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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.

Proboscis monkeys - PMF IAS

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: IUCNEN, CITESAppendix 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.
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