Borneo, the third-largest island in the world, is a biodiversity treasure trove and home to over 230 species of mammals, including around 50 endemic species found nowhere else on Earth. Its lush tropical rainforests—spanning across Malaysia, Indonesia, and Brunei—shelter iconic creatures such as the Bornean orangutan, pygmy elephant, proboscis monkey, and the elusive Sunda clouded leopard. Borneo is also the only place on the planet where all five of its native wild cat species—bay cat, flat-headed cat, marbled cat, leopard cat, and Sunda clouded leopard—can be found. From sun bears and binturongs to flying lemurs, civets, and slow lorises, the island offers exceptional opportunities for mammal watching. Whether trekking through primary rainforest or exploring rivers and lowland jungles, Borneo reveals an astonishing array of mammalian life both by day and under the cover of night.