Proboscis monkeys are most noticeable due to the males large nose. It is rounded and tapers to a point at the end. Females have a much smaller nose that does not have the large bulbous end.
They display sexual dimorphism with males being larger than females. Their fur is relatively long. On the back it is coloured brick red, bright orange, reddish brown or yellowish brown. On the underside, on the bottom of the legs and the rear the fur is light grey, yellow or light orange. The face and nose are bright red or pink. Around the neck is a collar of cream fur. The tops of the legs have a light brown spot. At the edges of the face are a grey spot and the top of the head is a similar colour to their back.
Both sexes have a pot belly and some of them have webbed toes.
Males measure between 66 and 76.2cm (26-30in). Females measure 53-62cm (21-24.4in). An average male weighs between 16 and 22.5kg (35-50lb) while females are about half that size at 7-12kg (15-26lb).
Their large stomach comes from the need to digest the cellulose using a special bacteria. This is a slow process and their stomach may comprise a quarter of this animals total body weight at any time. Most of their feeding takes place in the trees.
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Asia is the native home of the proboscis monkey. They can be found throughout Brunei, Indonesia and Malaysia.
Most proboscis monkey colonies live by the coast and along the inland rivers. They can be found in mangrove swamps, coastal mangroves, riverine forests and lowland forests. Most of their time is spent in the canopy. As these habitats are rare each bit that is deforested is a massive hit to the proboscis monkey populations.
When females become sexually mature their genitals will swell and become red or pink alerting males to her ability to mate. The breeding season appears to run from February to November. Both sexes initiate the mating. To entice a mate they will make pouty faces, males will vocalise and females will present their rear.
After a successful mating it takes 166-200 days for the young monkeys to be born. The infant will have dark coloured fur and a blue face until 3-4 months of age. After the birth the mother eats the placenta and then licks the infant clean.
All female members of the troop help to care for the infant. Solid foods are first introduced into their diet at six weeks old. At seven months of age they stop drinking milk and move fully to solid food.
If the resident male in the group dies another male will enter the group. Another male may then join the group and kill all the babies which belong to the other male.
At 4-5 years of age proboscis monkeys achieve sexual maturity.
Proboscis monkeys make numerous vocalisations. The males can emit honks to show the groups status. There is a special honk to reassure infants. They also use a number of alarm calls.
These animals form troops which normally contain an adult male, numerous females and their offspring. Some bachelor groups have also been found. They form a home range but do not defend it heavily. Some groups come together to sleep at night. Some troops come together in the day to form a band that will travel together. The bands are defined though with play and grooming only occurring between the animals from individual troops. A troop may contain 9-19 animals with bands containing up to 3 troops.
Male infants normally leave the group at maturity. On occasion females will also leave to avoid inbreeding.
Proboscis monkeys are able swimmers as they live close to water. Some have been seen swimming distances up to 20m (65ft).
Their main predator is the false gavial. Other predators include clouded leopards, monitor lizards, pythons, crested serpent eagles and estuarine crocodiles. A number of these can only take infants but on rare occasions will try for an adult. Occasionally orangutans and long tailed macaques will force proboscis monkeys away from feeding sites.
Occasionally this species is known as the Malay name, monyet balanda (“dutch monkey”) or orang balanda (“Dutchman”). This was due to some Indonesians comparing them to the Dutch colonisers who they said had large bellies and noses.
By Benjamint444 (Own work) [GFDL 1.2 (http://www.gnu.org/licenses/old-licenses/fdl-1.2.html)], via Wikimedia Commons
By U. Schröter (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
By einalem from Leeds (flickr:Juvenile Proboscis Monkey) [CC-BY-SA-2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons
Meijaard, E., Nijman, V. & Supriatna, J. 2008. Nasalis larvatus. The IUCN Red List of Threatened Species 2008: e.T14352A4434312. https://dx.doi.org/10.2305/IUCN.UK.2008.RLTS.T14352A4434312.en. Downloaded on 21 May 2020.
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