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.     

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.     

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.     

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.     

Neonatal weight in gibbons (Hylobates spp.)

Neonatal and birth weights of gibbons have mostly been reported for single individuals, and larger samples (n = 2–8) have apparently been published for only two species of gibbons (Hylobates lar and H. syndactylus). In addition, a critical examination of the few published neonatal weights of gibbons shows that several of them should not be used. Neonatal weights are here defined as weights taken on infants up to seven days old, whereas birth weights include only those taken on the day of birth. This paper presents neonatal weights for six representative species of gibbons (H. lar, H. leucogenys, H. moloch, H. muelleri, H. pileatus, H. syndactylus) and some of their hybrids. Most of our data stem from surviving animals that were subsequently hand-reared and include 80 infants, thus making the previously available dataset 5 times larger. Our neonatal weights fall roughly into three different classes: neonates of the lar group (about 390 g, n = 27), the concolor group (about 510 g, n = 7), and the siamang (about 540 g, n = 46). This grouping corresponds not only to taxonomic units within the hylobatids, but also to grouping of gibbons by adult body weight. No weight difference between males and females is evident in our sample, and hybrids of the lar group do not appear to differ in weight from pure species. True birth weights (i.e., weights recorded on the day of birth) are available for only a few individuals. These weights are, on average, 7% higher than neonatal weights, but the difference is not statistically significant. Additional samples of neonatal weights suggest that infants that die on the day of birth weigh, on average, 17% less, twins weigh 29% less, and infants born by Cesarean section weigh 19% more than our reference sample of neonates. © 1995 Wiley-Liss, Inc.     

Sociosexual behavior of captive sichuan golden monkeys (Rhinopithecus roxellana)

The Sichuan golden snub-nosed monkey, Rhinopithecus roxellana, is an endangered Colobine endemic to China. The social and reproductive behavior of this species are little known, and it has rarely been kept in captivity outside China. Results from observations of a captive golden monkey pair revealed some unusual details of sociosexual behavior. Both members of the pair mounted one other, and female mounting of the male occurred nearly as often as the reverse. Male mounts were always preceded by a stereotyped behavioral sequence that included distinct facial and vocal displays. Female mounts occurred in response to male solicitation (presenting). No evidence of ejaculation was observed in association with mounts, although mounts were only observed in a distinctly sexual context. Sexual behavior was associated with aggressive displays by one or both animals and with apparently proceptive behavior by the female. All sexual behaviors showed a peak near the end of the study, although the female was continually receptive throughout the 2-month study period.     

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.     

Human-type blood groups and Gm factors in gibbons (Hylobates)

Blood specimens from 69 gibbons (63Hylobates lar, 4Hylobates concolor, and 2Hylobates pileatus) were tested for human-type ABO, MN, and Rh blood groups. AmongH. lar, three phenotypes were noted in the ABO and MN blood groups respectively, but all fourH. concolor were grouped as AM. All group A gibbons were of subgroup A₁; subgroups A₂B and A₁₂B were observed at a low frequency in group AB gibbons. Le^b antigen was detected in about 30% of the red cell samples fromH. lar, but all the samples were negative for Le^a. All the gibbons tested had c(hr) antigen but no other Rh antigens (D, C, E, and e) in their red cells. Some selected blood samples fromH. lar were also tested for some other blood group antigens and for the Gm and Inv factors. The Jk^a antigen was detected in all the red cell samples tested, but the S, s, U, K, k, and Fy^a antigens were not. In the tests of plasma with anti-Gm (1),H. lar could be divided into two groups, i.e., Gm(1)^Gi and Gm(–1)^Gi; Gm(2), Gm(4), and Inv(1) were absent in the 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.     

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.     

Parental care in captive siamangs (Hylobates syndactylus)

Siamangs exhibit paternal care to the extent that the male of a monogamous unit carries its infant offspring beginning late in the infant's first year of life. Field studies have documented this but without behavioral detail. It has been hypothesized that the transfer is facilitated by a desertion of the infant by the female. An infant siamang born in captivity at the Washington Park Zoo, Portland, Oregon, was observed through its first year of life. The infant transfer to the male was documented in captivity and was associated with a high rate of infant-initiated contact with the male and a high rate of infant retrieval by the female. The study concludes that the infant probably played a key role in facilitating the transfer to the male.     

Hepadnavirus infection in captive gibbons

The recent isolation of a nonhuman primate hepadnavirus from woolly monkeys prompted an examination of other primates for potentially new hepadnaviruses. A serological analysis of 30 captive gibbons revealed that 47% were positive for at least one marker of ongoing or previous infection with a hepatitis B virus (HBV). The amino acid sequences of the core and surface genes of human and gibbon virus isolates were very similar. Phylogenetic analysis indicated that the gibbon isolates lie within the human HBV family, indicating that these HBV isolates most likely stem from infection of gibbons from a human source.     

