foods, the inability to forage individually, and the social relationships between members of the family, indicates that food sharing may assist the young in acquiring appropriate food habits, supplement their foraging capabilities, and may serve to reinforce the social bonds between adult and immature members of the family group.     

Linear enamel hypoplasia in gibbons (Hylobates lar carpenteri)

This study describes the expression of linear enamel hypoplasia (LEH), a sensitive dental indicator of physiological stress, in Thailand gibbons (Hylobates lar carpenteri). Previous studies of enamel hypoplasia in hominoids have focused on great apes, with little attention given to the expression of this stress indicator in gibbons. In that gibbons differ from both monkeys and great apes in numerous life history features, LEH expression in gibbons might be expected to show significant differences from both. In this study, 92 gibbon specimens from two sites in Thailand were compared with several samples of monkeys and great apes in their expression of LEH. The intertooth distribution of LEH in gibbons was compared to that of chimpanzees and rhesus monkeys. Gibbon populations from both sites exhibit LEH frequencies intermediate between those of the monkey samples, in which LEH prevalence is usually low, and those of the great ape samples, in which LEH prevalence is high. Gibbons differ significantly from monkeys, but not great apes, in the number of individuals whose teeth record multiple stress events. Multiple episodes of stress are rarely recorded in the teeth of monkeys, while multiple stress events occur with higher frequency in gibbons and great apes. Taxonomic variation in the duration of crown formation, the prominence and spacing of perikymata on dental crowns, life history features, and/or experience of physiological stress may explain these patterns. The intertooth distribution of LEH in gibbons is, for different reasons, unlike that of either chimpanzees or rhesus monkeys. The mandibular canines of gibbons have significantly more LEH than any of their other teeth. Aspects of crown morphology, perikymata prominence/spacing, enamel thickness, and crown formation spans are potential causes of taxonomic variation in the intertooth distribution of LEH.     

Homosexual behaviour in wild white-handed gibbons (Hylobates lar)

Homosexual mounting in apes and prosimians is rare. Male-male mounting was observed between an adult male white-handed gibbon and an adolescent male in the same group. The behaviour is discussed in terms of the gibbon group's social structure and the development of the adolescent. It is suggested that this behaviour provides reassurance for the adolescent gibbon.     

Grooming site preferences in wild white-handed gibbons (Hylobates lar)

During a four-month study of wild white-handed gibbons in Thailand one group was observed for 131 hr. The individuals spent 5.2% of their activity period allogrooming. Several body sites received more respectively less allogrooming than expected. Surface areas easy to clean by autogrooming such as the belly received significantly less allogrooming than expected. Upper body areas which are likely to be infested by parasites and other matter received significantly more allogrooming than lower body parts. Thus, grooming in gibbons seems to reflect a primarily hygienic function.     

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.     

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.     

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.     

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.     

Census and survey of wild black-crested gibbons (Hylobates concolor concolor) in Yunnan Province, People's Republic of China

Black-crested gibbons (Hylobates concolor concolor) inhabit the subtropical forests of Southern China and Northern Vietnam, and have never previously been the subject of any systematic behavioral or ecological study. This report presents the findings of a three-month census and survey of black-crested gibbons in the Wuliang and Ailao Mountain Game Reserves in Yunnan province, China. The censusing methods used here were similar to those techniques used during other census studies of gibbons. The sites visited were subtropical broadleaf evergreen forests, with trees belonging to the families Fagaceae, Theaceae, Magnoliaceae, Lauraceae and Elaeocarpaceae. A total of 23-25 groups of black-crested gibbons were documented from 4 sites visited, and a group density estimate from all sites averaged 0.82 groups/km2. These gibbons were found to be polygynous with an average family group size of 7-8 animals, comprising 1 adult male, 1-4 adult females and numerous offspring of various ages. Some of the ecological and evolutionary implications of these findings are also presented.     

