Hylobates Lar Research Articles

Context-specific close-range "hoo" calls in wild gibbons (Hylobates lar).

BackgroundClose range calls are produced by many animals during intra-specific interactions, such as during home range defence, playing, begging for food, and directing others. In this study, we investigated the most common close range vocalisation of lar gibbons (Hylobates lar), the ‘hoo’ call. Gibbons and siamangs (family Hylobatidae) are known for their conspicuous and elaborate songs, while quieter, close range vocalisations have received almost no empirical attention, perhaps due to the difficult observation conditions in their natural forest habitats.ResultsWe found that ‘hoo’ calls were emitted by both sexes in a variety of contexts, including feeding, separation from group members, encountering predators, interacting with neighbours, or as part of duet songs by the mated pair. Acoustic analyses revealed that ‘hoo’ calls varied in a number of spectral parameters as a function of the different contexts. Males’ and females’ ‘hoo’ calls showed similar variation in these context-specific parameter differences, although there were also consistent sex differences in frequency across contexts.ConclusionsOur study provides evidence that lar gibbons are able to generate significant, context-dependent acoustic variation within their main social call, which potentially allows recipients to make inferences about the external events experienced by the caller. Communicating about different events by producing subtle acoustic variation within some call types appears to be a general feature of primate communication, which can increase the expressive power of vocal signals within the constraints of limited vocal tract flexibility that is typical for all non-human primates. In this sense, this study is of direct relevance for the on-going debate about the nature and origins of vocally-based referential communication and the evolution of human speech.

Sexual Dimorphism in The Birth Canal and Non‐Obstetric Pelvis among Anthropoids: Effects of Cephalopelvic Proportions

As important as birth is hypothesized to be in shaping the primate pelvis, we actually know very little about how birth affects pelvic form across anthropoids. This study tests whether large cephalopelvic proportions are related to dimorphism in the obstetric and non‐obstetric pelvis. Landmark data for 118 anthropoid specimens were analyzed [Large cephalopelvic proportions: Ateles geoffroyi (n=4), Homo sapiens (n=40), Hylobates lar (n=22), Macaca fasicularis (n=10), Nasalis larvatus (n=13), Papio cynocephalus (n=10), Saimiri sciureus (n=38); Small cephalopelvic proportions: Alouatta palliata (n=5), Gorilla gorilla (n=21), Pan troglodytes (n=20), Pongo pygmaeus (n=12)]. Birth canal dimensions were significantly larger in females compared to males (p<0.05) within species having large cephalopelvic proportions, but not in species with small proportions. Birth canal shape, quantified using principal components analysis, was also dimorphic in species with large cephalopelvic proportions (all p‐values <0.05), but not for species with small proportions. Patterns of dimorphism, quantified using Euclidean Distance Matrix Analysis, were shared among phylogenetic groups (New World monkeys, Old World monkeys, apes). Finally, dimorphism in the non‐obstetric pelvis did not correspond predictably to the presence of birth canal dimorphism or to large cephalopelvic proportions.

Lar gibbon (Hylobates lar) great call reveals individual caller identity.

Gibbons (family Hylobatidae) produce loud, elaborate vocalizations (songs), often in well-coordinated male/female duets. The female's great call, the most conspicuous phrase of the gibbon vocal repertoire, functions primarily to mediate territorial defense. Despite the fact that lar gibbons (Hylobates lar) are the most widely distributed and well researched hylobatid species and produce a rich vocal repertoire, the individual-specificity of their great calls has not previously been quantified. In addition, spectral and temporal features of notes occurring at specific locations within the lar great call have not been described. Here we provide such a description, and test the hypothesis that great calls are statistically discriminable between a large sample of individual callers. We compared recordings of great calls from 14 wild lar females in Khao Yai National Park, Thailand. Our analyses of principal components derived from spectral and temporal measures, as well as spectrograms from the entire great call, indicate that acoustic variation is sufficient to allow identification of individual callers (83.5% discriminability based on principal components, and inter-individual call variation exceeding intra-individual variation in overall spectrogram). These vocalizations potentially allow individual recognition of animals.

