Gibbons () are apes in the family Hylobatidae (; hylobatids). The family historically contained one genus, but now is split into four extant genera and 20 species. Gibbons live in subtropical and tropical forests from eastern Bangladesh and Northeast India to Southeast Asia and Indonesia (including the islands of Sumatra, Borneo and Java).
Also called the lesser apes, gibbons differ from the great apes (chimpanzees, gorillas, orangutans and humans) in being smaller, exhibiting low sexual dimorphism, and not making nests. Like all of the apes, gibbons are tailless. Unlike most of the great apes, gibbons frequently form long-term pair bonds. Their primary mode of locomotion, brachiation, involves swinging from branch to branch for distances up to , at speeds as fast as . They can also make leaps up to , and walk bipedally with their arms raised for balance. They are the fastest of all tree-dwelling, nonflying mammals.
Depending on the species and sex, gibbons' fur coloration varies from dark- to light-brown shades, and any shade between black and white, though a completely "white" gibbon is rare.
Etymology The English word "gibbon" is a reborrowing from French and may originally derive from an Orang Asli word.
Evolutionary history Whole genome molecular dating analyses indicate that the gibbon lineage diverged from that of great apes around 16.8 million years ago (Mya) (95% confidence interval: 15.9–17.6 Mya; given a divergence of 29 Mya from Old World monkeys). Adaptive divergence associated with chromosomal rearrangements led to rapid radiation of the four genera 5–7 Mya. Each genus comprises a distinct, well-delineated lineage, but the sequence and timing of divergences among these genera has been hard to resolve, even with whole genome data, due to radiative speciations and extensive incomplete lineage sorting. An analysis based on morphology suggests that the four genera are ordered as (Symphalangus, (Nomascus, (Hoolock, Hylobates))).
A coalescent-based species tree analysis of genome-scale datasets suggests a phylogeny for the four genera ordered as (Hylobates, (Nomascus, (Hoolock, Symphalangus))).
At the species level, estimates from mitochondrial DNA genome analyses suggest that Hylobates pileatus diverged from H. lar and H. agilis around 3.9 Mya, and H. lar and H. agilis separated around 3.3 Mya. Whole genome analysis suggests divergence of H. pileatus from H. moloch 1.5–3.0 Mya. The extinct Bunopithecus sericus is a gibbon or gibbon-like ape, which until recently, was thought to be closely related to the hoolock gibbons.
Taxonomy
The family is divided into four genera based on their diploid chromosome number: Hylobates (44), Hoolock (38), Nomascus (52), and Symphalangus (50). Also, three extinct genera currently are recognised: Bunopithecus, Junzi, and Yuanmoupithecus.
Family Hylobatidae: gibbons Genus Hoolock Western hoolock gibbon, H. hoolock Eastern hoolock gibbon, H. leuconedys Skywalker hoolock gibbon, H. tianxing Genus Hylobates: dwarf gibbons Lar gibbon or white-handed gibbon, H. lar Bornean white-bearded gibbon, H. albibarbis Agile gibbon or black-handed gibbon, H. agilis Western grey gibbon or Abbott's grey gibbon, H. abbotti Eastern grey gibbon or northern grey gibbon, H. funereus Müller's gibbon or southern grey gibbon, H. muelleri Silvery gibbon, H. moloch Pileated gibbon or capped gibbon, H. pileatus Kloss's gibbon, Mentawai gibbon or bilou, H. klossii Genus Symphalangus Siamang, S. syndactylus Genus Nomascus: crested gibbons Northern buffed-cheeked gibbon, N. annamensis Concolor or black crested gibbon, N. concolor Eastern black crested gibbon or Cao Vit black crested gibbon, N. nasutus Hainan black crested gibbon, N. hainanus †Imperial gibbon, N. imperialis Northern white-cheeked gibbon, N. leucogenys Southern white-cheeked gibbon, N. siki Yellow-cheeked gibbon, N. gabriellae
Extinct genera Genus Bunopithecus Bunopithecus sericus Genus Yuanmoupithecus Yuanmoupithecus xiaoyuan
Hybrids Many gibbons are hard to identify based on fur coloration, so are identified either by song or genetics. These morphological ambiguities have led to hybrids in zoos. Zoos often receive gibbons of unknown origin, so they rely on morphological variation or labels that are impossible to verify to assign species and subspecies names, so separate species of gibbons commonly are misidentified and housed together. Interspecific hybrids, within a genus, are also suspected to occur in wild gibbons where their ranges overlap. No records exist, however, of fertile hybrids between different gibbon genera, either in the wild or in captivity.
Description
One characteristic aspect of a gibbon's anatomy is the wrist, which functions something like a ball-and-socket joint, allowing for biaxial movement. This greatly reduces the amount of energy needed in the upper arm and torso, while also reducing stress on the shoulder joint. Gibbons also have long hands and feet, with a deep cleft between the first and second digits of their hands. Their fur is usually black, gray, or brownish, often with white markings on hands, feet and face. Some species, such as the siamang, have an enlarged throat sac, which inflates and serves as a resonating chamber when the animals call. This structure can become quite large in some species, sometimes equaling the size of the animal's head. Their voices are much more powerful than that of any human singer, although they are at best half a human's height.
