SLEEPING POSTURES OF CAPTIVE CHIMPANZEES

Abstract

Most primates sleep about half of the day. However, most researchers have studied them while they are active and have not observed their behavior while they sleep. Chimpanzees sleep on beds made in trees at night and have prone, supine, and lateral sleeping postures. However, it is not clear which postures they prefer and how many times they change their sleeping posture at night. Therefore, we investigated the amount of time spent in a sleeping posture in captive chimpanzees and the number of times they rolled over. Six chimpanzees (two adult males, three adult females, and a juvenile) were observed at the Great Ape Research Institute, Hayashibara, on May 2008. The chimpanzees spent the night in the same room and they could make their own bed with straw on a floor covered with sawn wood. Their behaviors were recorded with infrared video cameras and a total of 95 hr of observation data were collected in two nights. Eight postures were defined based on direction, and we found that adult chimpanzees preferred the lateral posture. The chimpanzees changed their sleeping posture about five times an hour, and the duration of one posture ranged widely from 0.1 to 60.8 minutes. An average of 56% (range = 39-71) of the rolling over occurred about the same time as in the other chimpanzees. These results suggest that chimpanzees have both deep and shallow sleep and are aware of others while they are sleeping.