Gibbon (Hylobates lar) reintroduction success in Phuket, Thailand, and its conservation benefits.

We summarize the results from a long-term gibbon reintroduction project in Phuket, Thailand, and evaluate its benefits to conservation. Between October 2002 and November 2012, eight breeding families of white-handed gibbons (Hylobates lar) were returned to the wild in Khao Phra Thaew non-hunting area (KPT). Wild gibbons were extirpated from Phuket Island by the early 1980s, but the illegal wildlife trade has continued to bring young gibbons from elsewhere to the island's popular tourist areas as pets and photo props. The Gibbon Rehabilitation Project (GRP) has rescued and rehabilitated confiscated and donated captive gibbons since 1992 and aims to repopulate the island's last sizable forest area. Following unsuccessful early attempts at translocation in the 1990s, GRP has now developed specific methods for gibbon reintroduction that have led to the establishment of a small independent, reproducing population of captive-raised and wild-born gibbons on Phuket. Eleven infants have been born wild within the reintroduced population, including a second generation wild-born gibbon in September 2012. Benefits of the GRP project include restoration of the gibbon population on Phuket, rescue of illegally kept gibbons, public education, training of personnel in gibbon conservation work, and gaining experience which may prove useful in saving more severely threatened species. It is unlikely that gibbon (and other large primate) translocations will make a significant contribution to conservation of the species as a whole, and primate translocation projects should not be judged solely by this criterion.

テナガザル 2種（<i>Hylobates lar</i>，<i>Hylobates pileatus</i>）の Y染色体多型マーカーの確立

Genet ic assessment of gibbon populations is required to establish and develop the conservation management of endangered gibbon species. To assess the paternal genetic diversity of wild gibbons, we searched and developed six loci of the Y-chromosomal marker that consist of five microsatellites and three PCR fragment length polymorphisms in two species of gibbons, Hylobates lar (white-handed gibbons) and H. pileatus (pileated gibbons). Using these Y-chromosomal markers, no individual shared a Y-chromosomal haplotype with any other individual among captive H. lar (n = 12) and H. pileatus (n = 5). The application of the Y-chromosomal markers developed in this study will help us to uncover the genetic diversity of these two gibbon species.

Population Dynamics of a Reintroduced Population of Captive-Raised Gibbons (Hylobates lar) on Phuket, Thailand

Confiscated and donated white-handed gibbons ( Hylobates lar) originating from the local wildlife trade have been rescued by the Gibbon Rehabilitation Project (GRP) and rehabilitated on Phuket since 1992. Here we present some results of this long-term gibbon reintroduction project. Following unsuccessful early release attempts beginning in 1993, GRP has experi- mented with reintroduction methods and developed a protocol that has succeeded in re-establishing a small independent breeding population in a 22-km 2 forest fragment on Phuket. Eight breeding families of gibbons were released into the Khao Phra Thaew non-hunting area between October 2002 and November 2012 using a soft-release reintroduction method in which the gibbons were provisioned with fresh food in the trees for at least one year, or as long as they needed it. The adult pair in the first gibbon family that was reintroduced there has remained together for 10 years post-release, they have maintained the original pair-bond and raised three wild-born offspring. The reintroduced population has seen eleven infants born in the wild, including a second- generation gibbon born to the first wild-born female. Female interbirth intervals and ages at first reproduction in the reintroduced population are comparable with those in wild populations. Of the seven adult females originally released, two remain with their original mates and two remain in the wild paired with different mates. By the end of 2012, three of the six wild groups were not nuclear families, indicating a flexibility of group composition seen in well-studied wild populations.

