American Journal of Primatology 46:35–62 (1998) © 1998 Wiley-Liss, Inc. Sleeping Sites, Sleeping Places, and Presleep Behavior of Gibbons (Hylobates lar) ULRICH REICHARD* Center for Conservation Biology, Department of Biology, Mahidol University, Bangkok, Thailand The sleeping habits of wild white-handed gibbons (Hylobates lar) were investigated to assess the risk of predation and predation-avoidance be- havior. Sleeping sites were distributed throughout home ranges, includ- ing areas where they overlapped with neighbors, and appeared to be selected independently of habitat characteristics. Individuals did not build night nests or otherwise manipulate the vegetation around the sleeping place but slept on open branches. Group members usually slept in sepa- rate trees, and, except for females with infants, they never shared a sleep- ing place. Sleeping trees were entered several hours before dusk and were used for about 14–17 h. The majority of sleeping trees were used only once, and fewer were selected repeatedly by the same or other group members. Usually females with infants went into a sleeping tree first, then juveniles, and last were mostly subadult and adult males. Intra- group competition over access to a sleeping place was observed once. Av- erage time difference between the first and last group member to enter a sleeping tree was 13 min. The sequence of departure from sleeping trees was more variable. Gibbon sleeping habits seem to primarily reflect ad- aptations to minimize predation risk. The predation-risk hypothesis was indirectly supported by observations of mobbing pythons, alarm calls given in response to birdes of prey flying low over the canopy, and more impor- tantly by 1) the predominant use of large sleeping trees, which were among the tallest trees available, particularly by adult females with small infants and juveniles, 2) an unpredictable long-term pattern of reuse of sleeping places, and 3) inconspicuous presleep behavior. Am. J. Primatol. 46:35–62, 1998. © 1998 Wiley-Liss, Inc. Key words: gibbons; Hylobates lar; sleeping sites; sleeping habits; predation risk; predation-avoidance behavior INTRODUCTION Avoiding predators and finding food are crucial elements of survival strate- gies of primates and different adaptive responses to these pressures presumably led to the large array of social grouping arrangements seen in the primate order Contract grant sponsor: German Academic Exchange Service. *Correspondence to: Ulrich Reichard, Max-Planck-Institut für evolutionäre Anthropologie, Inselstraße 22, D-04103 Leipzig, Germany. Received 22 February 1997; revision accepted 3 March 1998