From high altitude swamp forest to dipterocarp forests, the lowland and montane forests of Borneo support a truly remarkable biodiversity. Most famously home to charismatic large mammals such as Sumatran rhinoceros and tiger, the small mammal fauna is equally impressive. Along with western Sumatra, northern Borneo is home to the highest density of globally threatened small mammals. Furthermore, northern Borneo is the region with the highest density of Data Deficient species, many of which are probably endangered. Some distinctive features of the Borneo forests small mammal fauna include the highest number of squirrels (34 species), tree-shrews (9 species) and old world porcupines (4 species) of any of the SMSG’s Key Regions. Notable examples of globally threatened species from this fauna are two Murids – the Rajah Spiny Rat and Whitehead's Spiny Rat – and three flying squirrels from the Petinomys genus - Whiskered Flying Squirrel, Temminck's Flying Squirrel and Vordermann's Flying Squirrel. There are no AZE trigger small mammals within this Key Region.

Loss of forest through industrial-scale logging, palm oil plantations and subsistence agriculture is the number one threat to these forests and its small mammals.

Borneo’s forests are among the highest conservation priorities in the world with multiple national and international NGOs working to try and save these forests to benefit both wildlife and people. This work ranges from direction protection of forests and wildlife, lobbying governments and other institutions to working with global business to improve the sustainability of the palm oil industry. The protected area network in Northern Borneo flags as being particularly inadequate for its coverage of threatened small mammal distributions. Its vital that the needs of many tens of globally threatened small mammals are taken into account in large-scale forest conservation initiatives in this region.

Northern Borneo has disproportionately the highest density of data deficient small mammal species in the world, for which we know almost nothing. Across the whole SMSG Key Region there are 18 DD species, second only to Sulawesi. Surveys are desperately needed to map the distributions of these species and to quantify population change in order to assess them for extinction risk. And new small mammal species are regularly being described from across Borneo, and no doubt there are many more to discover.