The reason is that these monkeys are hairy, have a potbelly, and a big nose, which was quite the picture they had of the first Europeans who came to Borneo. The males and females of the species are very different in several ways. The males are large, weighing anywhere between 16 to 22 kilograms, and the females weigh between 7 to 12 kilograms. The males have a large pendulous nose, whereas in the females the nose is smaller and upturned. These monkeys move in groups ranging from 3 to 32 individuals, and surprisingly they are not a very territorial species.
Group ranges overlap with the ranges of many other groups.
They usually stay near rivers, and being dependant on the habitat adjoining rivers, mostly stay close to them. They are very active from late afternoon to sundown, and often swim across rivers and streams in search of fruits etc. The proboscis monkeys use several forms of vocal communication. The males growl to calm other members of the group, honks from males is a threatening call, juveniles of both sexes shriek when excited, and finally come screams which are heard during feeding bouts and at night. They also have a social grooming system, in which bonding between individuals is improved through 1-5 minute grooming sessions.