You’ve seen the photos. It’s hard to miss. We’re talking about the proboscis monkey, that famous monkey with a funny nose that looks like it walked straight out of a Jim Henson fever dream. Most people just laugh and scroll past. But if you actually sit down and look at what’s happening with these animals in the mangroves of Borneo, the "funny" part starts feeling a lot more like a masterclass in weird evolution.
It’s big. It’s bulbous. It dangles.
The nose on a male Nasalis larvatus can grow longer than 10 centimeters. That is roughly four inches of flesh hanging over its mouth. Sometimes, they even have to push it out of the way just to eat. It’s ridiculous. But in the wild, being ridiculous is often the key to staying alive—or at least, the key to getting a date.
Why the nose is actually a high-tech megaphone
Biologists have spent decades trying to figure out the "why" behind the face. Is it for cooling down? Is it a snorkel? No. It’s a speaker system.
When a male proboscis monkey gets agitated or needs to warn the troop about a nearby clouded leopard, he honks. He doesn't just squeak like a marmoset. He lets out a resonant, deep blare. The massive nose acts as a specialized resonating chamber. Think of it like a subwoofer attached to his face. According to research published in Science Advances, there is a direct correlation between the size of a male’s nose and the volume of his calls.
Females love it.
It’s basically a biological billboard that says, "I am loud, I am healthy, and I have enough extra calories to grow this massive, useless-looking appendage." In the world of sexual selection, this is what we call an honest signal. If a monkey is sick or malnourished, he can’t maintain a trophy nose like that. So, the bigger the "funny" nose, the more attractive he is to the harem.
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Swimming with the locals
Most monkeys hate water. They’ll avoid a puddle like it’s lava. The proboscis monkey is the weirdo at the pool party who does cannonballs. These are arguably the most aquatic primates on the planet.
They have webbed feet. Seriously.
If you travel to the Kinabatangan River in Sabah, you’ll see them leaping from branches 50 feet in the air, belly-flopping into the water below. They do this to cross rivers while avoiding land-based predators. They can swim underwater for up to 20 meters. While they’re swimming, that huge nose stays above the surface like a fleshy periscope. It’s wild to watch.
They live in highly organized social groups. You have the "harem" groups—one dominant male and a bunch of females—and then you have the "bachelor pads." The bachelor groups are just a bunch of young males hanging out, growing their noses, and waiting for their chance to challenge a leader. It’s a constant soap opera played out in the treetops of the peat swamps.
The dietary struggle of a pot-bellied primate
If you look at a proboscis monkey, you’ll notice they all have massive pot bellies. It’s not because they’re lazy. It’s because their diet is incredibly difficult to digest.
They eat:
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- Unripe fruit (sugar makes them bloat and can actually kill them via fermentation)
- Mangrove leaves
- Seeds
- Certain flowers
Their stomachs are complex and multi-chambered, much like a cow’s. They rely on a massive colony of bacteria to break down the cellulose in the leaves. This process creates a lot of gas. A lot. So, you have a monkey with a funny nose that is also permanently bloated and frequently flatulent. It is the peak of primate comedy, yet it’s a highly specialized survival strategy for living in an environment where other monkeys would starve.
Where they are disappearing
We have to be real for a second. The proboscis monkey is Endangered. Their habitat is being shredded.
Borneo’s coastal mangroves and riverine forests are being converted into oil palm plantations at a staggering rate. Because these monkeys are "specialists"—meaning they can only live in these specific swampy areas—they can’t just move inland when their trees are cut down. When the mangroves go, the monkeys go.
Groups like the Danau Girang Field Centre and the Sabah Wildlife Department are trying to map out corridors so these troops aren't isolated. Isolation leads to inbreeding. Inbreeding leads to smaller noses and weaker immune systems. If we lose the habitat, we lose the most unique face in the animal kingdom.
Seeing them for yourself (responsibly)
If you want to see a proboscis monkey, don't go to a zoo. Go to Borneo. But don't just book any random tour.
- Sukau and Bilit: These villages on the Kinabatangan River are the gold standard. You take a small boat out at dusk. The monkeys come down to the riverbanks to sleep because it's safer than being deep in the jungle where cats can sneak up on them.
- Bako National Park: Located in Sarawak, this park is incredible because the monkeys are somewhat used to humans. You can see them foraging on the ground near the beach, which is rare.
- Labuk Bay: This is a private sanctuary. It’s a bit more "controlled," but if you have limited mobility, it’s the best way to see them up close without trekking through a swamp.
The evolution of the "Honk"
Interestingly, the nose continues to grow throughout the male's life. A young male has a cute, upturned button nose. He looks almost normal. But as the testosterone kicks in, the bridge of the nose begins to swell. By the time he’s a fully mature alpha, it’s a permanent fixture.
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There’s a lesson here about how nature doesn't care about "pretty." It cares about what works. A nose that looks like a giant sausage might seem like an evolutionary mistake, but it has kept this species thriving in the harsh, acidic swamps of Borneo for millions of years. It’s a tool for communication, a status symbol, and a megaphone all wrapped into one.
When you look at a proboscis monkey, you aren't just looking at a monkey with a funny nose. You’re looking at a creature that has perfectly adapted to a place where humans can barely survive for a few days without heavy equipment.
Next Steps for Primate Enthusiasts
If you're fascinated by the weird world of Bornean wildlife, your next move should be looking into the Heart of Borneo initiative. It’s a cross-border conservation effort that is actually working.
You can also support the Orphanage for Proboscis Monkeys at the Sepilok Rehabilitation Centre. They take in young monkeys whose mothers were lost to habitat loss or poaching. Most importantly, check the ingredients on your products. Avoiding unsustainable palm oil is the single most effective thing a person can do from their own kitchen to ensure the monkey with a funny nose still has a home in twenty years.