The Bornean gibbon (Hylobates muelleri), also called Mueller's gibbon, is the sound that defines dawn at Niah National Park. The great call — a rising, sustained duet between a mated pair that carries over a kilometre through primary forest — starts before sunrise and continues for 15–30 minutes. The female initiates the great call with a series of short notes that accelerate into a complex climactic phrase; the male intersperses shorter hoots. The pair coordinates so precisely that it is often impossible to separate their voices until you can see them moving in the canopy.
Gibbons are strictly arboreal and rarely, if ever, descend to the ground. They are the fastest canopy-moving mammal in Borneo — moving by brachiation (arm-swinging from branch to branch) at speeds that can exceed 55 km/h in short bursts. A gibbon crossing a 20-metre gap between canopy trees is a blur of fur and limb. They are also highly territorial: the great call advertisement serves to announce the pair's presence to neighbouring groups and deter encroachment. Two pairs with overlapping territories will often engage in hours of duelling calls before one retreats.
Seeing rather than just hearing a gibbon at Niah requires patience and positioning. The best strategy: be at the forest edge on the Bukit Kasut trail at first light (05:30–06:00). Listen for the call direction and move quietly toward it. Gibbons respond to movement below by going silent and freezing in the canopy — approach slowly and stop before entering the caller's line of sight. A 300mm or longer lens is necessary for any useful photography. A gibbon sitting quietly in the upper canopy can be nearly invisible against the backlit morning sky without binoculars.
Bornean gibbons are classified as Endangered on the IUCN Red List, with a population that has declined by more than 50% in recent decades due to deforestation across Borneo. Niah's protected forest is one of the relatively few sites where a healthy gibbon population persists in Sarawak's lowlands — most remaining populations are now fragmented in highland forests. The river boat at dawn sometimes offers canopy-level sightings as gibbons move through riverside trees, particularly at low tide when the riverbank trees are silhouetted against the sky.