PRIMATES, 24(4): 467-474, October 1983 467 Selecting Nest-sites of Sumatran Orang-utans, Pongo pygmaeus abelii in the Gunung Leuser National Park, Indonesia JITO SUGARDJITO National Biological Institute, Indonesia and State University of Utrecht ABSTRACT. In the Gunung Leuser National Park of Indonesia a population of orang-utans has been observed. The location of 172 night-nest sites has been examined with respect to their position in relation to the last food tree visited during the preceding day and with respect to their height above the ground. There were significant differences between the sex-age classes in the distribution of dis- tances between nest-sites and the last visited food tree. The more vulnerable animals nested at greater distances and also tended to nest at greater height. These aspects of nest-sites sdection have been interpreted in terms of an anti-predator strategy: the more vulnerable animals avoid nesting in fruit trees; instead they prefer more cryptic, unpredictable and inaccessible sites. INTRODUCTION All great apes (Pongidae) in the wild build nests for sleeping at night. Since a new nest is built everynight and nests are not used as a home base or for breeding, its function is as a sleeping place. For this reason, it is distinguishable from the more permanent nests of other mammals, birds and reptiles (HEIDIGER, 1977). Many field studies of wild orang-utans have been made in the last 20 years (SCHALLER, 1961 ; YOSHmA, 1964), but few reports have appeared on the nesting behaviour of this animal (DAVENPORT, 1967; HORR, 1972; RODMAN, 1973; MACKINNON, 1974; GALDIKAS-BRINOA- MOtrR, 1975; RIJKSEN, 1978). The nesting behaviour of semi-wild orang-utan has been de- scribed in more detail by HAI~ISSON (1969), whereas a comparison of nest building patterns and construction performances of the three great apes was tested under the relatively con- trolled laboratory conditions by BERNSTEIN (1969). Animals clearly select the site in which they sleep, but our understanding of how orang- utans make that selection is not yet clarified. SCHALLER(1961) suggests that nests may be placed anywhere in a tree as long as there is sufficient support and suitable quantity of small branches from which to construct a stable platform. MACKINNON (1974) proposed that some tree species are selected for nesting because of their leaf edibility. Many authors suggest that orang-utans select night nests on slopes at the edge of vegetation breaks, so as to achieve a good outlook (HARI~SSON, 1969; MACKINNON, 1974; RIJKSEN, 1978). It was our impression that the animals in the population differed in their selectiveness with respect to sleeping sites, the elder, less vulnerable animals being less selective than the younger more vulnerable animals. Whether this is indeed true is relevant in connection with the ques- tion, whether orang-utans are still subject to the selective pressures of predation, despite their arboreality and their large body size. In this paper we address the following question: Do the animals sleep in the last food tree that they happen to have visited or do they move on to an- other tree? Do the animals differ in this respect? Does their behaviour depend on the nature of the food tree last visited 9.