Moonlight: a neglected driver of primate sleep ecology

Sleep maintains physiological and neural homeostasis, but reduced responsiveness during sleep can increase vulnerability. For diurnal primates, sleeping site selection is key to survival, balancing predation risk, energetics, and comfort. However, sensory cues such as moonlight have been largely overlooked. According to the visual acuity hypothesis, increased lunar illumination should enhance the detection of predators and reduce perceived predation risk, whereas darker nights limit visual detection which may favor the use of safer sleeping sites. In our review of 179 studies of sleeping site selection by diurnal primates, predation was the most often reported driver of site choice, with tall trees being the most common response to this pressure, yet no study mentioned moonlight as a modulating factor of sleeping site selection. We tested if moonlight shapes site selection using 384 records of sleeping locations from four capuchin (Sapajus nigritus) groups in Argentina. Moonlight was a predictable, temporally structured cue shaping nocturnal trade-offs: capuchins selected higher canopy sites on darker nights, whereas brighter nights relaxed height preferences and increased selection for denser canopy. Consistent with the visual acuity hypothesis, darker conditions may elevate perceived predation risk, favoring vertical refuge. Further research on predator responses to lunar illumination will help clarify predator–prey dynamics. These moonlight-driven shifts in primates’ nighttime resource selection may influence broader ecological patterns, including dynamics of interspecific interactions and movement ecology. Diurnal and arboreal mammals, such as most primates, spend roughly half of their lives in sleeping sites. Decisions about where to sleep are strongly shaped by the risk of nocturnal predation. However, most studies have focused solely on the structural features of sleeping sites while overlooking the role of environmental cues such as moonlight. We investigated how black capuchin monkeys (Sapajus nigritus) select sleeping trees in a subtropical forest under varying levels of lunar illumination. Capuchins consistently chose higher canopy sites during darker nights and used lower, denser foliage during moonlit nights. These results indicate that moonlight is a key factor interacting with habitat features to shape sleeping-site decisions of arboreal diurnal species, likely structuring nocturnal predator–prey interactions and broader patterns of daily behavioral ecology.