The average body mass for an adult male proboscis monkey is between 16 and 22 kg, with females slightly smaller at 7-12 kg. This is a sexually dimorphic species, especially in body size. The proboscis monkey has interdigital webbing that makes it an excellent swimmer. Its favorite foods are fruits, seeds and young leaves. It also eats invertebrates including mosquitoes, caterpillars, and insect larvae. This is a diurnal and semi-terrestrial species. This species always sleeps near rivers, 0 to 15 meters from the river's edge.
The proboscis monkey moves through the forest and on land quadrupedally. This species avoids areas with heavy deforestation, such as agricultural land. The group sizes range from 3 to 32 individuals.