Evolution and systematics

The Sunda Shelf emerged out of the sea as a consequence of volcanic activity about 12 million years ago (mya). It owes its uniquely rich fauna and flora to an admixture of immigrants, first from the Indian subcontinent, the Siva-Malayan fauna, and then later from China, the Sino-Malayan fauna. The frequent changes of sea level during the latter part of the Pleistocene alternately exposed the Sunda Shelf as one landmass, and then flooded it, leaving numerous islands. The gibbon populations so isolated speciated and then migrated when land bridges were restored. After the initial spread of three of the genera into different parts of the Sunda Shelf, gibbon speciation occurred within the Shelf (not on mainland Asia), followed by subsequent, sequential spread back to the mainland, with the hoolock (fourth genus in the van). The pileated and lar gibbons followed, and the Kloss, Bornean, and Javan gibbons originated on the edges of the Shelf, with agile and lar gibbons in the center. During the dry periods, the key rainforest relicts, into which gibbons and other forest animals retreated and out of which they spread when sea level rose, were in eastern Indochina and southern China, northeast Borneo, west Java, north Sumatra, and southern Myanmar, as well as the Mentawai Islands.

To determine the pattern of speciation from the ancestral gibbon, there has been thorough reanalysis of all morphological and behavioral characters by multivariate techniques. It had been difficult to resolve whether siamang, concolor, or hoolock gibbon is the most primitive, but the most prudent picture has the hoolock gibbon evolving first, followed by concolor, and then siamang; Kloss follows, and then Mueller's, moloch, pileated, lar, and agile. Patterns vary according to whether one uses cranial and dental, pelage, song, or all variables. There are still burning issues to be resolved concerning the validity of species and subspecies, especially in the northeast. Apart from clarifying distribution and abundance from lesser-known areas, DNA analysis is the best way to resolve disputes.

The genus Hylobates has now been divided into four genera: Symphalangus, the siamang (H. syndactylus) of Sumatra and Peninsular Malaysia; Nomascus comprises at least three species of crested gibbons, each with several subspecies, from southern China, Vietnam, east Cambodia, and Laos, including H. concolor in the north; H. leucogenys in the center; and H. gabriellae in the south; Bunopithecus, the hoolock gibbon (H. hoolock) of Assam, Bangladesh, and Myanmar, extending across northern

A white-cheeked gibbon (Nomascus leucogenys) on a tree branch in Southeast Asia. (Photo by Animals Animals ©Michael Dick. Reproduced by permission.)

Thailand into the southwest corner of China; and Hylobates comprises five to six allopatric species, extending from Thailand through the islands of the Sunda Shelf, including H. klossi, confined to the Mentawai Islands off the west coast of Sumatra; H. pileatus of southeast Thailand and west Cambodia; H. moloch of Java, now confined to the west; H. lar, with two or three subspecies in Thailand and Yunnan, China, one in the Malay Peninsula, and one in north Sumatra; H. agilis, with one subspecies between two lar subspecies in the Malay Peninsula, one over all of Sumatra south of Lake Toba, and one in the southwest of Borneo, west and central Kalimantan, bounded by the Kapuas and Barito rivers); and H. muelleri, with three subspecies radiating around the rest of Borneo.

In view of the extensive hybridization between the last two species in the center of the island, it may be necessary to sink H. muelleri into H. agilis, as a fourth subspecies of the latter, but it has been argued that the agile is more similar to the lar gibbon. The four genera are partly justified by molecular data indicating a split as long ago as 8 mya. It is argued that male and female solos were ancestral, but another more persuasive claim is that solos are derived from an ancestor that dueted, that dueting occurred early in gibbon ancestry. In most species, the song is split into the distinctive male and female parts of the duet.

Two populations of hybrid gibbons have been well known for many years: between H. lar and H. pileatus in Thailand and between H. lar and H. agilis in west Malaysia. The former results from a lar gibbon isolate pushed up against the pileated population in the Khao Yai National Park in Thailand; the hybrid zone is narrow, mixed social groups unstable, and gene exchange limited. A small hybrid population was discovered in the northwest of Peninsular Malaysia between H. lar and H. agilis, where a dam built in 1968 had created a lake, so that males wandering across the Mudah River and its tributaries where they were narrow were trapped on the wrong side of the lake, and they mated with females of the other species. Given the distinctive appearance and songs of the taxa, it is agreed that they are not conspecific. For conservation purposes in particular, it is vital to promote such taxa, especially as gene exchange is so limited.

The third population of hybrid gibbons, between H. agilis albibarbis and H. muelleri in the Barito watershed in the center of Borneo, now presents a very different problem. When discovered in 1979, it seemed little different from the other two restricted populations, and to be of recent origin. The agile gibbon had supposedly entered Borneo from Sumatra during a glacial period, when the sea level was low, with the Bornean (or Mueller's) gibbon having retreated to the warmer

A lar gibbon (Hylobates lar) grooming session. (Photo by Animals Animals ©Michael Dick. Reproduced by permission.)

importance. This is especially important for brachiating out under the more flexible branches and for suspending to feed in the terminal branches, where the more nutritious plant foods such as flowers and fruit are most abundant.

Those who group most Hylobates forms into two species, H. klossi and H. lar, argue that the cranial features of gibbons are very similar, making it difficult to separate them into species. The field workers respond that once theories have evolved an animal to fill this particular suspensory niche, such anatomical differences are unlikely. It is calls and pelage color and markings that are so distinctive. Females have the most readily diagnostic call, the "great call," and species are either monochromatic (black in the west and gray in the southeast) or polychromatic (in the center), and asexually or sexually dichromatic (in the north in the more open semi-evergreen habitat)—an intriguing geographical pattern. Since they both have a genetic basis, such features, with profound behavioral significance for reproduction, should be taken seriously in classifying gibbons.

Siamang emit harsh barking and booming notes—a staccato song, with the resonating boom produced by air passing across the entrance to an inflated lateral laryngeal ventricle. The hoolock gibbon also has a hooting call, but not so harsh to the ear. By contrast, the other gibbons are much more and moister maritime influence of northeast Borneo (around present-day Sabah). When they spread toward each other they were separated by the Barito and Kapuas Rivers, except in the headwaters where tree crowns intermingled across the narrower rivers. It was concluded that there has been large-scale gene flow among hybrids for about 5,000-10,000 years over at least 1,930 mi2 (5,000 km2), so that agile and Bornean gibbons should be regarded as conspecific.

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