269 in which animals completely disappear. Second, nest building in prosimians is largely instinctive, whereas immature great apes tend to build better nests sooner when exposed to adults who build nests (Lethmate 1977; McGrew 2004; Videan 2006). This suggests an important role for learning and possibly innovation in great ape nest building, and therefore also opens up the possibility of cultural variation. Orangutans also regularly build nests (Galdikas 1978; Sugardjito 1983). Most commonly it is a new nest in a tree or a combination of trees, but on 19.1 Introduction Nest building is rare in the primate order. Of all primate taxa, only Prosimians and great apes build nests. Prosimians build nests that are used for longer periods of time and use them to sleep and keep their young. The nests of great apes dif- fer from those of prosimians in two major ways. First, great ape nests are flat platforms that tend to get built anew every day and tend to be used only once (Fruth and Hohmann 1996; McGrew 2004) rather than three-dimensional constructions CHAPTER 19 Nest building in orangutans Didik Prasetyo, Marc Ancrenaz, Helen C. Morrogh-Bernard, S. Suci Utami Atmoko, Serge A. Wich and Carel P. van Schaik 19-Wich-Chap19.indd 269 19-Wich-Chap19.indd 269 7/7/2008 4:06:41 PM 7/7/2008 4:06:41 PM