Nasalis larvatus also known as Long-nosed Monkey is a reddish-brown arboreal Old World monkey. It is the only species in monotypic genus Nasalis.
The most distinctive trait of this monkey is the male’s large protruding nose. The purpose of the large nose is unclear, but it has been suggested that it is a result of sexual selection. The female Proboscis Monkey prefers big-nosed male, thus propagating the trait.
Males are much larger than females, reaching 72 cm (28 inches) in length, with an up to 75 cm tail, and weighing up to 24 kg (53 pounds). Females are up to 60 cm long, weighing up to 12 kg (26 lb).
The Proboscis Monkey also has a large belly, as a result of its diet. Its digestive system is divided into several parts, with distinctive gut flora, which help in digesting leaves. This digestive process releases a lot of gas, resulting in the monkey’s “bloated” bellies. A side-effect of this unique digestive system is that it is unable to digest ripe fruit, unlike most other simians. The diet consists mainly of fruits, seeds and leaves.
Females become sexually mature at five years old. They experience sexual swelling, which involves the genitals becoming pink or reddened. At one site, matings largely take place between February and November, while births occur between March and May.
Copulations tend to last for half a minute. The male will grab the female by the ankles or torso and mount her from behind. Both sexes will encourage mating, but they are not always successful.When soliciting, both sexes will make pouted faces. In addition, males will sometimes vocalize and females will present their backsides. Mating pairs are sometimes harassed by subadults. Proboscis monkey may also engage in mounting with no reproductive purpose, such as playful and same-sex mounting.