Nasalis larvatus

Proboscis monkeys live in a fission–fusion social system with one-male, multi-female groups and all-male bands; groups forage and travel together but may split and re-join across the day. They are largely diurnal folivore-frugivores, often feeding on young leaves and fruits, travel along river corridors, sleep in adjacent trees at night, and use honks and other vocalizations plus visual displays for communication.

Females typically reach sexual maturity around five years; mating seasonality varies with local ecology but many populations show peak mating and birth periods tied to resource cycles. Gestation lasts roughly 166–200 days and usually produces a single infant; mothers provide intensive care while infants begin solid food within weeks and are weaned months later.

Marked sexual dimorphism: adult males are much larger, heavier-built and develop the pronounced proboscis used in visual/ acoustic display, while females are smaller with upturned noses. Proboscis monkeys have long tails, partially sacculated stomachs adapted for foregut fermentation of leaves, strong hindlimbs for leaping, and morphological specializations for an arboreal, riverine lifestyle.

Restricted to Borneo, proboscis monkeys have long occupied lowland riverine and coastal habitats; human pressures (logging, oil-palm conversion, hunting) accelerated in the 20th–21st centuries, fragmenting populations and reducing available habitat. Research over recent decades has detailed their ecology, social structure, and sensitivity to habitat change, informing conservation planning.

Listed as Endangered by the IUCN due to population declines from habitat loss, fragmentation and hunting; remaining populations are fragmented and concentrated in protected and riverine areas of Borneo. Conservation priorities include habitat protection and connectivity, protection of river corridors and mangroves, and reducing hunting and human-wildlife conflict.