The Endangered proboscis monkey Nasalis larvatus is endemic to the island of Borneo. Habitat loss is a major threat to this species, and an understanding of long-term demographic trends is crucial for its conservation. We assessed the population trends and group sizes of proboscis monkeys over 10 years in the Lower Kinabatangan floodplain in Sabah, Malaysian Borneo. Comparisons of observed populations between 2004 and 2014 revealed significantly reduced group sizes, which is probably a result of forest fragmentation. Three long-term studies over 34–73 months in specific areas showed fluctuating estimated densities in each area, but no overall population increase or decrease. Riparian forests are the most important habitat for these monkeys, and one reason for the relatively stable population could be that there were only minor losses of forest along rivers during 2004–2014 because protected areas have been established in the region in 2005. However, proboscis monkey habitat remains under threat in areas allocated for oil palm, and protection of these areas is paramount to maintaining this population.

The population of the threatened proboscis monkey Nasalis larvatus, a Bornean endemic, in the Pulau Kaget Nature Reserve, South Kalimantan, Indonesia, is extinct. Until 1997, this small, isolated population, estimated at c. 300 individuals, had been pushed towards the fringes of the reserve by illegal agricultural expansion. As food sources became depleted, the population apparently exceeded the decreasing carrying capacity of the reserve and was reported to be starving to death. As a solution, 84 animals were translocated to nearby, unprotected sites, resulting in 13 fatalities. An additional 61 animals were taken to a zoo, where 60 per cent died within 4 months of their capture. There was neither a proper pre-translocation assessment of the suitability of the release sites, nor a proper post-translocation monitoring programme for the released animals. We conclude that the Pulau Kaget reserve and its proboscis monkeys have been poorly managed. We provide some suggestions to improve the effectiveness of conservation efforts in Indonesia.