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From 1976 to 2005, we observed 100 captive gibbons (Hylobatidae) in all 4 genera at the Gibbon Conservation Center (GCC) and noted a behavior not previously thought to be part of the ethogram of gibbons-hostile presenting-in 25 of 66 individuals of Hylobates. We observed gibbon-to-gibbon hostile presenting, featuring anogenital presenting combined with agonistic behavior, only in parent-reared gibbons. When directed toward humans, the expression of the behavior varied with the displaying gibbon's rearing condition. We here describe the behavioral details of hostile presenting as expressed in captivity.

The results of a three year research project on the ecology, behaviour and conservation of the Sumatran orang utan are discussed. The 150 hectares Ketambe study area lie within the boundaries of the Gunung Leuser. reserve in Aceh Tenggara, and consists of mixed rainforest typical of hilly regions.The orang utan's food is irregularly distributed in quantity, space and time within this area. Although the orang utan is frugivorous he also eats leaves, insects, 'bark' and several other less important foods, such as bird's eggs. By far the largest part of the 114 food plants collected is typical primary rainforest growth. Figs make up an important part of the diet, particularly those known as the 'strangling' Ficus spp. As the fruiting seasons of the various strangling fig species are staggered over the whole year, and such trees are often enormous in size, they form particularly suitable food sources. Orang utans appear to have a good topographical knowledge of their 'home range'. There are indications that fig trees bearing ripe fruits are sometimes found by using the flight paths of flocks of hornbills, which also feed on figs. With respect to figs in particular, the orang utan must compete for its food with many other species, notably primates.The study area was regularly used by 22 individually recognized orang utans. Both males and females live in homeranges which overlap considerably and are 2-10 km sq. in extent. The population density of the Ketambe area is considerably higher than that reported for other areas in Indonesia, being approximately 5 individuals per square kilometre. Orang utans live a 'limited gregarious' lifestyle, that is, adult males spend the largest part of their time alone, and avoid encounters with other adult males. The adult females and their offspring stay together for the first 4-5 years of life (the birth interval is at least 3 years.). Adolescent social groups form after the mother-offspring bond becomes weaker. This social phase conti