Investigating constraints on the survival of orangutans across Borneo and Sumatra

Abstract

Orangutans are highly endangered. In order to implement effective conservation strategies for these species, it is crucial to understand fully what constrains their distribution. Here, we use a previously developed time budget model to investigate the factors constraining the orangutans’ ability to inhabit different areas of Borneo and Sumatra, as well as the social group size they are potentially able to adopt in their habitats. This model uses data from 13 field sites, together with climate and environmental data, to predict the amount of time individuals would need to spend in each of four main activities - feeding, moving, resting, and socializing. We found that resting time, which was linked to both dietary constraints and forest cover, was a key behavioral limitation on orangutan distribution. The maximum possible group size predicted for orangutans was low over the entire range, indicating that ecological constraints limit orangutan sociality, and that even small changes to habitat quality may further reduce the maximum possible group size. The fact that the values are already approaching one in many locations and are low throughout both Borneo and Sumatra, suggests that orangutans are vulnerable to extinction over their entire current distribution.