Borneo, the third-largest island globally, is a massive biological repository and a globally recognized biodiversity hotspot in Southeast Asia. The vast, ancient tropical rainforests of Borneo, some estimated to be over 100 million years old, have provided the stable conditions necessary for immense biological richness to evolve. The unique array of habitats, from lowland peat swamps to montane cloud forests, supports an incredible variety of plant and animal species. This distinct geographical setting has fostered a natural world where many life forms exist only within the island’s boundaries.
The Definition of Endemic Species
The unique nature of Borneo’s fauna is best understood through the concept of endemism, which describes species found exclusively in a specific geographical area. Borneo’s long history of isolation from mainland Asia has allowed its wildlife to evolve along its own distinct evolutionary path. During periods of lower sea levels, Borneo was connected to the continent as part of the Sundaland landmass, but rising waters after the last Ice Age separated the island, leading to the speciation of many populations.
This geographical phenomenon resulted in a high percentage of Bornean species being endemic across nearly all taxonomic groups. For example, of the estimated 222 land mammal species on Borneo, 44 are endemic. Among the approximately 620 bird species, around 10% are found only here, which includes 61 endemic species.
The island’s amphibian life shows an even higher rate of uniqueness, with roughly 73% of the 178 known amphibian species being endemic. This high level of endemism extends to plants, where an estimated 28.4% to 37% of Borneo’s flora is found solely on the island. The concentration of unique species highlights why Borneo’s continued conservation is important for global biodiversity.
Borneo’s Iconic Primate and Apex Predator Populations
Borneo is home to some of the world’s most recognizable large mammals, many exclusive to the island’s rainforests.
Bornean Orangutan
The Bornean Orangutan (Pongo pygmaeus), often called the “person of the forest,” is the largest arboreal mammal on Earth. It is characterized by its coarse, reddish-brown coat and extremely long arms. Mature males develop large, fleshy cheek pads (flanges) and a throat sac used to produce booming, long-distance calls to mark their territory. These intelligent great apes spend most of their lives in the tree canopy, foraging for over 500 species of fruit and acting as important seed dispersers.
Proboscis Monkey
Another remarkable primate, endemic to Borneo, is the Proboscis Monkey (Nasalis larvatus), instantly identifiable by the male’s large, pendulous nose. This arboreal species is found in coastal mangrove forests and lowland areas close to rivers, feeding primarily on young leaves and unripe fruit. Males use their sizeable noses to attract females and amplify calls meant to intimidate rivals.
Bornean Pygmy Elephant
Borneo is also home to the Bornean Pygmy Elephant (Elephas maximus borneensis), the smallest subspecies of elephant globally. Found in the northeastern part of the island, these elephants are generally less than 8.2 feet tall and are distinguishable by their relatively large ears and round bellies. These social animals live in matriarchal herds and primarily inhabit lowland forests and river valleys.
Sunda Clouded Leopard
The island’s largest terrestrial predator is the Sunda Clouded Leopard (Neofelis diardi), a secretive cat native to both Borneo and Sumatra. This medium-sized predator is recognizable by its coat, which is marked with large, irregular, cloud-like patterns. The cat possesses the longest canine teeth relative to its skull size of any living cat species. Its long tail and flexible ankle joints make it a highly effective climber, enabling it to hunt prey in the dense canopy.
Hidden Diversity of Avian, Reptilian, and Amphibian Life
Borneo’s terrestrial habitats host a stunning array of avian, reptilian, and amphibian species.
Avian Life
The island is home to eight species of Hornbills, which are culturally significant to the Dayak people and function as primary seed dispersers. The largest species, such as the Rhinoceros Hornbill, are characterized by a large, hollow structure called a casque atop their beaks.
Reptiles
The island’s reptiles include the bizarre, endemic Earless Monitor Lizard (Lanthanotus borneensis). This semi-aquatic lizard has a sleek, elongated body and lacks external ear openings. Another unusual reptile is the “flying” Draco lizard, which glides between trees by extending skin flaps attached to its highly mobile ribs.
Amphibians
Borneo is a center of amphibian diversity. This includes the visually striking Wallace’s Flying Frog (Rhacophorus nigropalmatus), which uses its large, fully webbed feet to glide between trees. A remarkable example is the Bornean flat-headed frog (Barbourula kalimantanensis), the world’s only known lungless frog, which respires entirely through its skin. These amphibians are highly sensitive to environmental changes, making them important indicators of the rainforest’s health.
Fauna Specific to Aquatic and Cave Ecosystems
Borneo’s intricate network of rivers, estuaries, and limestone cave systems supports specialized fauna distinct from general rainforest life.
Aquatic Life
The major river systems are home to a critically endangered, freshwater population of the Irrawaddy dolphin (Orcaella brevirostris), locally known as pesut. This population is genetically distinct from its oceanic counterparts. The island’s waterways are also patrolled by two major crocodilians: the Saltwater Crocodile (Crocodylus porosus) and the slender-snouted False Gharial (Tomistoma schlegelii), a fish-eater preferring freshwater swamps and rivers. Montane streams host specialized fish, such as the Sucker Fish (Gastromyzon species), which uses modified fins to cling to rocks in fast-flowing currents.
Cave Ecosystems
Extensive limestone caves, such as the Gomantong system, form subterranean ecosystems reliant on external resources. Millions of bats and Cave Swiftlets use the caves for roosting, leaving behind guano that forms the base of the food chain. This guano supports a large community of invertebrates, including cockroaches and crickets, which attract predators like the Cave Racer snake. The swiftlets construct their nests from their own saliva, a delicacy harvested for centuries.