Hylobates Lar Research Articles

Interspecific variation at the Y‐linked RPS4Y locus in hominoids: implications for phylogeny

Within- and between-species variation in restriction endonuclease recognition sites was examined at the Y-linked RPS4Y locus of six hominoid species: human (Homo sapiens), gorilla (Gorilla gorilla), chimpanzee (Pan troglodytes), bonobo (Pan paniscus), orangutan (Pongo pygmaeus), and gibbon (Hylobates lar). RPS4Y is an expressed gene that maps to the non-recombining region of the Y chromosome. An approximately 1,490 base pair fragment of the RPS4Y gene, including all of intron 3, was amplified by PCR from DNA extracted from each of the six species. Forty-seven restriction sites were identified on the six-species composite map derived from double-digest restriction analyses of the amplified fragment. As expected, maximum parsimony analysis indicated that chimpanzee and bonobo are the two most closely related living hominoids. The same analysis suggested that the closest living relative of Homo is Gorilla, not Pan, although support for this relationship was relatively weak. These results disagree with recently published phylogenies based on analyses of mtDNA sequences (Horai et al. [1995] Proc. Natl. Acad. Sci. U.S.A. 88:7401-7404) and the Y-linked ZFY locus (Doris et al. [1995] Science 268:1183–1185). A combined data set derived from three distinct Y-linked loci—RPS4Y, SRY, and ZFY—was also analyzed. The maximum parsimony topology for the combined data provided only weak support for a shared common ancestor for Homo and Pan subsequent to divergence from the Gorilla lineage. Taken together, the data from the Y chromosome do not provide unequivocal support for any single, dichotomously branching species tree linking Homo, Pan, and Gorilla. © Wiley-Liss, Inc.

Pair Bonds in Monogamous Apes: a Comparison of the Siamang Hylobates Syndactylus and the White-Handed Gibbon Hylobates Lar

AbstractDescriptions of the social systems of gibbons (Hylobates, Hylobatidae) have typically emphasized generically uniform attributes such as 'monogamy' and 'territoriality'. This has prevented testing of the hypothesis that pair bonds differ in the siamang (Hylobates syndactylus) and the white-handed gibbon (Hylobates lar) (Chivers, 1972). I replace a description of sociality based on mating system and group size/composition with quantitative measurement of social interactions and spatial relations between wild adult males and females in three heterosexual pairs of siamang and two pairs of white-handed gibbons studied for 2.5 years at the Ketambe Research Station (Sumatra, Indonesia). Siamang pair bonds show greater heterosexual cohesion than those of white-handed gibbon as reflected in higher rates of affinitive interactions such as close proximity, relaxed physical contact, embraces, and communal use of sleep trees. Although males are more responsible than females for the maintenance of close proximity in both species, sex differences in intra-pair allogrooming suggest divergent mechanisms maintaining pair bonds in the two species. In white-handed gibbons, the female rarely initiates grooming and grooms her mate significantly less than he grooms her, partly because she solicits grooming from him at higher rates while simultaneously ignoring more of his 'presents' for grooming. In siamang, the contributions of the sexes to grooming are more equivalent and reciprocal. Taken together, these results suggest that investment of the sexes in maintaining the pair bond is more asymmetrical in white-handed gibbons (i.e. males contribute relatively more than females) and more mutual in siamang. Although mate guarding may have been the selective force behind the origin of pair bonds in both species, greater intra-group feeding competition in the gibbon and substantial paternal care in the siamang may account for the evolution of more reciprocal and stronger pair bonds in the latter. Future research on more groups is necessary to clarify the proposed species differences in light of existing intraspecific variation in social behavior.

