AbstractDescriptive and quantitative observations were made of wild Guinea baboons (Papio papio) at a regularly used sleeping tree (Ceiba pentandra) in Senegal. Observations concentrated on the transition from darkness before sunrise until the baboons had left the tree. Behavior at sleeping sites is affected by a variety of social and nonsocial factors. Sleeping postures were adjusted during the night and to suit weather conditions. The baboons began to leave the tree earlier before sunrise in the dry season, especially on moonlit mornings. This might reflect increased foraging demands in the dry season. Sleeping parties were larger in the wet season, the difference being mainly due to juveniles. Huddling in the sleeping tree was little affected by weather conditions. The most common sleeping huddles consisted of adult females and young; huddles rarely contained more than one adult male. Although the sleeping tree was a safe refuge from leopards, the first baboon to leave the tree in the morning was usually an adult male. It has been hypothesized that the buildup of intestinal parasites in feces below sleeping trees influences the use of the trees by baboons; this did not occur here.