Restricted to the deciduous monsoon and evergreen rainforests of Southeast Asia, the lesser apes – gibbons and siamangs – form the family Hylobatidae, which includes four genera and at least sixteen species.
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Similar to the great apes, gibbons lack a tail and have the same dental formula. But they differ in their smaller size, slender form, exceptionally long arms, longer canine teeth and the presence of sitting pads. In most species, males and females are the same size but are often sharply distinct in their coloration. Unique to the family, the basal part of the thumb is freed from the palm, allowing for superior locomotion through the trees.
Almost entirely arboreal, the gibbons propel themselves through the trees by brachiation, suspended from their uniquely long arms – the longest relative to body size of any primate. Using their hands to hook over branches, they swing themselves in daredevil leaps through the forest canopy.
Gibbons can spring nine meters or more in a single bound, moving swift as a bird with fluid grace. (However, one study found that as many as 30 percent of adult gibbons had suffered broken bones.) When they need to cross short distances on the ground, gibbons walk bipedally with their arms held upright.
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Until recently all gibbons were thought to be monogamous, much like marmosets; but as with marmosets, new research has shown that their social arrangements can be much more involved. A family group of gibbons often consists of an adult pair and two immature offspring. Bonded pairs are strongly territorial, defending a compact home range with singing calls and aggressive displays.
Gibbon songs are unique to each species, and males and females have their own distinctions as well. Bonded males and females will often combine their songs in an elaborate duet that announces their territory and reinforces the pair-bond. Duets are usually sung for less than half an hour each day, but males may call on their own for two hours or more – and loudly enough to be heard from as far as two miles away.
Threats to Lesser Apes
As elsewhere, human populations continue to increase in Southeast Asia, and the accelerating destruction of habitat
for logging and agriculture has had a devastating effect on every species of gibbon. Each of the sixteen species of gibbon is threatened with extinction, some of them direly so. A recent survey for one of the rarest subspecies, the Yunnan white-handed gibbon, concluded that the populations in southernmost China have been completely extirpated, and it is unknown whether they still survive anywhere else.
It is estimated that there are no more than 250 adult male individuals of the silvery Javan gibbons (Hylobates moloch), occupying barely two percent of their original habitat. The world’s most endangered primate is the Hainan gibbon, Nomascus hainanus, of which no more than 17 individuals are known to survive.
If gibbons are to survive the next few years, the highest priority must be given to establishing and enforcing protected areas in suitable habitat for their remaining populations. Breeding centers and broad-based campaigns of conservation education, as well as stricter and better-enforced land management, will be needed to prevent the wholesale extinction of these exceptional primates.
LEARN MORE: Discover the threats for all primates