Physical description[edit]
Closeup of a proboscis monkey face
The proboscis monkey is a large species, being one of the largest monkey species native to Asia. Only the Tibetan macaque and a few of the gray langurs can rival its size. Sexual dimorphism is pronounced in the species. Males have a head-body length of 66 to 76.2 cm (26.0 to 30.0 in) and typically weigh 16 to 22.5 kg (35 to 50 lb), with a maximum known weight of 30 kg (66 lb). Females measure 53.3 to 62 cm (21.0 to 24.4 in) in head-and-body length and weigh 7 to 12 kg (15 to 26 lb), with a maximum known mass of 15 kg (33 lb).[5][6][7] Further adding to the dimorphism is the large nose or proboscis of the male, which can exceed 10 cm (3.9 in) in length,[8] and hangs lower than the mouth.[9][10] Nevertheless, the nose of the female is still fairly large for a primate. The proboscis monkey has a long coat; the fur on the back is bright orange, reddish brown, yellowish brown or brick-red.[9][10] The underfur is light-grey, yellowish, or greyish to light-orange.[9][10] The face is orange-pink. The male has a red penis with a black scrotum. Both sexes have bulging stomachs that give the monkeys what resembles a pot belly. Many of the monkeys' toes are webbed.[9]
Behavior[edit]
Social behavior[edit]
Proboscis monkeys generally live in groups composed of one adult male, some adult females and their offspring.[11][12][13] All-male groups may also exist.[14] Some individuals are solitary, mostly males.[15] Monkey groups live in overlapping home ranges, with little territoriality,[11][12] 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.[11][12][13] Groups gather during the day and travel together, but individuals only groom and play with those in their own group.[13] One-male groups consist of 9–19 individuals, while bands can consist of as many as 60 individuals.[11][15] One-male groups typically consist of three to 12 individuals, but can contain more.[14] Serious aggression is uncommon among monkeys but minor aggression does commonly occur.[16] Overall, members of the same bands are fairly tolerant of each other. A linear dominance hierarchy exists between females.[12] 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.[14] Upon reaching adulthood, males leave their natal groups and join all-male groups.[11][15] Females also sometimes leave their natal groups, perhaps to avoid infanticide or inbreeding, reduce competition for food, or elevation of their social status.[14][15]
Proboscis monkey pair
Reproduction[edit]
Females become sexually mature at five years old. They experience sexual swelling, which involves the genitals becoming pink or reddened.[14][17] At one site, matings largely take place between February and November, while births occur between March and May.[18] Copulations tend to last for half a minute.[12][14] The male will grab the female by the ankles or torso and mount her from behind.[12] Both sexes will encourage mating, but they are not always successful.[17] When soliciting, both sexes will make pouted faces. In addition, males will sometimes vocalize and females will present their backsides.[11][17][18] Mating pairs are sometimes harassed by subadults.[17] Proboscis monkeys may also engage in mounting with no reproductive purpose, such as playful and same-sex mounting. Gestation usually last 166–200 days or slightly more.[18] Females tend to give birth at night or in the early morning. The mothers then eat the placenta and lick their infants clean.[19] 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.[12][18][19] When a resident male in a one-male group is replaced, the infants are at risk of infanticide.[20]