Sterile pyuria in a population of wild white‐handed gibbons (Hylobates lar)

Urinalysis is an emerging method for monitoring the health and energy balance of wild primates. Here, we report the first urinalysis of wild gibbons. We used multi‐reagent test strips to monitor the health status of 52 individual white‐handed gibbons (Hylobates lar) inhabiting Khao Yai National Park, Thailand. Most urinary reference values were within normal ranges; however, regardless of age‐ and sex‐class or monthly fruit productivity, we found unexpectedly high rates of urinary leukocytes (50% and 90% of individuals in 2001–2003 and 2006, respectively). In contrast to previous studies of African apes, this finding is coupled with the near absence of urinary nitrites, demonstrating pervasive levels of sterile pyuria. This result is the first reported case of sterile pyuria in a population of wild primates. The etiology of human sterile pyuria is diverse, but in all cases it is diagnostic of systemic inflammation. We discuss the potential causes of sterile pyuria in the gibbons of Khao Yai. Am. J. Primatol. 71:880–883, 2009. © 2009 Wiley‐Liss, Inc.     

Cross-species amplification of human microsatellite markers using noninvasive samples from white-handed gibbons (Hylobates lar)

Analysis of the population genetic structure and reproductive strategies of various primate species has been facilitated by cross-species amplification (i.e., the use of microsatellite markers developed in one species for analysis of another). In this study we screened 47 human-derived markers to assess their utility in the white-handed gibbon (Hylobates lar). Only eight produced accurate, reliable results, and exhibited levels of polymorphism that were adequate for individual identification. This low success rate was surprising given that human microsatellite markers typically work well in species (such as macaques) that are evolutionarily more distant from humans than are gibbons. In addition, we experienced limited success in using a set of microsatellite markers that have been reported to be useful in the closely-related H. muelleri, and applying our set of microsatellite markers to samples obtained from one H. pileatus individual. Our results emphasize the importance of extensively screening potential markers in representatives of the population of interest.     

Genetic analysis of hybridization between white-handed (Hylobates lar) and pileated (Hylobates pileatus) gibbons in a contact zone in Khao Yai National Park,Thailand

Primates - Natural hybridization has played various roles in the evolutionary history of primates. Its consequences range from genetic introgression between taxa, formation of hybrid zones, and...     

Field playback of loud calls to gibbons (Hylobates lar): territorial,sex-specific and species-specific responses

We present the results of an experimental study of territoriality among monogamous lar gibbons (Hylobates lar), scoring vocal and movement responses to field playback of their loud calls. Playback of neighbouring groups' male-female duets on territory borders, and of these duets and music controls in territory centres, indicated a general territorial response. Responses to playback of a strange male's solo and of a strange female's solo in territory centres indicated that resident males would evict deeply intruding males and pairs but not necessarily females, and that resident females would evict deeply intruding females but probably not males or pairs. Vocal responses also implied that, although males carry the burden of routine territory defence in border disputes involving chases and special calls, the female part of the daily duet also contributes to routine exclusion of outside females. Responses of lar females to playback of the solo of a female capped gibbon (H. pileatus) suggested that the females of one species cannot sex the calls of females of the other species. This could partly account for the existence of bigamous mixed-species groups at the interface between the two species. This result also implies that female calls contribute to routine exclusion of conspecific females, and thus to the maintenance of monogamy.     

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.     

Karyotype of the gibbons hylobates lar and h. moloch inversion in chromosome 7.

A karyotype of the gibbon, Hylobates, has been prepared based on the chromosome banding patterns produced by quinacrine, trypsin-Giemsa, and centromeric heterochromatin stains. The banding patterns of H. lar and H. moloch are virtually identical. No brilliant quinacrine-fluorescent areas are present. The banding pattern of most of the gibbon chromosomes show less resemblance to those of the human, chimpanzee, gorilla, or orangutan than the chromosomes of the higher primates do to each other, suggesting a relatively large evolutionary separation of the gibbon from the higher primates. A pericentric inversion of chromosome 7 is present in one gibbon.     

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.