A dominant male proboscis monkey has an enormous, pendulous nose. These monkeys are also known for their potbellied physiques and partially webbed feet, which make them incredible swimmers. These endangered primates are endemic to the island of Borneo.

A large male’s nose can reach up to 17 cm (6.5 in) in length. The supersized noses amplify the males’ calls to attract mates and intimidate rivals. Scientists have found that males with larger noses also tend to be larger overall, with more developed digestive organs and testes.

Only male proboscis monkeys develop such an elongated nose. Females and juveniles have much smaller, upturned noses. This difference between sexes is a classic example of sexual dimorphism. The males’ noses help project loud calls through the rainforest. These low, resonant calls serve to mark territory and keep the group together, especially when visibility is low in the mangrove forests.

Proboscis monkeys are found only on Borneo, an island in Southeast Asia divided among Malaysia, Indonesia, and Brunei. They stick to coastal and riverside habitats, including lush mangrove forests, peat swamps, and jungle riversides. These monkeys are arboreal and superb leapers, often seen springing through the canopy. At dusk, different groups frequently come together by the riverbanks to sleep in the safety of numbers.

Living in swampy jungles means proboscis monkeys are never far from water. They travel along rivers and mangroves in search of food, rarely straying far from the water’s edge.

One of the most surprising facts about proboscis monkeys is that they are excellent swimmers. Unlike other monkeys that avoid water, proboscis monkeys frequently leap from high tree limbs into rivers. They have evolved partially webbed hands and feet to propel themselves efficiently through water. This helps them outpace predators, a crucial skill when crocodiles lurk in the rivers below. Observers have seen proboscis monkeys swim underwater for up to 20 meters (66 feet) in a single breath-holding dash, using a kind of modified doggy paddle. Adults try to slip into the water with minimal splash, unlike the young ones’ belly flops, to avoid attracting crocodiles.

Their comfort in water allows proboscis monkeys to cross rivers and travel between mangrove islands in search of food. If threatened by a predator on land, they will dive into a river to escape. This swimming talent is extremely rare among primates.

Another standout feature of the proboscis monkey is its big belly. That pot belly is due to a specialized digestive system. Proboscis monkeys are primarily folivores, meaning they eat a lot of leaves, especially tough mangrove leaves, along with seeds, unripe fruits, and the occasional insect. Their diet is high in fiber and sometimes in toxic compounds, so they are equipped with a multi-chambered stomach full of symbiotic bacteria to ferment and break down these tough foods. Their complex stomachs function a bit like a cow’s rumen.

Just like bovines, proboscis monkeys regurgitate and re-chew their food to extract every bit of nutrients. They are the only primates known to chew cud. This slow, thorough digestive process produces a lot of gas, contributing to their bloated look. An adult’s stomach can account for up to one-quarter of their body weight. Because their gut flora ferments their food, too much sugary ripe fruit can cause dangerous bloating, so they prefer unripe fruits, which are easier on their stomachs. After a large meal, they often rest before continuing to leap or swim.

Proboscis monkeys live in groups often described as harems. A typical group, or band, has one adult male, several adult females, and their offspring. The dominant male protects and leads the troop, while females help rear the young and maintain group cohesion. These family troops are generally peaceful and easygoing. Unlike some other monkeys, they are not very territorial and rarely engage in serious fights. Males do not rely on sharp canine teeth for combat. Instead, dominance is often shown through louder calls or more prominent noses.

There are also bachelor groups of younger males without females. Individuals sometimes move between groups, and multiple troops may gather at night by the riverside to sleep near one another. In the morning, each group splits off to forage on its own. This flexible social system and their mild temperament make proboscis monkey society relatively harmonious. Females even practice alloparenting, allowing others to handle and play with their infants.

Newborn proboscis monkeys have dark, almost black fur and a bright blue face. As they grow, their fur lightens to the adult orange-brown coat, and their blue face fades to a beige-pink tone. Babies spend their first year clinging to their mothers, nursing, and learning what to eat.

Young proboscis monkeys are playful and often chase each other in the trees or leap into the water. By age four or five, they reach sexual maturity. Males usually leave to join bachelor groups. Adult males can grow over 70 cm (2.3 ft) tall and weigh more than 22 to 24 kg (50+ lbs). Females are smaller and lighter. With age comes the development of the iconic nose in males and the potbelly from their diet. In the wild, proboscis monkeys can live around 15 to 20 years.

The proboscis monkey is classified as Endangered. Populations have dropped by more than 50 percent in the last 40 years. The biggest threat is habitat destruction. Borneo’s coastal forests and mangroves are being cleared for timber, human development, and especially oil palm plantations. As their habitat is fragmented, proboscis monkeys are forced to travel more on the ground, exposing them to predators and conflict. In some areas, they are also hunted.

Several reserves aim to protect their remaining habitat. Sanctuaries along the Kinabatangan River in Sabah and in Kalimantan offer refuge for surviving populations. Ecotourism has also helped raise awareness and support for their conservation.

Preserving this strange, semi-aquatic monkey means protecting one of Borneo’s last great river forests.