Arboreal locomotor habit and the morphology of the epaxial muscles

The superficial epaxial muscles of a dusky lutong and a white-handed gibbon were dissected to reveal the arboreal morphology of these muscles. The results were compared with those of the terrestrial primate species which have been previousl y reported. Additional dissection was made on the epaxial muscles of a dog and a cat in order to specify the general rule for the organization of the epaxial muscles. Although the epaxial muscle morphology differed in the cat and the dog, the patterns of their origin-insertion relationship were similar to any primate species. Thus the morphology of these muscles could be discussed from the view point of the locomotor adaptation. The morphological differences between arboreal and terrestrial primates were observed in the lumbar architecture of the longissimus system. The strong additional muscle bundles that originated at the lumbar mamillary process joined to the longissimus system in the terrestrial patas monkey and hamadryas baboon. These muscle bundles were not found in the arboreal dusky lutong, spider monkey and white-handed gibbon. However, the erector spinae muscles morphology varied in these arboreal species. The erector spinae aponeurosis extended to the cranial direction in the spider monkey and the gibbon, but that of the dusky lutong attached only in the relatively caudal range. The erector spinae muscles of the dusky lutong and the spider monkey less developed in lumbar region, though that of the gibbon well developed. These results would suggested that the differential strategy for the arboreal life would exist, which reflects on the epaxial muscle morphology.

Relationship of incisor size to diet and anterior tooth use in sympatric sumatran anthropoids.

Researchers often relate anthropoid incisor size to diet and ingestive behaviors. It is suggested that primates that frequently consume large, tough foods (i.e., fruits) require large incisors to process these items. This idea has been difficult to test because of a lack of data on anterior tooth use in wild primates, and a lack of understanding concerning the relationships between food properties and ingestive behaviors. The first field study of primate ingestive behaviors has recently been completed for four species of Sumatran anthropoids: Hylobates lar, Macaca fascicularis, Pongo pygmaeus, and Presbytis thomasi [Ungar, American Journal of Physical Anthropology 95:197-219, 1994; International Journal of Primatology 16:221-245, 1995]. This paper documents both relative and absolute incisor row width differences among these taxa, and evaluates the relationships between incisor size and feeding behaviors for specific taxa. Results indicate that differences in incisor size among these species cannot all be explained by degree of frugivory, food item size, or even degree of incisor use in ingestion alone. It is therefore suggested that inferences of dietary differences based on largely or solely on differences in incisor sizes of specific fossil anthropoid taxa should be approached with caution. © 1996 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.

Fruit preferences of four sympatric primate species at Ketambe, northern Sumatra, Indonesia

Recent researchers have drawn attention to fruit preferences in a variety of primates;for instance, in 1991, Davies noted that monogastric primates prefer the flesh of succulent, sugar-rich fruits, while colobines more often consume and digest large seeds of drier fruits. I compare fruit preferences in four sympatric primates— Hylobates lar, Macaca fascicularis, Pongo pygmaeus,and Presbytis thomas—which I studied concurrently at the Ketambe Research Center in northern Sumatra. I collected continuous focal animal data during 40–50 hr per taxon per month for 10 months and recorded fruit species size,pH, and descriptive attributes, including degree of ripeness, hardness, pericarp type, and number of seeds. The langurs prefer dry fruit seeds while the monogastric primates more often consume acidic, succulent fruit flesh. Further, H. lar, M. fascicularis,and P. pygmaeusvary significantly in preferences for fruits vis-a-visthe characteristics examined.

Comparative mapping of the immunoglobulin Cε1 gene (IGHE) in five species of nonhuman primates by fluorescence in situ hybridization

The orthologous immunoglobulin C epsilon 1 gene (IGHE) of the common chimpanzee, pygmy chimpanzee, orangutan, white-handed gibbon, and Japanese macaque was assigned to the human chromosome 14 homologue in each species and regionally mapped by fluorescence in situ hybridization to PTR15q32 (common chimpanzee), PPA15q32 (pygmy chimpanzee), PPY15q32 (orangutan), HLA17qter (white-handed gibbon), and MFU7q29 (Japanese macaque). The gene localized to the terminal region of the chromosome in each species, and so this probe provides a new telomeric DNA marker for nonhuman primates.

