Deep dive 3 min read
Proboscis Monkey Nose: Why Is It So Big?
It's long, floppy and hangs over his mouth. The proboscis monkey's nose looks like a joke, but for the ladies of Borneo, it's the sexiest thing in the jungle.
Some monkeys have big muscles. Some have colourful faces. The male proboscis monkey has a nose so big it flops over his mouth, and he has to push it aside to eat. It looks ridiculous to us. But in the swamps of Borneo, that nose is a serious business.
How big is the nose?
An adult male’s nose can grow more than 10 cm long, hanging down below his chin like a fleshy pear. Females and youngsters have much smaller, upturned noses. So the giant nose is clearly a “boys only” feature, and that’s the first clue to what it’s for.
It’s all about the ladies
Proboscis monkeys live in groups where one male has several females, called a harem. Competition for those females is fierce, and the nose is the male’s way of advertising himself.
In 2017, scientists studied proboscis monkeys in Borneo and found that males with bigger noses also tended to have bigger bodies and bigger testes, and lived with more females. In other words, the nose is an honest signal: a big nose means a big, healthy, successful male. The females seem to agree.
In Borneo, the biggest nose gets the most girlfriends.
A built-in loudspeaker
The nose isn’t just for looks. When a male calls, his nose acts as a resonating chamber, making his honks louder and deeper. A big nose means a deep, impressive voice that tells rivals and females exactly who’s boss. During excitement or anger, the nose can even straighten out or swell up.
Strange but trueProboscis monkeys are excellent swimmers, with partly webbed feet. They sometimes jump from trees straight into rivers, belly-flopping into the water.
The pot belly is part of the look
That big round belly isn’t from overeating. Proboscis monkeys eat mostly leaves and unripe fruit, which are hard to digest. Their stomach has several chambers full of bacteria that slowly ferment the food, a bit like a cow’s. The result is a permanent beer-belly look.
Where do they live?
Proboscis monkeys live only on the island of Borneo, in mangroves, swamps and forests along rivers. Males weigh around 20 kg, about twice as much as females. They’re listed as Endangered, mainly because their riverside forests are being cleared for palm oil plantations.
The verdict
Why is the proboscis monkey’s nose so big? Because the females like it, and because it turns his voice into a jungle loudspeaker. It’s a perfect example of how evolution sometimes chooses style over comfort. See more amazing schnozzes in our Weird Noses & Faces collection.
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