Tropical Wildlife. Jane Whitten

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      Gibbons

      There are nine species of gibbon in the forests of Southeast Asia. Eight of these species weigh around 6 kg, and the larger siamang weighs about 10 kg. The smaller species arc geographically separated by seas and rivers, but the siamangs overlap with them in the forests of Peninsular Malaysia and North Sumatra. Gibbons are uniquely adapted to life in the forest canopy and are almost never found on the ground. They travel quickly and efficiently by swinging from branch to branch, often leaping between supports. The siamang's diet consists almost equally of fruit and leaves, while the smaller gibbons eat mainly fruit, which they pick carefully, leaving unripe fruit to ripen. They also eat leaves, shoots and insects.

      Gibbons live in monogamous families into which a single offspring is born every two or three years. Infants are weaned in their second year, but stay with their parents until they are about eight. In siamang groups, the male takes care of the infant after the first year.

      A gibbon family lives within a well-defined home territory, which the animals know well, visiting trees as they come into fruit. A gibbon group travels about 1.5 km in a day whereas the larger siamangs travel less than a kilometer. The groups sleep in tall, emergent trees, safe from predators. Before dawn, the males of some species sing to advertise the presence of the group and their occupancy of that area of forest, About a third of the active day is spent feeding and another third traveling between feeding trees.

      Siamang groups are very cohesive, with individuals rarely more than 10 m from each other, whereas individuals in a group of small gibbons are more spread out, coming together to feed in larger fruiting trees. Adults groom each other, which helps maintain the pair bond. Most gibbon pairs also perform an elaborate duet that lasts for about 15 minutes. These stereotyped songs differ between species and are thought to advertise the presence of a pair in the territory and to reinforce pair bonding.

      Mentawai gibbon, Hylobates klossii

      Javan gibbon, Hylobates moloch

      Siamang, Hylobates syndactylus

      Müller's gibbon, Hylobates muelleri

      Proboscis and other

       Leaf Monkeys

      Leaf monkeys are found through most of Southeast Asia although some species, like the proboscis monkey, have very restricted ranges. Most are relatively slender monkeys with long tails and largely arboreal lifestyles. The species are distinguished by coat color and hair patterns on the head.

      Some leaf monkeys eat nothing but leaves, but most eat flowers, buds, seeds and shoots as well. They have complex stomachs, with bacteria-filled fermenting chambers that break down leaves, releasing the normally indigestible sugars and deactivating the leaf toxins. Because the leaf monkey eats such poor quality food, it has to forage for much of the day, and its stomach contents may constitute a quarter of the animal's weight.

      Leaf monkeys live mostly in large groups comprising one male and a number of females with their offspring. An infant is born with its eyes open and is strong enough to hold onto the mother's fur while she travels. Females frequently tolerate and even suckle each other's Small infants, Excess males form groups and look for opportunities to take over from breeding males. Incoming males usually kill all the infants fathered by their predecessors, although the mothers sometimes manage to protect their infants.

      The proboscis monkey is found only in the coastal swamp forests of Borneo. The males have remarkable faces, with long pendulous noses, the functions of which have long been the source of speculation. In the past, suggestions have been made that they enable the large male to give off heat or that the noses assist in swimming by acting like some kind of snorkel. It is now thought that they serve to attract females, like the peacock's tail. Proboscis monkeys feed mostly on young leaves and travel further than most forest monkeys to obtain this scarce food. These monkeys swim well and cross large rivers very quietly, with no splashing that could attract crocodiles.

      Silvered leaf monkey, Semnopithecus cristata

      Female and juvenile proboscis monkey, Nasalis larvatus

      Male proboscis monkey, Nasatis larvatus

      Banded leaf monkey, Presbytis melalophos

      Javan leaf monkey, Presbytis comma

      Macaques

      Macaques are the typical monkey—gregarious, active and curious. They are primarily fruit eaters, but will also eat other plant parts and small animals, from insects to birds and mammals, if they can catch them. Where they come into contact with people, they will expand their diets to include garbage, peanuts and other offerings, or stolen crops. They spend more time on the ground than the leaf monkeys, but are excellent climbers and swimmers.

      Macaques are long-lived and give birth to a single infant each year, after a gestation of five or six months. The infant is born with fur and open eyes, and can immediately travel clinging to its mother's belly fur, with a nipple in its mouth to support its head. Nursing becomes infrequent after the first few months, but usually continues until the next infant is born. These social animals spend long periods in mutual grooming, which helps reinforce group cohesion.

      The long-tailed or crab-eating macaque is found in a variety of habitats, especially coastal forests and offshore islands where it frequents low trees and scrub. The pig-tailed macaque, named for the shorter tail which it carries curled over its back, is found more in inland hill forests. It is mostly arboreal, but is unique in that it will descend to the ground to flee from people. Pig-tailed macaques seem to learn more readily than other macaques, and captive pig-tails are trained throughout the region to pick coconuts. Experienced individuals are able to choose ripe fruit without help, while less experienced individuals are directed by their handlers. Both species are found in groups of up to forty individuals, but these large groups often split up to feed.

      On the island of Sulawesi there are lour distinct species of macaque, all of which are found nowhere else. They all resemble the pig-tailed macaque, hut differ in fur pattern and color.

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