Erythrocyte metabolism in the gibbon, the elephant and the lion

Erythrocyte metabolism was studied in the white-handed gibbon (Hylobatus leucogenys), the Asian elephant (Elephas maximus) and the lion (Panthera leo). Aspects of metabolism studied were activities of the enzymes associated with glycolysis and the antioxidant defence systems, glycolytic rates using four different substrates and the susceptibility of haemoglobin to three different oxidising agents, acetylphenylhydrazine (APH), hydrogen peroxide (H202) and sodium nitrite (NaNO2). The main findings were as follows. First, compared with humans, the erythrocytes from the gibbon were more resistant to H202-induced haemolysis and had very high activity of NADH-methaemoglobin reductase. Oxidation of haemoglobin by NaNO2 was largely blocked by 0.1 mM GSH. Second, the erythrocytes from the lion had a very high activity of pyruvate kinase (PK). The rate of methaemoglobin formation by APH was also very high in these erythrocytes. Third, the erythrocytes of the elephant had very low levels of hexokinase and PK and were more resistant to oxidant stress than those of the lion and humans.

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.

Phylogenetic relationships among Homo sapiens and related species based on restriction site variations in rDNA spacers.

A rapid method, using 12 restriction enzymes, was employed to analyze variations in ribosomal DNA (rDNA) spacers in a study of phylogenetic relationships between Homo sapiens and related species. We mapped restriction sites in the external and internal spacer regions and compared the arrangements of sites. The estimated sequence divergence between Homo sapiens and Pan troglodytes, Pan paniscus, Gorilla gorilla, Pongo pygmaeus, Hylobates lar, H. agilis, and Macaca fuscata was 2.7, 2.3, 3.8, 7.3, 6.8, 7.8, and 14.1%, respectively. The genetic relationships inferred from these distances generally correspond to those inferred from analyses of other molecular markers in the literature. The divergence between H. lar and H. agilis and between H. lar and H. syndactylus was 0.34 and 2.4%, respectively.

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.

Coraco-clavicular joint: normal variant in humans

The coraco-clavicular joint is a true synovial joint that may become painful in some patients after trauma. Among the descriptions of this entity is the assertion that the coraco-clavicular joint is routinely seen in gorillas and gibbons. We undertook to assess the incidence of this variant among gorillas, gibbons, and other non-human primates. All available radiographs of large primates performed at the International Wildlife Conservation Park/Bronx Zoo (IWCP) over the past 10 years were reviewed by a musculoskeletal radiologist (human radiology). All radiographs were taken during the normal clinical care of the non-human primate population of the IWCP and are a part of each animal's clinical record. Eighty-one non-human primate radiographs were suitable for study as they contained the region of interest. The 81 radiographic examinations included 14 different species of non-human primates. The coraco-clavicular joint was seen in 4 out of 9 silver-leaf langur, 2 out of 8 lowland gorilla, and in 1 out of 6 white-handed gibbon. In all non-human primate cases where the coraco-clavicular joint occurred, it was bilateral. In 1 out of 8 mandrill, there were very wide distal clavicular ends that articulated both with the coracoid and with the acromion. The coraco-clavicular joint differs from an ossified coraco-clavicular ligament. The radiographic appearance is characteristic and is found in both humans and some non-human primate species. It may rarely become painful following trauma. When symptomatic in humans, resection of this anomalous articulation is curative.

Temple and pet primates in Thailand

In Thailand, a predominantly Buddhist country, isolated populations of long-tailed macaques (Macaca fascicularis) especially may be found at temples in both rural and urban areas where they are provisioned by people making merit although the monkeys may be shot as agricultural pests in adjacent rice or corn fields in rural areas. At the same time, macaques and gibbons, especially the lar gibbon (Hylobates lar), have been kept as pets in both rural and urban areas. Gibbons even are reported to be the « favorite pet for Thais ». The number of primates being offered for sale has risen dramatically as legal and illegal timbering have reduced Thailand’s forest cover to perhaps no more than 12 %, peripherali-zing primate populations and making them more vulnerable to hunting. A provision in Thai law permitted individuals to possess as many as two primates or other wild animals of the same species — « a potential pair to promote breeding » — even though their capture and sale was prohibited. Changes in Thailand’s wildlife laws governing private possession of wild animals have caused owners to abandon pets to government agencies and temples. Private rescue centers offer acute care and permanent shelter for some of these primates but cannot cope with the numbers involved. Pilot studies suggest that rehabilitation and réintroduction may be feasible alternatives for some former captive gibbons.

