Long-term Population Dynamics and Conservation of Proboscis Monkeys (Nasalis larvatus) in the Lower Kinabatangan, Sabah, Malaysia

Abstract

Long-term ecological monitoring is essential for evaluating the effectiveness of conservation actions for long-lived tropical primates, yet few studies have integrated repeated population censuses with habitat-change analyses. We evaluated long-term population and habitat dynamics of the endangered proboscis monkey (Nasalis larvatus) in the Lower Kinabatangan, Sabah, Malaysian Borneo, by integrating a landscape-wide population survey conducted in 2025 with previous surveys from 2005 and 2014, long-term monitoring at three representative sites, and spatial analyses of land-use and land-cover change. Standardized late-afternoon boat-based surveys covering approximately 223 km of the Kinabatangan River and its tributaries recorded an estimated 2,420 individuals in 174 social groups, representing an increase of approximately 66% and 72% respectively compared with the 2005 survey. Group sizes (number of individuals per social group) remained largely unchanged despite the overall increase in population size. Long-term monitoring at three representative sites indicated stable population trajectories throughout the study period. Habitat analyses further revealed an increase in natural vegetation and limited conversion of forest to oil palm within protected areas between 2005 and 2025. Together, these independent lines of evidence indicate that the Lower Kinabatangan proboscis monkey population has recovered while maintaining overall demographic stability under long-term habitat protection. Nevertheless, substantial areas of suitable habitat remain outside the protected-area network, highlighting opportunities for additional habitat restoration and improved landscape connectivity. Our findings demonstrate the value of integrating long-term ecological monitoring with landscape-scale habitat assessment to inform adaptive conservation management in fragmented tropical landscapes.