When the Lights Go Out: Studying Sex Differences in Sleep Latency, Sleep Quality, and Social Behavior in Captive...

Assessments of nonhuman primate (NHP) affiliation are typically restricted to daytime behavior. There is, however, a growing interest in understanding NHP behavior and welfare over the 24-h period. To assess the sleep quality and sociality of rhesus macaques, we measured the time it took pair-housed macaques to fall asleep (sleep latency), the proportion of time macaques spent asleep at night, and whether these behaviors were mediated by sex and daytime affiliation. We studied the behavior of six female-female and six male-male rhesus macaque pairs (24 animals). Five daytime observations were done cage-side per subject, and nighttime recordings captured 5 nights/pair. The pairs of macaques fell asleep in a coordinated fashion-the sleep latency of cagemates was significantly correlated-but males fell asleep almost three times faster than females. Despite the sex difference in time to fall asleep, there was no difference in the proportion of the night that males and females were observed asleep, and all pairs were observed to be asleep the majority of the time at night. At night, both male and female macaques were significantly more likely to be asleep than awake when in contact with their partner and they spent a greater proportion of time in proximity to their cagemate at night than during the day. This study emphasizes the value of co-housing NHP for their wellbeing and given our finding that daytime sociality did not predict nighttime social behavior, highlights the need to consider the behavior of NHP over 24-h.