To discover more about these water-loving primates, conservation biologist Amalia Rezeki recently spent a few weeks on Indonesian Borneo's Bakut Island filming the animals and their behavior in mangrove forests. (Read about swimming pigs and other surprising animals that love water.)

"The main primate in the area that people think of are orangutans. We thought that a video about these monkeys' swimming abilities would help bring some positive attention," says Rezeki, who works with Sahabat Bekantan Indonesia, a nonprofit that works to protect proboscis monkeys, also called bekantan.

Due to loss of their mangrove habitat and hunting, proboscis monkeys are listed as endangered, with fewer than 7,000 animals left in the wild.

Rezeki's expedition discovered several monkeys on