Sleep and social status in captive gelada baboons ( Theropithecus gelada)

Sleep was investigated in 10 captive gelada baboons ( Theropithecus gelada), belonging to two harem groups by continuous infrared video recording ( n=4 males, n=3 females, n=3 juveniles). The aim was to investigate the relation between sleep and social status. Social status was assessed during daytime activities, when the two harem groups interacted. Three behavioral states (waking, transitional sleep and relaxed sleep) as well as sleep fragmentation were scored based on movements and body posture. The individuals belonging to each of the harem groups spent most of the night huddled closely together within a sleeping cluster. Sleep was considerably fragmented in all adult and subadult individuals. No relation was found between sleep latency or sleep fragmentation and social rank. Total sleep time was 11.4±0.5 h per night ( n=10) and was negatively correlated with age. In the four males sleep duration was unrelated to their social rank, whereas both within the females and the juveniles it increased with decreasing rank. The amount of relaxed sleep was lower in the dominant males and the dominant females compared to the corresponding low-ranking ones. In contrast, dominant males had the highest amount of transitional sleep, while in the females no rank-association was evident. These results indicate that the high-ranking geladas engaging less in a relaxed sleeping posture may be maintaining a larger degree of alertness that would enable them to react quickly to nocturnal dangers.