Wild orangutans maintain sleep homeostasis through napping, counterbalancing socio-ecological factors that interfere...

Sleep is a vital physiological process that lab-based studies of model species, including humans, have shown is homeostatically regulated-i.e., pressure to sleep builds during wakefulness and dissipates during sleep. However, how wild animals maintain sleep homeostasis and how socio-ecological pressures interfere with their sleep remain understudied. Here, we investigated sleep homeostasis and the factors that influence sleep duration among wild Sumatran orangutans (Pongo abelii), leveraging a comprehensive long-term dataset of their behavior, sociality, and ecology. We quantified sleep in 53 adult individuals using the time that an individual spent in a sleeping nest-i.e., its sleep period-as an indicator of time spent sleeping. We found that, after shorter nighttime sleep periods, orangutans' next-day cumulative nap period duration was longer and that shorter nap periods were associated with a higher number of naps on the same day. We also found that orangutans had shorter sleep periods (night and day) when they associated with more conspecifics. Orangutans also had shorter nighttime sleep periods when they traveled farther the day before, and they had longer cumulative nap periods on days when (1) they ate fewer calories, (2) the ambient temperature was cooler, and (3) it rained. Our results suggest that multiple factors shape wild orangutans' sleep behavior and that orangutans compensate for lost sleep via daytime napping. This supports the hypothesis that social and ecological pressures interfere with sleep among wild animals and that they must balance the costs and benefits of sleep with those of other critical activities.