Dynamic Pair Bonds in Hylobatids: Implications Regarding Monogamous Social Systems

AbstractDescriptions of monogamy in the gibbons (Hylobates spp., Hylobatidae) have generally emphasized features derived from inference rather than direct observation: 1) adults mate for life; 2) the genetic structure of groups corresponds with a ''nuclear family'' pattern of parents and their mutual offspring; 3) first mate acquisition is achieved when two ncwly dispersed, unaccompanied, young adults of opposite sex establish a new pair and territory together. The six-year history of mating associations of adults in three groups of sympatric, wild siamang (Hylobates syndactylus) and three groups of white-handed gibbon (Hylobates lar) at the Ketambe Research Station (Sumatra, Indonesia) challenge the generality of these assumptions. These longitudinal sociodemographic data suggest that gibbon pair bonds are not necessarily life-long and that adults may leave mates permanently or temporarily, sometimes in order to exploit new reproductive opportunities. Of 11 heterosexual pair bonds that existed at some time during the study, only two were terminated by death of one adult, whereas five were terminated when one adult left its current mate. In some cases, the ''deserting'' individual left its mate and joined a neighboring adult of opposite sex who had recently lost its own mate through death or desertion. This turnover in pair bond membership generated stable, ''non-nuclear'' family groups in which the replacement mate was not the parent of the resident nonadults. Mechanisms of mate acquistion observed at Ketambe included two processes that may also promote non-nuclear families. A young adult may obtain its first mate by: 1) replacing an adult of the same sex in a neighboring group or in the group in which it resides; 2) establishing a new pair bond (and territory) with another unmated young adult of opposite sex who is, however, accompanied in dispersal by juvenile conspecifics from its group. Finally, observed extra-pair copulations (in siamang) suggested an additional, but as yet unspecified potential for variable relatedness between adult males and resident nonadults. Review of other long-term histories of pair bonds in wild gibbons and of studies of socio-sexual behavior in captivity reinforce the conclusion that the apparent temporal stability and underlying behavioral inflexibility of pair bonds has been over-emphasized. Although data on behavior and reproduction are limited, they nevertheless suggest strongly that the mating and social systems of these monogamous primates are considerably more dynamic and complex than previously appreciated.

Unique Pre-S Sequence in a Gibbon-Derived Hepatitis B Virus Variant

A unique hepatitis B virus (HBV) variant has been identified in a gibbon (Hylobates lar) which could be passed to a chimpanzee by experimental inoculation. This HBV variant had been shown to have no reactivity to a monoclonal anti-preS2 antibody (preS2 mAb 116-34) differentiating it from all human HBV specimens tested. This gibbon sera also was not recognized by an anti-preS1 mAb which binds the preS1 hepatocyte receptor region, amino acids 27-35. In this paper, we report the DNA sequence of the gibbon HBV PreS gene. The lack of preS2 mAb (116-34) binding can be explained by a unique nucleotide substitution of A for C in the second codon of the preS2 region leading to the replacement of glutamine with lysine. Two other unique changes were observed at the seventh and 24th amino acid positions in the preS2 gene leading to a substitution of a valine for threonine and alanine, respectively. Unlike all human derived HBV sequences in the preS1 region, the gibbon HBV had a glutamic acid instead of an aspartic acid at amino acid residue 27. Another unique substitution was a leucine for alanine at preS1 position 33. These amino acid changes in the gibbon HBV may explain its unique preS mAb reactivity.

The bronchial tree and lobular division of the lung of the white handed gibbon

The bronchial tree and lobular division of the lungs of four white handed gibbons (Hylobates agilis) were examined from the viewpoint of comparative anatomy, based upon the fundamental structure of the bronchial ramifications of the mammalian lung (Nakakuki, 1975, 1980). The right lung of the white handed gibbon consists of the upper, middle, lower, and accessory lobes, whereas the left lung consists of the middle and lower lobes. Each lobe is separated by the interlobular fissure, on both sides. The right and left lungs have the dorsal bronchiole system, lateral bronchiole system, and ventral bronchiole system. The medial bronchiole system is lacking on both sides. In the right lung, the upper lobe is formed by the first branch of the dorsal bronchiole system. The middle lobe is formed by the first brach of the lateral bronchiole system, and the accessory lobe by the first branch of the ventral bronchiole system. The remaining bronchioles constitute the right lower lobe. In the left lung, the upper lobe bronchiole, which is the first branch of the dorsal bronchiole system, is lacking. Therefore, the middle lobe bronchiole, i.e. the first branch of the lateral bronchiole system, is well developed. The accessory lobe bronchiole, the first branch of the ventral bronchiole system, is also lacking. The remaining bronchioles constitute the left lower lobe. These features were compared with those of other apes and man.

