Manoeuvring mangroves: a preliminary study on the use of a mangrove reserve by Philippine tarsiers (Carlito syrichta) on Siargao Island, Philippines

Mangrove forests are highly threatened ecosystems, regarded as important habitats for many non-human primates, including tarsiers. However, for Philippine tarsiers (Carlito syrichta; formerly Tarsius syrichta), our understanding of their relationship with mangroves remains poor. Here, we explored how Philippine tarsiers used mangrove habitats by locating their sleeping sites, describing their sleeping behaviours, and estimating their home ranges. We surveyed Philippine tarsiers from 24 June to 10 July 2025 in a mangrove forest within the Del Carmen Mangrove Reserve on Siargao Island. We were able to record 44 sleeping sites from six Philippine tarsiers that were radio-tracked. The tarsiers frequently slept on screw pines, non-mangrove trees, and thickets, although they also used true mangrove species (Rhizophora sp. and Nypa fruticans) within the flooded portions of the site. Adult males were skittish and consistently slept solitarily, whereas the adult females were often accompanied by their juvenile offspring. From the three adult males we fitted with GPS tags, we estimated minimum home range sizes (±SE) of 3.16 ± 0.59 ha based on minimum convex polygons and 4.22 ± 1.34 ha based on kernel density estimates, with percentages of mangrove overlap ranging from 17.56 to 72.99%. Our observations underscore the value of mangrove habitats as potential refugia for supporting insular primate populations against predators, disturbances, and stochastic events, and as evolutionarily significant ecosystems that, along with their behavioral flexibility, possibly contributed to their dispersal across open oceans. We recommend extending ecological research of tarsiers in mangroves and integrating mangrove protection in tarsier conservation.