The proboscis monkey (Nasalis larvatus) or long-nosed monkey, known as the bekantan in Malay, is a reddish-brown arboreal Old World monkey that is endemic to the south-east Asian island of Borneo. monkey is sexuality dimorphic. Males have a head-body length averaging 75.5 cm (29.7 in) and weigh on average 20kg (44.1 lb). Females average 62 cm (24.4 in) in length and weigh have as much as the males. Further adding to the dimorphism is the large nose or proboscis of the male, which can exceed 10cm (3.9 in) in length dwarfing that of the female, and hangs lower than the mouth. Nevertheless, the nose of the female is still fairly large for a primate. The proboscis monkey has a nearly long coat. Dorsally, the fur is is bright orange, reddish brown, yellowish brown, or brick-red. The monkey lives only in lowland habitats such as coastal and riparian habitat many of which are flooded by tides. It prefers dipterocarp forests, mangrove forests, and riverine forests. It can also be found in swamp forests, stunted swamp forests, rubber forests, rubber plantations, limestone hill forests, nipa swamps, nibong swamps, and tall swamp forests, tropical heath forests and steep cliffs. This species is rarely found more than a kilometer from water sources. It is perhaps the most aquatic of the primates and is a fairly good swimmer, capable of swimming up to swimming up to 20m (65.6 ft) when completely submerged.
Females become sexuality mature at 5 years old. They show sexual swelling which involves the genitals becoming pink or reddened. At one site, mating peak in mid-year and births peak between March and May. Copulations tend to last for half minute. The male will grasp the female by the ankles or torso and mount her from the rear. Both sexes will solicit mating but they are not always successful. When soliciting, both sexes will make a pouted face. In addition, males will sometimes vocalize and females will present her backside to a male. Copulating pairs are sometimes harassed by sub-adults. Proboscis monkey may also engage in non-reproductive mounting, such as playful and same-sex mounting. Gestation usually last 166-200days or slightly more. Births tend to take place at night or in the early morning.
the proboscis monkey eats primarily fruit and leaves. It also eats flowers seeds and insects to a lesser extent. At least 55 different plant species have been recorded used as food sources, with a marked preference for Eugenia sp., Ganua motleyana and Lophopetalum javanicum. Young leaves are preferred over mature leaves and unripe fruits are preferred over ripe fruit. Being a seasonal eater, the proboscis monkey eats mostly fruit from January to May and mostly leaves from June to December. Groups usually sleep in one or several trees that are nearby. In habitats near rivers, monkey sleep near the river. Proboscis monkeys will start the day feeding and then move further inland. Proboscis monkeys seed most of the daily resting, traveling, feeding and keeping vigilant.