[13][14] Infants are born with a blue coloured face that at 2.5 months darkens to grey.

[15] Both sexes have bulging stomachs that give the monkeys what resembles a pot belly.

[13] Further adding to the dimorphism is the large nose or proboscis of the male, which can exceed 10.2 cm (4.0 in) in length,[16] and hangs lower than the mouth.

The skull of the proboscis monkey has specialized nasal cartilages that support the large nose.

It is perhaps the most aquatic of the primates and is a fairly good swimmer, capable of swimming up to 20 m (66 ft) underwater.

[21] Aside from this, the proboscis monkey is largely arboreal and moves quadrupedally and by leaps.

[20][26] Females also sometimes leave their natal groups, perhaps to avoid infanticide or inbreeding, reduce competition for food, or elevation of their social status.

[6] Researchers believe this may be a result of the two species being confined to a small patch of riverine forest due to deforestation in order to plant oil palm trees.

[25][28] At one site, matings largely take place between February and November, while births occur between March and May.

[30] The young begin to eat solid foods at six weeks and are weaned at seven months old.

[23][29][30] When a resident male in a one-male group is replaced, the infants are at risk of infanticide.

Nonvocal displays include leaping-branch shaking, bare-teeth open mouth threats and erection in males, made in the same situations.

[23] As a seasonal folivore and frugivore, the proboscis monkey eats primarily fruit and leaves.

At least 55 different plant species are consumed, "with a marked preference for Eugenia sp., Ganua motleyana and Lophopetalum javanicum".

[23] Being a seasonal eater, the proboscis monkey eats mostly fruit from January to May and mostly leaves from June to December.

Proboscis monkeys will start the day foraging and then rest further inland.

Their daily activities consist of resting, traveling, feeding and keeping vigilant.

Predators (potential or confirmed) of the proboscis monkey include crocodilians like false gharials and saltwater crocodiles, the Sunda clouded leopard, sun bears and reticulated pythons as well as, for probably young or sickly monkeys, large eagles (such as the crested serpent eagle or black eagle), large owls, and monitor lizards.

Its total population has decreased by more than 50% in the past 36–40 years to 2008 due to ongoing habitat loss because of logging and oil palm plantations, and hunting in some areas due to the species being treated as a delicacy, as well as its use in traditional Chinese medicine.