Exploring Strategic Functions of Sleeping Sites in Crested Macaques (Macaca nigra): Evidence from Intergroup Encounters

In gregarious animals, competition for resources, such as water, food, and shelter, is common. Sleeping sites are one such resource that also may serve strategic functions, such as food or group defense. We investigated whether groups of wild crested macaques (Macaca nigra) compete for sleeping sites and two potential strategic functions of these sites: food defense and group protection. We gathered data from three habituated groups (Pantai Batu 1, Rambo 1, and Rambo 2) in Tangkoko Nature Reserve in North Sulawesi, Indonesia. We collected information on sleeping site use between October 2015 and June 2016, for 192, 156, and 41 nights, for each group respectively. We recorded 229, 207, and 183 travel routes for the three study groups and used these to calculate their home ranges and core areas using Brownian Bridge Movement Models. We also documented 304 intergroup encounters (IGE) among the three habituated groups and their neighbors. We found that suitable sleeping sites for crested macaques are limited, frequently reused, and shared sequentially, but not used simultaneously. There was a strong link between core areas and sleeping site density. However, we found no significant relationship between high IGE risk areas and sleeping site density, nor between core or high IGE risk areas and sleeping site reuse frequency. The study found no correlation between the distance traveled post-encounter to sleeping sites and the encounter's outcome or intensity. Overall, our study adds to the evidence that primates are selective in choosing sleeping sites, whose additional functions might be population specific.