Using primate models to study the evolution of human locomotion: concepts and cases

This note introduces the types of models used in functional morphological research in order to clarify certain semantic issues and to present some examples of the use of extant model species to contribute to our understanding of bipedal locomotion. The existing models fall into two broad categories: “abstraction models” are simplifications/abstractions of living organisms, whereas “comparative models” are extant organisms used as models or analogues for other organisms (e.g. extinct species). In a palaeoanthropological context, comparative models may be selected for their close (but always imperfect) resemblance to the organism of interest, but some atypical model species can also produce insights not despite their imperfect resemblance, but because of it. We present three examples of our own work on comparative primate models during studies of terrestrial bipedal locomotion in bonobos (Pan paniscus), olive baboons (Papio anubis), and white-handed gibbons (Hylobates lar), showing how they each provide insights into the evolution of human bipedal locomotion.

White‐handed Gibbon (Hylobates lar) Core Area Use Over a Short‐Time Scale

AbstractCore areas represent small regions within animal home‐ranges intensively used during a given period of time. We assessed the quality of core areas relative to the rest of the home‐ranges (i.e., non‐core areas) of 11 groups of the territorial and highly frugivorous white‐handed gibbon (Hylobates lar) over a short‐time scale to reflect temporarily available resources. Weekly core areas included consistently higher densities of the gibbons' important foods compared to non‐core areas. Gibbon core areas partially overlapped at 46 percent with a model comprising the best hypothetical core areas based on a concentration map of available food sources. In addition, a maximization ratio estimated by dividing the cumulative dbh covered by gibbon core areas and the model reached an intermediate value. Gibbon core areas only partially mirrored food distribution probably because they represent a trade‐off between covering regions with important food locations and areas needed for other biologically relevant activities such as territory defense. Results do not support the concept that core areas can represent the minimum area requirement that would allow a gibbon group to survive and reproduce successfully irrespective of the time period considered, which indicates that core areas alone should not be treated as a conservation target.

The interpretive power of infraorbital foramen area in making dietary inferences in extant apes.

The infraorbital foramen (IOF) is located below the orbit and transmits the sensory infraorbital nerve (ION) to mechanoreceptors located throughout the maxillary region. The size of the IOF correlates with the size of the ION; thus, the IOF appears to indicate relative touch sensitivity of maxillary region. In primates, IOF size correlates well with diet. Frugivores have relatively larger IOFs than folivores or insectivores because fruit handling/processing requires increased touch sensitivity. However, it is unknown if the IOF can be used to detect subtle dietary differences among closely related hominoid species. Hominoids are traditionally grouped into broad dietary categories, despite the fact that hominoid diets are remarkably diverse. This study examines whether relative IOF size is capable of differentiating among the dietary preferences of closely related species with overlapping, yet divergent diets. We measured IOF area in Hylobates lar, Symphalangus syndactulus, Pongo pygmaeus spp., Pan troglodytes, Gorilla gorilla, Gorilla beringei graueri, and Gorilla beringei beringei. We classified each species as a dedicated folivore, mixed folivore/frugivore, soft object frugivore, or hard object frugivore. The IOF is documented to be larger in more frugivorous species and smaller in more folivorous taxa. Interestingly, G.b. beringei, had the largest relative IOF of any gorilla, despite being a dedicated folivore. G.b. beringei does have unique food processing behavior that relies heavily on maxillary mechanoreception, thus this finding is not entirely unsupported behaviorally. The results of this study provide evidence that the IOF is an informative feature in interpretations of fossil apes.