Distribution of the pulmonary artery and vein in the lung of the white handed gibbon

The lungs of four white handed gibbons (Hylobates agilis) were examined. The right pulmonary artery runs across the ventral side of the right upper lobe bronchiole, and then traverses the dorsal side of the right middle lobe bronchiole. Thereafter, it runs along the dorso-lateral side of the right bronchus, between the dorsal bronchiole system and the lateral bronchiole system, and gradually follows the dorsal side of the right bronchus. During its course, it gives off arterial branches which run along each bronchiole. The left pulmonary artery runs across the dorsal side of the left middle lobe bronchiole and then along the left bronchus as in the right lung. The branches of the pulmonary artery run mainly along the dorsal or lateral side of the bronchiole, while the pulmonary veins run mainly the medial side of the bronchioles or between them. However, in a few portions, the pulmonary veins run the lateral side of the bronchioles. Finally, they enter the left atrium with four large veins i.e. the common trunk of the right upper lobe vein and right middle lobe vein, right lower lobe pulmonary venous trunk, left middle lobe vein, and left lower lobe pulmonary venous trunk.

Lethal territorial aggression in a white-handed gibbon.

During an intergroup conflict an adult male white-handed gibbon (Hylobates lar) from one group was attacked and wounded by the adult male of a neighboring group. The wounded male's calling behavior, general activity level, feeding, and participation in territorial defense declined dramatically in the days following the injury as he instead spent long periods resting and tending the wound. The normal and healthy appearance of this male prior to injury, the circumstances of the fight that caused the injury, the resultant deterioration in normal maintenance and social behavior, and finally the apparent infection of the wound by insect larvae all suggested that his disappearance 24 days later was the result of death due directly or indirectly to the wound he had suffered. The observations reported here suggest that the ritualization of territorial aggression in this species has not eliminated risks of serious injury and death. © 1993 Wiley-Liss, Inc.

Tandemly duplicated alpha globin genes of gibbon.

The alpha globin gene locus of the common gibbon (Hylobates lar) was isolated, and it contains two closely linked alpha globin genes that share the same arrangement as that found for the homologous genes of other catarrhine primates. The nucleotide sequences of the gibbon alpha globin genes were determined and compared with the alpha globin gene sequences from other primate species (human, chimpanzee, orangutan, baboon, and rhesus); the prosimian primate, galago, alpha A and alpha B globin genes provided the out-group for this analysis. The degree of divergence for both synonymous and nonsynonymous substitutions among the alpha globin genes are generally smaller for intraspecies than interspecies comparisons, which is indicative of concerted evolution between the paired alpha globin genes of each catarrhine species. The pattern of differences is suggestive of gene conversions. This possibility is supported by both site-by-site and branch-swapping parsimony analyses. The site-by-site parsimony analysis was also used to determine the 3' boundary of each catarrhine species-specific conversion. These alpha globin gene conversion events encompass most of the transcriptional region, including the promoter, exons, introns, and about one-half of the 3'-untranslated region. However, the paired gibbon alpha globin genes show the highest degree of divergence, differing by five nucleotides in their coding regions, three of which encode amino acid replacement substitutions. It appears that the last convergence event between the paired gibbon alpha globin genes is relatively ancient, having occurred approximately 11 million years ago. The remaining 3'-untranslated region and flanking DNAs show no evidence of involvement in any recent conversion event as the parsimony analysis of these sequences group all the alpha 1 globin genes of the different catarrhine species into one clade and all the alpha 2 globin gene of these species into a separate clade. The sequence comparison among the alpha globin genes of different primate species also provides interesting implications regarding the evolution and functional domains of the 3'-untranslated regions.

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.