What Makes Wild Chimpanzees Wake Up at Night?

I examined the possible cause of night awakening among wild chimpanzees (Pan troglodytes) in Mahale Mountains National Park, Tanzania. Chimpanzee vocalizations and activity-related sounds (CVSs) were used to indicate awakening because I was unable to visually observe them. Over a 5-night observation period, CVSs (n = 128) were heard every night, and most (n = 91) were observed within 5 min of previous CVSs. Chimpanzees use CVSs as social communication to maintain spatial contact with other chimpanzees who occasionally travel at night. The first sound in a sequence of CVSs (CVS bout) was heard immediately following the vocalization or sound of another animal (n = 11), defecation or urination by a chimpanzee (n = 7), or unknown (n = 19). CVS bouts were longer when preceded by defecation or urination than when preceded by the vocalization or sound of other animals or an unknown factor. This suggests that the degree of wakefulness varies according to the possible cause of the disturbance. CVSs at night may be provoked by various factors, and awakening during the night is probably common among diurnal primates.