Why the Proboscis monkey has the largest nose of any primate

Why the Proboscis monkey has the largest nose of any primate

One Earth's “Species of the Week” series highlights the flagship species of each of the 844 unique ecoregions contained within Earth’s bioregions.

With a protruding belly, a tawny-red fur patch on the forehead that resembles a unibrow, and the largest nose of any primate, the proboscis monkey might not be a looker to you. However, that’s exactly what evolution intended as the bigger the nose and belly, the more attracted the females are to mating. 

The proboscis monkey is the flagship species of the Borneo Peat Swamp Forests ecoregion, located in the Borneo Tropical Forests & Sundaland Heath Forests bioregion (IM16).

Proboscis monkeys are only found in the jungles on the island of Borneo — which is politically divided among three countries: Malaysia and Brunei in the north, and Indonesia to the south. They live in the coastal mangroves, swamps, and river areas. Leaping from tree limb to tree limb, they have frequently been spotted hitting the water with an entertaining belly flop. Considered the ‘primate world’s most prolific swimmers’, they’ve even evolved webbed feet and hands. Their potbellies are also a result of their ability to adapt to this remote part of the planet. In these dense mangroves, the leaves are tough and full of toxins. Many species cannot digest in such a manner, but the proboscis monkey can thanks to their protruding stomachs as they have several filled bacteria chambers designed to break down the toxic leaves. 

Sexual dimorphism is pronounced in the species, meaning that the males and females' outward appearance have different characteristics. Males can grow up to 30 inches in length and weigh up to 50 pounds, while females only grow to a maximum height of 25 inches and weigh 15 to 26 pounds. The most prominent feature that differentiates the genders, however, is the nose or proboscis. In males, it can exceed 4 inches in length and hangs lower than the mouth. Females and young males still have a unique look though, with nostrils that turn up at the end and remind one of Dr. Suess’s Whos.  

Living in organized harem groups consisting of a dominant male, several females, and their offspring, the large nose of the male serves them in communication with the group and with their mating call. Scientists believe that their huge, flappy nose serves as an echo chamber amplifying their sound. To call a female, the male tries to make the lowest, loudest noise he possibly can. His mouth opens as wide like a lion that is about to roar, complete with his canines sticking out and tongue flat. Although, looks like you will need to cover your ears, the sound is more of a deep, resonating growl. 

Females become sexually mature at the age of five years and matings largely take place between February and November. Birthing occurs March through May and although the mother is the primary caregiver, she will allow other members of her group to hold her offspring. Due to how long it takes a female to fully mature and even more so to the rampant clearing of their habitat for timber and oil palm plantations, their population has plummeted. 

Currently, proboscis monkeys are protected from hunting or capture in Borneo and efforts are being made to further conserve their habitat and educate locals. In Indonesia, it has been rumored that the monkey is called monyet belanda ("Dutch monkey") or orang belanda ("Dutchman") as Indigenous peoples thought the Dutch who colonized the area often had similarly large bellies and noses. 

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