Proboscis monkeys (Nasalis larvatus)

• Context (DTE): Nasalis larvatus monkey is a topic of study for its large, bizarre nose.
• One of the largest monkey species in Asia, Proboscis monkeys are endemic to the island of Borneo.

Credits: DTE

Nose: A status symbol

• Unique nose: Enormous, pendant, tongue-shaped noses of adult males; those of juveniles and females are shorter and upturned.
• The monkey’s enormous nose, along with its pink face and rotund belly, led people in the Indonesian half of Borneo to call them ‘Dutchman monkeys’.
• Social groups tend to be large-nosed male-led harems of 6-16 individuals. Neighbouring groups sometimes meet and eat together.
• These older, dominant and large-nosed males don’t easily tolerate other large-nosed males, often trying to ward them off aggressively with deep honks and “nasal roars”.
• Young adult males with smaller noses often live in all-male bachelor groups and don’t tend to fight aggressively with each other. Bachelor males on getting older and large (and large-nosed) enough to compete with males that are part of a breeding group.

Evolution of large nose

• The bony chamber of the skull sits behind the fleshy nose.
• Previous research that looked at the bulbous nose in males suggests it evolved to advertise status.
• A new study has linked the shape of the nose to “honks”, the roar sounds. Male nasal cavities were low and long compared to females.
• This allows males to build up resonance (sound vibration) in their nasal cavities, allowing them to emit deeper and louder calls through their noses.
• Nasal aperture shape in males resembles eggplant, while in females, it resembles upside-down pear.
• This further supports the idea that the nasal cavity of male proboscis monkeys underwent an evolutionary change for the purpose of making certain sounds.

Other physical properties and behaviour

• Habitat: Coastal mangroves, peat swamps and riverine forests.
• Conservation status: IUCN – EN, CITES – Appendix I.
• The male sexual organ is permanently erect. The pelt is greyish-white ventrally and reddish dorsally.
• Sexual dimorphism is pronounced in body size and nose shape. Males are heavier. Babies are dark-furred with a bluish face.
• Strongly arboreal and Folivorous (favourite diet is leaves).
• They can swim quite well and have webbed fingers and toes.