Proboscis monkey

Nasal cartilages that were left intact in a specimen skull in order to show the structures that support the nose

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,[15] and hangs lower than the mouth. Theories for the extensive length of their nose suggest it may be sexual selection by the females, who prefer louder vocalisations, with the size of the nose increasing the volume of the call.[16][12][13]

The nose is smaller in the female and is upturned in the young.[17] Nevertheless, the nose of the female is still fairly large for a primate. The skull of the proboscis monkey has specialized nasal cartilages that support the large nose.[3]

Jumping (composite image, from right to left), Labuk Bay, Sabah

A group of Proboscis monkeys by the river Kinabatangan

Proboscis monkeys generally live in groups composed of one adult male, some adult females and their offspring.[19][22][23] All-male groups may also exist.[24] Some individuals are solitary, mostly males.[25] Monkey groups live in overlapping home ranges, with little territoriality,[19][22] in a fission-fusion society, with groups gathering at sleeping sites as night falls. There exist bands which arise when groups come together and slip apart yet sometimes groups may join to mate and groom.[19][22][23] Groups gather during the day and travel together, but individuals only groom and play with those in their own group.[23] One-male groups consist of 3 to 19 individuals,[24][19] while bands can consist of as many as 60 individuals.[19][25] Serious aggression is uncommon among the monkeys but minor aggression does occur.[26] Overall, members of the same bands are fairly tolerant of each other. A linear dominance hierarchy exists between females.[22] Males of one-male groups can stay in their groups for six to eight years. Replacements in the resident males appear to occur without serious aggression.[24] Upon reaching adulthood, males leave their natal groups and join all-male groups.[19][25] Females also sometimes leave their natal groups, perhaps to avoid infanticide or inbreeding, reduce competition for food, or elevation of their social status.[24][25]

In Sabah, Malaysia, proboscis monkeys have been observed in mixed-species groups with silvery lutungs, and interspecific mating and a possible hybrid has been observed.[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.[6]

Females become sexually mature at the age of five years. They experience sexual swelling, which involves the genitals becoming pink or reddened.[24][27] At one site, matings largely take place between February and November, while births occur between March and May.[28] Copulations tend to last for half a minute.[22][24] The male will grab the female by the ankles or torso and mount her from behind.[22] Both sexes will encourage mating, but they are not always successful.[27] When soliciting, both sexes will make pouted faces. In addition, males will sometimes vocalize and females will present their backsides and shake their head from side to side.[19][27][28][14] Mating pairs are sometimes harassed by subadults.[27] Proboscis monkeys may also engage in mounting with no reproductive purpose, such as playful and same-sex mounting, and females will attempt to initiate copulation even after they have conceived.[14] Gestation usually last 166–200 days or slightly more.[28] Females tend to give birth at night or in the early morning. The mothers then eat the placenta and lick their infants clean.[29] The young begin to eat solid foods at six weeks and are weaned at seven months old. The nose of a young male grows slowly until reaching adulthood. The mother will allow other members of her group to hold her infant.[22][28][29] When a resident male in a one-male group is replaced, the infants are at risk of infanticide.[30]

Proboscis monkeys are known to make various vocalizations. When communicating the status of group, males will emit honks. They have a special honk emitted towards infants, which is also used for reassurance. Males will also produce alarm calls to signal danger. Both sexes give threat calls, but each are different. In addition, females and immature individuals will emit so-called "female calls" when angry.[31] Honks, roars and snarls are made during low-intensity agonistic encounters. Nonvocal displays include leaping-branch shaking, bare-teeth open mouth threats and erection in males, made in the same situations.[22]

As a seasonal folivore and frugivore, the proboscis monkey eats primarily fruit and leaves.[22] It also eats flowers, seeds and insects to a lesser extent. At least 55 different plant species are consumed, "with a marked preference for Eugenia sp., Ganua motleyana and Lophopetalum javanicum".[32] Young leaves are preferred over mature leaves and unripe fruits are preferred over ripe fruit.[22] Being a seasonal eater, the proboscis monkey eats mostly fruit from January to May and mostly leaves from June to December.[32] Groups usually sleep in adjacent trees.[33] Monkeys tend to sleep near rivers, if they are nearby. Proboscis monkeys will start the day foraging and then rest further inland. Their daily activities consist of resting, traveling, feeding and keeping vigilant.[22] Occasionally, they chew their cud to allow more efficient digestion and food intake.[34] As night approaches, the monkeys move back near the river and forage again. Predators (potential or confirmed) of the proboscis monkey include crocodilians likely false gharials and saltwater crocodiles, Sunda clouded leopard, sun bears and reticulated pythons as well as, for probably young or sickly monkeys, eagles, possibly larger Nisaetus, Haliaeetus, crested serpent eagle or black eagles), large owls and monitor lizards.[35][36] Monkeys will cross rivers at narrows or cross arboreally if possible. This may serve as predator avoidance.[37]