Opportunistic behaviour or desperate measure? Logging impacts may only partially explain terrestriality in the Bornean orang-utan Pongo pygmaeus morio

There is a lack of information on how the Endangered Bornean orang-utan Pongo pygmaeus morio moves through its environment. Here we report on a camera-trapping study carried out over 2.5 years to investigate the orang-utan's terrestrial behaviour in Wehea Forest, East Kalimantan, Indonesia. We set 41 camera trap stations in an area of secondary forest, 36 in recently logged forest immediately adjacent to Wehea Forest, and 20 in an area of primary forest in the heart of Wehea Forest. A combined sampling effort of 28,485 trap nights yielded 296 independent captures of orang-utans. Of the three study sites, orang-utans were most terrestrial in recently logged forest, which may be only partially explained by breaks in the canopy as a result of logging activity. However, orang-utans were also terrestrial in primary forest, where there was a closed canopy and ample opportunity for moving through the trees. Our results indicate that orang-utans may be more terrestrial than previously thought and demonstrate opportunistic behaviour when moving through their environment, including using newly constructed logging roads for locomotion, possibly indicating some degree of resilience to human disturbance. This finding is important because of the potential role of sustainably logged forests for orang-utan conservation.