Gibbon skulls and teeth resemble those of the great apes, and their noses are similar to those of all catarrhine primates. The dental formula is .Myers, P. 2000. Family Hylobatidae, Animal Diversity Web. Accessed April 05, 2011-04-05. The siamang, which is the largest of the 18 species, is distinguished by having two fingers on each foot stuck together, hence the generic and species names Symphalangus and syndactylus.
Behavior
Like all primates, gibbons are social animals. They are strongly territorial, and defend their boundaries with vigorous visual and vocal displays. The vocal element, which can often be heard for distances up to , consists of a duet between a mated pair, with their young sometimes joining in. In most species, males and some females sing solos to attract mates, as well as advertise their territories. The song can be used to identify not only which species of gibbon is singing, but also the area from which it comes.Glover, Hilary. Recognizing gibbons from their regional accents , BioMed Central, EurekAlert.org, 6 February 2011.
Gibbons often retain the same mate for life, although they do not always remain sexually monogamous. In addition to extra-pair copulations, pair-bonded gibbons occasionally "divorce". About 10% of gibbon groups studied in the wild contained more than two adults. In these cases, the limitation of food availability on group size may be relaxed, allowing more adults to congregate together without a significant increase in competition.
Gibbons are among nature's best brachiators. Their ball-and-socket wrist joints allow them unmatched speed and accuracy when swinging through trees. Nonetheless, their mode of transportation can lead to hazards when a branch breaks or a hand slips, and researchers estimate that the majority of gibbons suffer bone fractures one or more times during their lifetimes. They are the fastest of all tree-dwelling, nonflying mammals.Attenborough, David. Life of Mammals, "Episode 8: Life in the Trees", BBC Warner, 2003. On the ground, gibbons tend to walk bipedally, and their Achilles tendon morphology is more similar to that of humans than that of any other ape.
Diet Gibbons' diets are about 60% fruit-based,Gibbon - Monkey Worlds Retrieved Feb-12-2015 but they also consume twigs, leaves, insects, flowers, and occasionally birds' eggs. Levels of frugivory vary between populations and species of gibbons and are best predicted by local fruit availability. The most folivorous gibbon species come from the genus Nomascus, whose higher reliance on leaves is thought to be because they live in high altitude seasonal habitats that lack year-round abundant fruits. Long-term field studies by David Chivers in Bornean rainforests revealed gibbons' strong seasonal preference for non-fig fruits, informing reintroduction and habitat protection efforts for lesser apes.
Genetics
Gibbons were the first apes to diverge from the common ancestor of humans and other great apes about 16.8 Mya. With a genome that has a 96% similarity to humans, the gibbon has a role as a bridge between Old World monkeys, such as macaques, and the great apes. According to a study that mapped synteny (genes occurring on the same chromosome) disruptions in the gibbon and human genome, humans and other great apes are part of the same superfamily (Hominoidea) with gibbons. The karyotype of gibbons, however, diverged in a much more rapid fashion from the common hominoid ancestor than other apes.
The common ancestor of hominoids is shown to have a minimum of 24 major chromosomal rearrangements from the presumed gibbon ancestor's karyotype. Reaching the common gibbon ancestor's karyotype from today's various living species of gibbons will require up to 28 additional rearrangements. Adding up, this implies that at least 52 major chromosomal rearrangements are needed to compare the common hominoid ancestor to today's gibbons. No common specific sequence element in the independent rearrangements was found, while 46% of the gibbon-human synteny breakpoints occur in segmental duplication regions. This is an indication that these major differences in humans and gibbons could have had a common source of plasticity or change. Researchers view this unusually high rate of chromosomal rearrangement that is specific in small apes such as gibbons could potentially be due to factors that increase the rate of chromosomal breakage or factors that allow derivative chromosomes to be fixed in a homozygous state while mostly lost in other mammals.
The whole genome of the gibbons in Southeast Asia was first sequenced in 2014 by the German Primate Center, including Christian Roos, Markus Brameier, and Lutz Walter, along with other international researchers. One of the gibbons that had its genome sequenced is a white-cheeked gibbon (Nomascus leucogenys, NLE) named Asia. The team found that a jumping DNA element named LAVA transposon (also called gibbon-specific retrotransposon) is unique to the gibbon genome apart from humans and the great apes. The LAVA transposon increases mutation rate, thus is supposed to have contributed to the rapid and greater change in gibbons in comparison to their close relatives, which is critical for evolutionary development. The very high rate of chromosomal disorder and rearrangements (such as duplications, deletions or inversions of large stretches of DNA) due to the moving of this large DNA segment is one of the key features that are unique to the gibbon genome.
A special feature of the LAVA transposon is that it positioned itself precisely between genes that are involved in chromosome segregation and distribution during cell division, which results in a premature termination state leading to an alteration in transcription. This incorporation of the jumping gene near genes involved in chromosome replication is thought to make the rearrangement in the genome even more likely, leading to a greater diversity within the gibbon genera.