Sleeping Site Selection, Sleep-related Positional Behavior, and Social System of Philippine Tarsiers (Carlito syrichta) Inhabiting a Protected Forest Fragment in Mindanao Island, Philippines

Sleep and sleep-related behaviors are integral components of primate behavior and ecology that are linked to various habitat-specific factors and often are modified in response to environmental fluctuations and threats. Despite the growing literature on primates, information on sleep-related behaviors of tarsiers remains sparse, especially for Philippine tarsiers, where our current understanding is hampered by the paucity of studies on wild populations. We radio-tracked eight Philippine tarsiers inhabiting a protected forest fragment between December 2016 and February 2018 to assess their sleeping site selection, sleep-related positional behavior, and social system. From 266 instantaneous sampling bouts, where each tarsier was observed a maximum of once per day, we found that the tarsiers frequently used trees with thickets of lianas and at least 15 plant species as sleeping sites. They slept from ground level to above 8 m, with most sightings at 3 to 6 m, and recurrent detections above 8 m, and preferred a flexed resting posture, on angled or horizontal, small-sized lianas or branches. Although frequently solitary, they also slept in pairs and in small groups of up to four individuals. We inferred polygyny based on observations of one adult male who associated and huddled with two nonassociated adult females, and direct male care through huddling and accompaniment. Our study revealed increased height use and gregariousness not previously described for Philippine tarsiers, shedding light on potential flexible behaviors by nocturnal primates in fragmented landscapes. Sleep-related behaviors present another avenue of research that has important implications on their conservation.