Gibbons span the semi-evergreen rainforests of mainland Asia north of the isthmus of Kra and the evergreen rainforests of the islands of the Sunda Shelf. The latter comprise the main gibbon habitat, but significant numbers of taxa and individuals occur in the more seasonal forests of mainland Asia, concentrated in pockets of evergreen forest, surviving in the moister areas under maritime influence, including Indochina, Thailand, Myanmar, and Bangladesh. The evergreen rainforest zone in the wet humid tropics is characterized by the main northeast monsoon early in the year, with a milder southwest monsoon in April or May, and an annual rainfall of 200 in (5,000 mm). Trees of the family Dipterocarpaceae are typical of most gibbon habitats, ranging from 1% to 43% of forest composition; there are in total about 400 trees per 2.5 ac (1 ha). Moraceae (figs) and Euphorbiaceae are the most common tree families used as food sources in gibbon habitats.
The siamang occurs more frequently in higher altitude forests. Otherwise, gibbons prefer the lowland forests, where diversity and density of fruit trees is greatest. The main features of increasing altitude are a decrease in the size of trees and in species diversity. Those smaller species that are common at higher altitude clearly provide sufficient food for sia-mang, in terms of leaves if not fruit. Altitudinal zonation is more compressed in isolated massifs such as the Malay Peninsula. At lower altitudes, there is a greater biomass and diversity of trees and, hence, of animals.
Sadly, lowland forests are the first to be cleared, as they are more accessible and have better soils. Nevertheless, gibbons can live in surprisingly small patches of surviving forest, and they cope very well (albeit at half the density initially in some areas) with selectively logged forest, since the colonizing tree species in the gaps and the proliferation of lianas provide abundant food. In a more detailed study before, during, and after selective logging in Sungai Tekam, west Malaysia, it was found that gibbon density was much less reduced than that of langurs.
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