Proboscis monkeys, Nasalis larvatus (Wurmb, 1787), are indigenous to the island of Borneo and are considered one of its most emblematic species. Today the conservation status of this primate is classified as Endangered on the the IUCN Red List and listed under Appendix I of CITES, prohibiting all international commercial trade. In the Malaysian state of Sabah, the species is listed as totally protected and cannot be hunted. Continuing studies suggest that the number of proboscis monkeys has been decreasing in recent years. These studies have identified various factors contributing to this decline and its potential consequences. In order to carry out a thorough assessment of the conservation status of the species it is essential to have a good understanding of the animal`s ecology and habitat requirements and to use research-based approaches. One of such are species distribution models (SDMs) which in recent decades have become widely used tools in ecology by relating species occurrences to environmental data so as to gain ecological insights. In this work we specifically evaluated the effect of environmental parameters such as cloud cover to predict the potential distribution of the proboscis monkey in Sabah. Cloud cover, a seemingly simple atmospheric phenomenon, exerts a profound influence on a wide range of ecological biological processes, yet the assessment of its importance has remained remarkably limited. For modeling purposes the ‘flexsdm’ R (v. 3.3.3) modeling package was employed for testing out the Maximum Entropy (Maxent) algorithm, one of the most widely used SDM modeling methods. Model evaluation gave satisfactory results and the resulting model found a high level of suitability for proboscis monkeys in nearshore areas. A concerning discovery is that perhaps less than 13% of Sabah's area is suitable for proboscis monkey habitats, raising questions about their long-term viability. Cloud cover, particularly average annual cloudiness, is a key environmental factor influencing the distribution of proboscis monkeys in Sabah. The conversion of Borneo's forests to oil palm plantations can negatively impact cloud properties, potentially threatening the monkeys' habitat.