Canine crown morphology and sexual dimorphism in the <i>Hylobates </i><i>lar </i>

小型類人猿シロテテナガザル（Hylobates lar）について犬歯形態の詳細な記載と大きさの性的二型性を明らかにすることを目的に研究を行った。テナガザルの犬歯は従来いわれているように性的二型性が小さく，雌雄間で形態が非常によく似ている。歯冠頬側面の概形は上顎犬歯でサーベル形，下顎犬歯は不正四辺形を呈する。オスに較べてメスの形態特徴を挙げると，1）サイズが小さい，2）歯冠浮彫像の発達が弱く，全体に丸みを帯びている，3）下顎犬歯の近心shoulderの位置が歯冠高の約1/2にある，4）歯頚隆線がよく発達していることである。歯冠サイズによる犬歯の性差では，上・下顎の歯冠基底部のサイズや歯冠高でオスの方が有意に大きい。一方，下顎犬歯では歯冠近遠心径に対する歯頚部エナメル質の膨らみはメスの方が大きく，有意に強い性差を示す。歯冠の高径，とくに下顎犬歯の尖頭から近心shoulderまでの距離が最も強い性差を示す。犬歯の形態やサイズに性的二型がみられることはペア社会を構成するテナガザルでもある程度雌雄の違いが大きさや形にも存在することを示す。

Cues to Androgens and Quality in Male Gibbon Songs

Animal vocal signals may provide information about senders and mediate important social interactions like sexual competition, territory maintenance and mate selection. Hence, it is important to understand whether vocal signals provide accurate information about animal attributes or status. Gibbons are non-human primates that produce loud, distinctive and melodious vocalizations resembling more those of birds than of other non-human primates. Wild gibbons are characterized by flexibility in social organization (i.e., pairs and multimale units) as well as in mating system (i.e., monogamy and polyandry). Such features make them a suitable model to investigate whether the physiology (hormonal status) and socio-demographic features find their correspondence in the structure of their songs. By combining male solo song recordings, endocrine outputs using non-invasive fecal androgen measures and behavioral observations, we studied 14 groups (10 pair-living, 4 multimale) of wild white-handed gibbons (Hylobates lar) residing at Khao Yai National Park, Thailand. We collected a total of 322 fecal samples and recorded 48 songs from 18 adult animals. Our results confirmed inter-individuality in male gibbon songs, and showed a clear correlation between androgen levels and song structures. Gibbons with higher androgen levels produced calls having higher pitch, and similarly adult individuals produced longer calls than senior males. Thus, it is plausible that gibbon vocalizations provide receivers with information about singers' attributes.

Fibre type composition in the lumbar perivertebral muscles of primates: implications for the evolution of orthogrady in hominoids

The axial musculoskeletal system is important for the static and dynamic control of the body during both locomotor and non-locomotor behaviour. As a consequence, major evolutionary changes in the positional habits of a species are reflected by morpho-functional adaptations of the axial system. Because of the remarkable phenotypic plasticity of muscle tissue, a close relationship exists between muscle morphology and function. One way to explore major evolutionary transitions in muscle function is therefore by comparative analysis of fibre type composition. In this study, the three-dimensional distribution of slow and fast muscle fibres was analysed in the lumbar perivertebral muscles of two lemuriform (mouse lemur, brown lemur) and four hominoid primate species (white-handed gibbon, orangutan, bonobo, chimpanzee) in order to develop a plausible scenario for the evolution of the contractile properties of the axial muscles in hominoids and to discern possible changes in muscle physiology that were associated with the evolution of orthogrady. Similar to all previously studied quadrupedal mammals, the lemuriform primates in this study exhibited a morpho-functional dichotomy between deep slow contracting local stabilizer muscles and superficial fast contracting global mobilizers and stabilizers and thus retained the fibre distribution pattern typical for quadrupedal non-primates. In contrast, the hominoid primates showed no regionalization of the fibre types, similar to previous observations in Homo. We suggest that this homogeneous fibre composition is associated with the high functional versatility of the axial musculature that was brought about by the evolution of orthograde behaviours and reflects the broad range of mechanical demands acting on the trunk in orthograde hominoids. Because orthogrady is a derived character of euhominoids, the uniform fibre type distribution is hypothesized to coincide with the evolution of orthograde behaviours.

