Why the male proboscis monkey evolved a fleshy, pendulous nose that hangs below its mouth.

Nature · Published 3 October 2026 Swipe more like this →

In short: The proboscis monkey is a master of the Borneo wetlands, but its most striking feature is the male's oversized nose. Far from a mere curiosity, this fleshy appendage acts as a biological echo chamber, amplifying vocalizations to intimidate rivals and attract mates in the dense, riverine jungles they call home.

Endemic to the island of Borneo, the proboscis monkey (Nasalis larvatus) is a highly specialized primate that rarely strays from coastal mangroves, swamps, and riverbanks. These monkeys are among the largest in Asia, with males reaching weights of up to 50 pounds, while females typically attain about half that size. Their survival is intrinsically linked to these fragile landscapes, where they navigate the canopy with agility and occasionally descend to the ground to forage.

The most distinctive trait of the species is the male's pendulous nose, which develops only after maturity. Scientists hypothesize that this outsized organ functions as an acoustic amplifier. By creating an internal echo chamber, the nose increases the volume and resonance of the monkey's calls, serving as a powerful tool for sexual selection and territorial defense. Females, who possess smaller, upturned noses, appear to favor the louder, more impressive vocalizations produced by these structures, while rival males are often intimidated by the display.

Beyond their unique morphology, these primates are exceptionally adapted to their aquatic environment. They possess webbed hands and feet, making them the most prolific swimmers in the primate world. They are frequently observed leaping from tree limbs into the water, often with a comical belly flop, to travel between forest patches or escape land-based predators like crocodiles. Despite these adaptations, the species faces a precarious future. Rampant deforestation for timber and oil palm plantations has fragmented their habitat, forcing them to travel longer distances and increasing their vulnerability to human encroachment and hunting. Over the last 40 years, populations have plummeted, leaving this iconic primate classified as endangered.

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