Do Predators and Thermoregulation Influence Choice of Sleeping Sites and Sleeping Behavior in Azara's Owl Monkeys (Aotus azarae azarae) in Northern Argentina?

The spatiotemporal aspects of sleeping behavior are indicative of the ecological pressures that primate species face. We investigated the potential influence of predation and thermoregulatory constraints on sleeping site choice and sleep-related behaviors in a population of cathemeral owl monkeys (Aotus azarae azarae) in Formosa, Argentina. During 10 mo, we recorded data on 153 diurnal sleeping bouts (N = 5 groups), sleeping tree physical characteristics (diameter at breast height [DBH], height, foliage), sleeping sites within the tree, and grouping and positional behavior while sleeping. We also conducted a vegetation survey of potential sleeping trees. Our study groups used only 17 of 58 available tree species, slept in the top fifth of trees, and slept at sites midway between the trunk and crown exterior. Tree DBH, height, and sleeping site height varied among groups according to the forest subtypes within their home ranges. Group members slept in a huddle when temperatures were between 5A degrees and 35A degrees C, and slept separately only with temperatures > 20A degrees C. During the wet, hot summer, they slept more frequently under dense foliage; in the dry, cold winter, they preferred sites with light foliage and direct sun exposure, potentially to facilitate sunbathing, which occurred almost exclusively during the winter. While several sleeping site characteristics were consistent with minimizing predation risk, our results also suggest a tradeoff between predator exposure and warmer conditions at sleeping sites, especially during winter. Our results support the predation avoidance and thermoregulatory constraint hypotheses, furthering the possibility that these pressures were also contributing factors in the evolution of their cathemeral activity pattern.