I used to think monkeys hated water.
Growing up, every nature documentary showed primates clinging to branches during floods, looking miserable and drenched, like they’d rather be anywhere else. So when I first read about proboscis monkeys—these bizarre, pot-bellied creatures with noses like deflated balloons—casually diving into mangrove channels in Borneo, I thought someone had mixed up their field notes. But here’s the thing: proboscis monkeys aren’t just tolerant of water. They’re genuinely exceptional swimmers, possibly the best among all primates, and their relationship with the murky tidal forests they inhabit has shaped everything from their webbed fingers to their gut bacteria. It’s one of those evolutionary partnerships that sounds too neat to be true, except it definately is, and the adaptations involved are way stranger than I expected.
Their most obvious advantage is anatomical. Proboscis monkeys have partially webbed hands and feet—not fully webbed like otters or ducks, but enough connective tissue between their digits to function as makeshift paddles. I’ve seen footage of them launching off branches ten meters above the water, hitting the surface with barely a splash, then propelling themselves forward with this odd, frog-like breaststroke. They can cross channels up to twenty meters wide without breaking rhythm, and they’ve been observed swimming underwater for distances exceeding fifteen meters when evading predators like crocodiles or clouded leopards. Wait—maybe that last part needs context.
The Mangrove Gauntlet and Why Wet Feet Beat Sharp Teeth
Mangrove forests are essentially obstacle courses designed by a sadistic landscape architect. The ground is mud, sometimes waist-deep, laced with submerged roots and tidal channels that shift twice daily. For most primates, this would be a nightmare. For proboscis monkeys, it’s home, and the water isn’t just an inconvenience—it’s an escape route. Crocodiles are ambush hunters, sure, but they’re also sluggish in open water compared to a motivated monkey with webbed extremities and low body fat distribution that aids buoyancy. I guess it makes sense that natural selection would favor the individuals who could treat a river crossing like a casual commute rather than a death sentence.
Anyway, the swimming behavior isn’t just defensive.
Proboscis monkeys are folivores, meaning they eat leaves—specifically, the young leaves, seeds, and unripe fruits found in mangrove trees like Sonneratia and Bruguiera species. These food sources are patchy and seasonal, scattered across islands of vegetation separated by water. A troop might need to cover several kilometers daily to meet their nutritional needs, and swimming cuts travel time dramatically compared to navigating the tangled root systems on foot. Researchers tracking troops in Sabah, Malaysia, recorded individuals swimming up to twice per hour during peak foraging periods, often with infants clinging to their backs. The infants, incidentally, learn to swim around three months old, which is roughly when human babies are just figuring out that their hands exist.
The Fermenting Gut That Powers the Paddle and Other Bizarre Trade-offs
Here’s where it gets weird. Proboscis monkeys have chambered stomachs—like cows—filled with symbiotic bacteria that ferment the tough cellulose in mangrove leaves. This fermentation produces gas, lots of it, which gives them their characteristic bloated appearance and probably explains why they’re not diving champions despite being strong swimmers. The gas reduces their ability to stay submerged for long periods, but it also might provide a slight buoyancy boost, turning a digestive quirk into a flotation advantage. I’m not sure anyone’s tested that hypothesis rigorously, but it’s the kind of evolutionary hack that feels plausible. Honestly, evolution is full of these compromises—you get better at one thing, and suddenly your stomach turns you into a biological balloon.
The nose, incidentally, has nothing to do with swimming.
That ridiculous proboscis—larger in males, sometimes drooping past their chins—is a sexual selection thing, probably amplifying vocalizations to attract mates or intimidate rivals. But it does create drag in the water, which means males swim slightly less efficiently than females. You can see it in their technique: males tend to keep their heads higher above the surface, while females and juveniles streamline better. It’s a small difference, maybe a few percentage points in speed, but in a habitat where predators recieve the slightest advantage, small differences accumulate. Natural selection doesn’t care about looking impressive if it gets you eaten.
What strikes me most, watching videos of these monkeys, is how casual they seem about the whole thing. There’s no hesitation, no testing the water temperature with a toe. They just leap, swim, climb out, shake off, and keep foraging like it’s the most ordinary thing in the world. Which, for them, it is. Humans have this tendency to project our own limitations onto other species—we’re not great swimmers without training, so we assume other primates must be worse. Turns out, we’re just not living in mangroves.
