Nocturnality and activity budgets of owl monkeys in tropical ecosystems

The subtropical populations of owl monkeys (Aotus) and some lemurs have flexible patterns that include activity during both day and night. Early studies on the activity patterns of tropical owl monkeys in seminatural conditions evidenced their nocturnal behavior and a strong influence of moonlight on their activity. In this chapter, we review previous and ongoing studies of activity budgets in tropical owl monkeys. Prior field studies provided valuable but biased accounts on the activity budgets of tropical owl monkeys. Recent data on their behavior in the wild suggest they spend almost half of the night resting and have peak activity at twilight with their activity being strongly influenced by moonlight. Studies on activity budgets of owl monkeys in tropical areas are key to understand the evolution of their nocturnal behavior in tropical ecosystems and how they secondarily became cathemeral in subtropical ecosystems, as an adaptive response to environmentally stringent conditions.