Masturbation in captive Hylobates (gibbons)

Masturbation was observed in 9 of 52 captive adult Hylobates (gibbons) over a period of 17 years at the International Center for Gibbon Studies (ICGS). Four females and 5 males masturbated with varying degrees of regularity. The behavior may be related to a disruption in early rearing experiences of Hylobates. This is the first report on masturbation in these animals and documents that this behavior occurs infrequently in captive Hylobates. © 1994 Wiley-Liss, Inc.     

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.     

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.     

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.     

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.     

Siamangs (Hylobates syndactylus) and White-Cheeked Gibbons (Hylobates leucogenys) Show Few Behavioral Differences Related to Zoo Attendance

The effect of visitors on behavior and welfare of nonhuman animals in the zoo has been an active research topic during the last few decades. Although research has variously shown negative or positive impacts of zoo visitors on animals in captivity, previous primate research at Disney's Animal Kingdom® suggests the importance of visual barriers in allowing animals to cope with large crowds. Examining this further, this study monitored the behavior of white-cheeked gibbons (Hylobates leucogenys) and siamangs (Hylobates syndactylus) in large, open exhibits. Behavioral data showed rates of social behavior and percentage of time engaged in solitary behavior did not differ between low and high visitor-attendance days. Both gibbons and siamangs spent more time in areas away from the public on high attendance days. Supporting previous findings, results imply visual barriers and ability to retreat from crowds may have provided these animals with choice and minimized potential negative visitor impact. Future research should focus on the relationship between attendance and actual crowds at exhibits; it should utilize multi-institutional methodologies to control for variance and look for individual and demographic differences between individuals.     

Extra-pair Copulations in a Monogamous Gibbon (Hylobates lar)

This study, conducted in a rainforest in Thailand, presents the first evidence for extra-pair copulations in white-handed gibbons - Asian apes that live in monogamous groups. The ratio of in-pair copulations versus extra-pair copulations as observed among three free-ranging, well-habituated groups was 88 versus 12 % and involved one adult paired female and three adult paired males. Extra-pair copulations may be explained as an effort to breed with a partner of superior quality to the current mate and/or may be part of a strategy to forestall infanticide.     

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.     

Long-Distance Vocal Signaling in White-Handed Gibbons (Hylobates lar)

International Journal of Primatology - Few primates are characterized by strict territoriality, pair-bonding, and loud, complex vocalizations. Gibbons (Hylobatidae) are amongst them, with mated...     

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.     

Behavioural development of twin siamangs (Hylobates syndactylus)

Multiple births are very rare among gibbons. The birth of siamang twins at the Zürich Zoo in 1992 therefore presented a valuable opportunity to observe the development of the twins and to contrast it with a survey of previous reports on the development of single offspring of siamang and gibbons of thelar group. Furthermore, the hypothesis that the presence of twins among siamang may facilitate the occurrence of helping behaviour (defined as the care of offspring by individuals other than their parents) was re-examined (Dielentheis et al., 1991). The Zürich twins (one male and one female) were observed for a total of 74 hr during their first year of life. The results show that: (1) The twins exhibited more rapid behavioural development than that reported for single offspring. (2) A clear difference between the twins was observed: the female twin developed more rapidly than the male. (3) Neither the twins' father nor their older sisterRama was ever observed carrying the twins. The hypothesis ofDielentheis et al. (1991) is hence not supported by the present study, although it is possible that the older sisterRama did not carry the twins because she was younger than the juvenile in that study. (4) Siamangs may have a longer maturation period than gibbons of thelar group.     

Female reproductive parameters in the Javan gibbon (Hylobates moloch)

Javan gibbons (Hylobates moloch) are one of the most endangered gibbon species in the world. Data on the reproductive biology of the species are almost nonexistent, and a general understanding of the female reproductive biology of this species is important for both ex situ and in situ conservation. Using 18 years of data from 11 captive individuals, we provide new information on the reproductive biology of Javan gibbons based on sexual swelling and menstrual bleeding, including reproductive development, interbirth intervals, and ovarian cycle lengths. Menarche and the onset of sexual swelling occurred at 6.2 and 6.5 years respectively, followed by a period of adolescent sterility of about 1.5 years. Average age at first birth was 8.8 years, and interbirth intervals were about 2.3 years, decreasing to 1.0 year during cases of infant mortality at or shortly after birth. Ovarian cyclicity was measured through periods between menstrual bleeding and sexual swelling. Menstrual bleeding indicates the start of a new ovarian cycle, while sexual swelling normally occurs near the time of ovulation. Menstrual bleeding intervals gave a cycle length of 25.6 days, while sexual swelling intervals gave a cycle length of 27.3 days. These both correspond closely to cycle lengths in other gibbon species, as well as hormonal studies in Javan gibbons. In particular, observing the presence/absence of swellings was found to be a useful and easy method to monitor female ovarian cycles, and could be a practical noninvasive technique for caretakers and researchers. Zoo Biol 29:449–456, 2010. © 2009 Wiley‐Liss, Inc.     

Selection of sleeping trees in pileated gibbons (Hylobates pileatus)

Selection and use patterns of sleeping sites in nonhuman primates are suggested to have multiple functions, such as predation avoidance, but they might be further affected by range defense as well as foraging constraints or other factors. Here, we investigate sleeping tree selection by the male and female members of one group of pileated gibbons (Hylobates pileatus) at Khao Ang Rue Nai Wildlife Sanctuary, Thailand. Data were collected on 113 nights, between September 2006 and January 2009, yielding data on 201 sleeping tree choices (107 by the female and 94 by the male) and on the characteristics of 71 individual sleeping trees. Each sleeping tree and all trees ≥40 cm diameter at breast height (DBH) in the home range were assessed (height, DBH, canopy structure, liana load) and mapped using a GPS. The gibbons preferentially selected tall (mean=38.5 m), emergent trees without lianas. The majority of the sleeping trees (53.5%) were used only once and consecutive reuse was rare (9.5%). Sleeping trees were closer to the last feeding tree of the evening than to the first feeding tree in the morning, and sleeping trees were located in the overlap areas with neighbors less often than expected based on time spent in these areas. These results suggest avoidance of predators as the main factor influencing sleeping tree selection in pileated gibbons. However, other non‐mutually exclusive factors may be involved as well. Am. J. Primatol. 72:617–625, 2010. © 2010 Wiley‐Liss, Inc.     

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.     

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.     

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.