Population and Habitat of Proboscis Monkeys (Nasalis larvatus) in Northwest Sabah, Borneo, Malaysia

Abstract Habitat loss, degradation, and fragmentation continue to threaten primate conservation worldwide by altering species abundance, distribution, and connectivity. The proboscis monkey ( Nasalis larvatus ), endemic to Borneo, faces increasing pressure from these threats. We assessed its conservation status in northwestern Sabah, Malaysian Borneo. We conducted systematic boat-based surveys at 18 wetland forest sites, key habitats for the species. We measured vegetation structure, including canopy cover, tree height, and basal area, to examine how habitat quality affects monkey abundance. We also analysed land use and land cover to estimate remaining suitable habitat, levels of protection, and threats. We recorded proboscis monkeys at only seven sites (39%), with at least 75 individuals in 13 groups, mostly confined to isolated, fragmented areas with poor connectivity. The vegetation analysis showed wide variation in vegetation structure at sites where we found proboscis monkeys but no significant relationship between vegetation structure and monkey abundance. Land-use and land-cover analysis showed that only 54% of surveyed areas still provide suitable habitat, and just 43% lie within protected areas. These results suggest that while the species can tolerate some disturbance, its small population size, patchy distribution, and limited connectivity increase the risk of local extirpation. Agricultural expansion, settlement growth, planned aquaculture in mangroves, and major infrastructure projects such as the Pan Borneo Highway continue to drive habitat loss and fragmentation. Our findings highlight the urgent need for targeted conservation actions to protect remaining habitats, improve habitat quality, restore connectivity, and address site-specific threats to ensure the species’ survival in northwestern Sabah.