Sexual dimorphism in relative sacral breadth among catarrhine primates

As the sacrum contributes to the size and shape of the birth canal, the sexually dimorphic sacrum of humans is frequently interpreted within obstetric contexts. However, while the human sacrum has been extensively studied, comparatively little is known about sacral morphology in nonhuman primates. Thus, it remains unclear whether sacral sexual dimorphism exists in other primates, and whether potential dimorphism is primarily related to obstetrics or other factors such as body size dimorphism. In this study, sacra of Homo sapiens, Hylobates lar, Nasalis larvatus, Gorilla gorilla, Pongo pygmaeus, Pan troglodytes, and Pan paniscus were evaluated for sexual dimorphism in relative sacral breadth (i.e., the ratio of overall sacral breadth to first sacral vertebral body breadth). Homo sapiens, H. lar, N. larvatus, and G. gorilla exhibit dimorphism in this ratio. Of these, the first three species have large cephalopelvic proportions, whereas G. gorilla has small cephalopelvic proportions. P. pygmaeus, P. troglodytes, and P. paniscus, which all have small cephalopelvic proportions, were not dimorphic for relative sacral breadth. We argue that among species with large cephalopelvic proportions, wide sacral alae in females facilitate birth by increasing the pelvic inlet's transverse diameter. However, given the small cephalopelvic proportions among gorillas, an obstetric basis for dimorphism in relative sacral breadth appears unlikely. This raises the possibility that sacral dimorphism in gorillas is attributable to selection for relatively narrow sacra in males rather than relatively broad sacra in females. Accordingly, these results have implications for interpreting pelvic dimorphism among fossil primates, including hominins.

Extra‐pair paternity confirmed in wild white‐handed gibbons

Knowledge of the genetic mating system of animal species is essential for our understanding of the evolution of social systems and individual reproductive strategies. In recent years, genetic methods have uncovered an unexpected diversity of paternal genetic contributions across diverse animal social mating systems, but particularly in pair-living species. In most pair-living birds, for example, genetic and behavioral observations have confirmed a previously unknown significance of extra-pair copulations (EPCs) and extra-pair paternity. Among mammals, white-handed gibbons (Hylobates lar) are also known to live in pairs and are traditionally believed to be single-male single-female breeders. However, at Khao Yai National Park, Thailand, behavioral observations have confirmed the occurrence of both EPCs and functional multi-male grouping, but knowledge about the genetic mating system is still unavailable. In this study, we genotyped 89 white-handed gibbons of the Khao Yai population based on fecal samplings and were able to determine paternity for 41 offspring through short tandem repeat analysis. We found that females' stable social partners sired the majority (90.5%) of offspring (N = 38), while only a few (7.1%) offspring (n = 2 confirmed cases; n = 1 inferred case) were conceived with extra-pair partners. The paternity of one offspring remained inconclusive (2.4%), because the offspring's genotype did not mismatch with the genotypes of two potential sires. Like other predominantly pair-living species, gibbons appear to follow a mixed-reproductive strategy. The genetic mating system of wild white-handed gibbons is best described as flexible, primarily monogamous and opportunistically promiscuous. Inc.

La sécurité respiratoire des voies aériennes supérieures par une étude endoscopique comparative in live d’un échantillon de primates : hominidés et humains : l’« hypothèse de contact : In-Out »

