Living in organized harem groups consisting of a dominant male, several females, and their offspring, the large nose of the male serves them in communication with the group and with their mating call. Scientists believe that their huge, flappy nose serves as an echo chamber amplifying their sound.

Serious aggression is uncommon among the monkeys but minor aggression does occur. Overall, members of the same bands are fairly tolerant of each other. A linear dominance hierarchy exists between females. Males of one-male groups can stay in their groups for six to eight years.

Their large pot bellies encase a four-chambered stomach that helps them digest a cellulose-rich diet, and a close-up on their hands and feet reveals webbed digits, an adaptation rarely found in primates! Webbed fingers and toes help proboscis monkeys to navigate their river and coastal environments.

Proboscis monkeys have declined at a rate of approximately 50% in the last 40 years, according to the IUCN Red List of Threatened Species. This is as a result of habitat destruction for logging and oil palm plantations, but hunting for food and traditional Chinese medicine also occur.