Population dynamics of hoolock gibbons (Hylobates hoolock) in Assam,India

Data are presented on group size, composition, and reproduction of several hoolock gibbon groups studied in different areas of Assam, India, between 1986 and 1988. Presence of fewer young than reported in earlier studies signals a grim future for these gibbons unless strict conservation measures are undertaken.     

Protective and territorial behavior in captive siamangs (Hylobates syndactylus)

Protective and territorial behavior was observed in 14 heterosexual pairs of adult siamangs in 11 zoos for a total of 1,155 h. The study shows that the quality of protective and territorial behavioral patterns was similar in the wild and in captivity. Under zoo conditions, the behavioral response (except calling) to conspecific and human rivals was similar. Males could be found more often at the front of the enclosure and were more active in protective and territorial behavior than were females. Males were more attentive to happenings outside their enclosures than were their mates, whereas females concentrated their protective and territorial activities on specific people or females of a conspecific group. Although the duration of each song and the average number of duet sequences as well as the temporal distribution of calling throughout the day were similar in the wild and in the zoo, the total duetting rate differed remarkably: it was much higher in captivity. Siamangs in acoustical and visual contact with neighboring conspecifics spent more time singing than did siamangs without such contacts. In captivity, pairs without young seemed to be more engaged in protective and territorial behavior patterns than were parents. Zoo Biol 16:309–325, 1997. © 1997 Wiley-Liss, Inc.     

Visual co-orientation and expectations about attentional orientation in pileated gibbons (Hylobates pileatus)

This study investigated attentional processes in a sample of captive gibbons. An initial aim of the research was to examine subjects' ability to co-orient with photographic images of both conspecific and human models. The gibbons' expectancies about the focus of another's attention was then also assessed, with an expectancy violation paradigm revealing subjects' sensitivity to an incompatibility between visual orientation and the position of a target object. The gibbons were exposed to two conditions; consistent sequences in which the stimulus individual directed attention towards a target object, and inconsistent sequences in which the model's attentional focus was incompatible with the location of this article. Analyses of the subjects' responses were made according to the direction of gazes and the time spent inspecting the depicted model in each of these conditions. The results reveal a tendency for visual co-orientation with both conspecific and human models, suggesting that gibbons are competent in detecting the visual orientation of other species as well as their own. Furthermore, the subjects' tendency to look longer and check back to the depicted model in response to violations in the relationship between an agent and object (target appearing in an opposite direction to model's gaze), suggests that they possess some knowledge of how visual gaze direction relates to external stimuli.     

Plasma insulin-like growth factor-I,testosterone and morphological changes in the growth of captive agile gibbons ( Hylobates agilis) from birth to adolescence

We examined growth changes in concentrations of plasma insulin-like growth factor-1 (IGF-1) and testosterone, and somatometric parameters in two captive male agile gibbons from birth to about 4 years of age, to examine the evolution of growth patterns in primates. Plasma IGF-1 concentrations in agile gibbons generally increased with age with values ranging from 200 to 1,100 ng/ml. The growth profiles in plasma IGF-1 in the gibbons were similar to those reported for chimpanzees. The highest concentrations of plasma testosterone (230 and 296 ng/dl) were observed within the first 0.3 years from birth, then the concentrations rapidly decreased and fluctuated below 100 ng/dl. Continuously higher IGF-1 concentrations were observed after 2.6 and 3.5 years of age. The profiles of plasma testosterone in these gibbons also resembled those of other primates including humans. However, their plasma testosterone levels in both neonate and adult stages (60 ng/dl) were lower than those reported for macaques and chimpanzees of respective stages. The obtained growth profiles of plasma IGF-1 and testosterone suggest that the adolescent phase starts around 2.6 or 3.5 years of age in male agile gibbons. The growth trend in many morphological parameters including body weight showed a linear increase without a significant growth spurt at approximately the onset of puberty. Head length and first digit length had reached a plateau during the study period. Brachial index, which indicates the relative length of forearm to upper arm, significantly increased gradually through the growth period. This result indicates that forearm becomes relatively longer than the upper arm with growth, which may be an evolutionary adaptation for brachiation.     

Individuality in the songs of wild agile gibbons (Hylobates agilis) of Peninsular Malaysia

Recordings were made and analyzed of the female dominated duet songs by eight adult pairs of wild agile gibbons from two sites in Peninsular Malaysia. A statistical analysis of their songs revealed that individuality occurred throughout the females' songs from both sites. Individuality in gibbon songs may allow singing individuals to more efficiently and effectively locate and identify neighboring nonmate conspecifics.     

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.