Nucleotide sequences of the parathyroid (PTH) gene of 12 species of primates belonging to suborder Anthropoidea were examined. The PTH gene contains one intron that separates two exons that code the sequence of prepro and PTH, respectively. The intron of the PTH gene in Cebus apella, Callithrix jacchus, and Saguinus oedipus was 102 bp long, whereas a 103-bp intron was observed in the remaining species. Phylogenetic analysis using the nucleotide sequences of PTH revealed that these 12 species of primates of suborder Anthropoidea could be divided into two groups of the infraorder Platyrrhini (C. apella, C. jacchus, and S. oedipus) and the infraorder Catarrhini (Macaca fascicularis, Macaca fuscata, Cercopithecus aethiops, Papio hamadryas, Presbytes obscura, Hylobates lar, Pongo pygmaeus, Pan troglodytes, and Pan paniscus). The latter infraorder could be further subdivided into two subgroups belonging to the superfamily Cercopithecoidea (M. fascicularis, M. fuscata, C. aethiops, P. hamadryas, and P. obscura) and the superfamily Hominoidea (H. lar, P. pygmaeus, P. troglodytes, and P. paniscus). The deduced amino acid sequences of PTH gene between 12 species of nonhuman primates and human revealed no amino acid substitution in mature PTH among orangutans, chimpanzees, and humans. The results indicated that the PTH gene is very conserved among primates, especially between great apes and humans. The apes are the most suitable animals to be used for studying the bone metabolism and applying the knowledge to clinical use in humans.

Effects of social status and stress on patterns of gastrointestinal parasitism in wild white‐handed gibbons (Hylobates lar)

Although gibbons (family Hylobatidae) are typically monogamous, polyandrous groups occur regularly. Stress associated with elevated intragroup competition among males in polyandrous groups may increase susceptibility to infectious disease. To better understand this interplay, as well as to provide the first comprehensive assessment of parasitism in free-ranging gibbons, we characterized the richness of gastrointestinal parasites and examined their prevalence in males from 14 groups (10 pair-living, 4 multi-male) of white-handed gibbons (Hylobates lar) at Khao Yai National Park, Thailand. From September 2008 to May 2009, 324 fecal samples were collected from 23 individually recognizable male gibbons and screened for gastrointestinal helminths and protozoa after isolation via fecal floatation, sedimentation, and immunofluorescent antibody detection. A total of 10 parasite species recovered, including seven nematodes, two protozoans, and one trematode. Parasite richness and species-specific prevalence were examined relative to social organization (pair-living vs. multi-male), male status (primary vs. secondary), age (subadult, adult, senior), fecal glucocorticoid levels, and time of the year. No relationship was found between parasite richness and sociodemographic or physiological factors. Similarly, prevalence of infection with parasite species was not associated with the majority of sociodemographic factors; however, Ternidens sp. and Balantidium coli varied seasonally and Trichuris sp. decreased with increasing age. Moreover, observational data suggest that competition is low in this gibbon population, and our findings are consistent with those observations in that cooperative defense may offset stress and reduce susceptibility to infection.

Locomotion behavior of cao vit gibbon (Nomascus nasutus) living in karst forest in Bangliang Nature Reserve, Guangxi, China

The cao vit gibbon is a critically endangered species. Only approximately 110 individuals remain in degraded karst forest along the China-Vietnam border. Karst forest is unusual gibbon habitat. Currently, the canopy height of cao vit gibbon habitat is approximately 10 m. Research on the locomotor behavior of gibbons living in this particular forest type might provide important insight into locomotor stability and variability of gibbons. We used 5 min scan samples to record the locomotion mode, support use and canopy strata of gibbons in 3 groups for 2096 h between January 2008 and December 2009. Although cao vit gibbon habitat has a lower canopy in comparison to that of other forests inhabited by gibbons, cao vit gibbons displayed a similar overall locomotor pattern to other gibbon species (Symphalangus syndactylus, Hylobates lar and Hylobates agilis) in which brachiation dominate their locomotor behavior. Cao vit gibbons spent most of their time travelling on inclined branches (2-10 cm) in the middle stratum through the forest canopy. Adult females appear to more often employ safer modes of locomotion (bridging more often and brachiation less), while adult males choose riskier modes (leaping more and climbing less). As gibbons increased in body weight, as they grew from infant to adult, they tended to use larger supports. This research documented that locomotor behavior in Hylobatidae is strongly determined by anatomical characters, but cao vit gibbons also show the ability to use various supports, enabling them to survive in karst forest.

Soprano singing in gibbons

Diversifications in primate vocalization, including human speech, are believed to reflect evolutionary modifications in vocal anatomy and physiology. Gibbon song is acoustically unique, comprising loud, melodious, penetrating pure tone-like calls. In a white-handed gibbon, Hylobates lar, the fundamental frequency (f(0) ) of song sounds is amplified distinctively from the higher harmonics in normal air. In a helium-enriched atmosphere, f(0) does not shift, but it is significantly suppressed and 2f(0) is emphasized. This implies that the source is independent of the resonance filter of the supralaryngeal vocal tract (SVT) in gibbons, in contrast to musical wind instruments, in which the filter primarily determines f(0) . Acoustic simulation further supported that gibbons' singing is produced analogously to professional human soprano singing, in which a precise tuning of the first formant (F(1) ) of the SVT to f(0) amplifies exclusively the f(0) component of the source. Thus, in gibbons, as in humans, dynamic control over the vocal tract configuration, rather than anatomical modifications, has been a dominant factor in determining call structure. The varied dynamic movements were adopted in response to unique social and ecological pressures in gibbons, allowing monogamous gibbons to produce pure-tonal melodious songs in the dense tropical forests with poor visibility.

Sociodemographic Correlates of Fecal Androgen Levels in Wild Male White-Handed Gibbons (Hylobates lar)

Despite the central role of testosterone in influencing many aspects of the male life cycle, information on the influence of social and behavioral factors associated with androgen output is available for only a few primate species, mainly those living in multimale–multifemale societies. We collected 322 fecal samples and measured fecal testosterone metabolite levels in 19 adult male white-handed gibbons (Hylobates lar) from 13 groups living in the Khao Yai National Park, Thailand, to examine the extent to which androgen output is related to sociodemographic variables (social organization, social status, presence of infants, age) in a socially flexible species, i.e., pair-living/multimale groups. We predicted that androgen levels would be higher in males 1) that live in potentially more unstable single-male/single-female units; 2) that are primary (dominant) vs. secondary; 3) that live in groups with dependent infants; and 4) that are of prime vs. senior age. We found marked differences in androgen levels among males and a significant effect of time of the year. Males living in pairs exhibited significantly higher androgen concentrations vs. males living in multimale groups, and males residing in groups with infants showed a significant increase in androgen levels throughout the sampling year. However, we found no relationship with male age or social status. The increased androgen output found in both pair-living males and those residing with infants might be a physiological mechanism that facilitates male aggression to cope with potential threats posed by social challenges and the demand for infant/group defense, respectively. More generally, our data indicate that in socially flexible species male androgen output is less strongly linked to key life-history variables than in species that live permanently in multimale–multifemale groups.

Sex Ratio Bias in Managed Populations of Hylobatids

A number of explanations have been proposed for the occurrence of sex ratio bias in primates, including the Trivers-Willard hypothesis on differential investment, local resource competition or enhancement as a result of sex-biased dispersal, dominance-related advantages conferred on one sex but not the other, and the fragile male hypothesis. However, none of these theories was thought to be applicable to monogamous primate species. Here, we examine data on zoo-housed populations of three hylobatid species to test the null hypothesis of equal sex ratio. We analyzed over 40 years of demographic data on Nomascus leucogenys, Hylobates lar and Symphalangus syndactylus. We identified a strong male-biased birth sex ratio in N. leucogenys. Male infant mortality was consistently higher than female infant mortality for N. leucogenys, but this difference was not significant. We found that prime-age N. leucogenys females (aged 13–20) produced significantly more male offspring than female offspring, and young S. syndactylus females (<13 years) produced significantly more female offspring. Recent field evidence of more flexible mating systems in H. lar and S. syndactylus may also be occurring in N. leucogenys. However, N. leucogenys has not been well studied in the wild. Ecological differences among species may further contribute to these observed patterns.