Northern Sumatran leaf monkey (Presbytis thomasi)

Also known as:	Sumatran grizzled langur, Thomas' leaf monkey, Thomas’ langur
French:	Semnopithèque De Thomas
Spanish:	Langur De Thomas

Kingdom	Animalia
Phylum	Chordata
Class	Mammalia
Order	Primates
Family	Cercopithecidae
Genus	Presbytis (1)
Size	Head-body length: 42 – 61 cm (2) Tail length: 50 – 85 cm (2)
Weight	5 – 8 kg (2)

Classified as Vulnerable (VU) on the IUCN Red List (1) and listed on Appendix II of CITES (3).

The Northern Sumatran leaf monkey is a very striking primate, with a vertical black stripe running down its forehead, contrasting with the white patches of fur either side of the stripe. The fur on the face is a lighter grey, with a black strip over the top of the mouth and long, white fur on the chin, surrounding a flesh-coloured muzzle (4). Smooth, grey fur covers the back and the tops of the limbs, and creamy, white fur covers the chest, belly and the underside of the arms and legs. The tail is pale on the underside and grey on the top, and the hands and feet are black. Juvenile Northern Sumatran leaf monkeys have pure creamy-white fur (4).

As its name suggests, the Northern Sumatran leaf monkey inhabits north Sumatra, Indonesia, in the remote Aceh Province. Its range stretches north from the Wampu and Simpangkiri rivers (1).

The Northern Sumatran leaf monkey lives in tropical primary and secondary forest, and has also been recorded in rubber tree plantations (1). It occurs from sea level up to elevations of around 1,500 metres (1).

The Northern Sumatran leaf monkey feeds mainly on young leaves, but may also take fruits, flowers, toadstools and stalks of coconuts (1), using its sharp teeth to pierce through the outside of hard fruits or through thick handfuls of leaves (4). Sometimes it will even eat gastropods such as ground snails (1). The stomach of this monkey is specially adapted to enable it to digest cellulose in the leaves that it eats; microbes in the forestomach break down the cellulose into nutrients that can be utilised by the monkey (4).

This species is semi-arboreal, meaning it often frequents the trees, but will also spend time on the ground, typically coming down to the ground once a day to feed (4). The Northern Sumatran leaf monkey moves through the horizontal branches of trees on all fours, but has also been known to leap through the forest (4). Female Northern Sumatran leaf monkeys give birth to single offspring, which may be born at any time of the year (1).

The Northern Sumatran leaf monkey is typically found in uni-male groups (one male with lots of females) (4), although all-male groups also occur. These all-male groups receive a lot of aggression from males in uni-male groups, and so tend to live lower in the forest (4). Aggression can also occur between females when food is scarce, and it has been suggested that a hierarchy exists within the group according to the individual’s age (4). The reticulated python, the clouded leopard and the tiger are all predators of the Northern Sumatran leaf monkey (4).

Like so many of Indonesia’s primates, the Northern Sumatran leaf monkey is severely threatened. Widespread deforestation in Sumatra, to make way for lucrative commercial oil palm plantations, has resulted in extensive habitat loss for this species (5). There is also concern that the tendency of this monkey to feed on local crops, combined with a local increase in firearm ownership, is catalysing an unofficial culling of this species by farmers (6). Furthermore, the Northern Sumatran leaf monkey may be impacted by the primate trade; many primates are being captured and sold in so-called ‘bird markets’ for use in traditional medicine (7).

This species is protected by law in Indonesia and occurs in at least one protected area, Gunung Leuser National Park (1). 

To learn more about palm oil and deforestation see:

• Greenpeace:
  http://www.greenpeace.org.uk/forests/palm-oil
• Roundtable on Sustainable Palm Oil:
  http://www.rspo.org

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:
arkive@wildscreen.org.uk

• Arboreal: an animal which lives or spends a large amount of time in trees.
• Cellulose: the primary structural component of green plants.
• Gastropods: a group of molluscs that have a well-defined head, an unsegmented body and a broad, flat foot. They can possess a single, usually coiled, shell or no shell at all. Includes slugs, snails and limpets.
• Primary forest: forest that has remained undisturbed for a long time and has reached a mature condition.
• Secondary forest: forest that has re-grown after a major disturbance, such as fire or timber harvest, but has not yet reached the mature state of primary forest.

1. IUCN Red List (March, 2010)
   http://www.iucnredlist.org
2. Nowak, R.M. (1999) Walker's Mammals of the World. The Johns Hopkins University Press. Baltimore, Maryland.
3. CITES (March, 2010)
   http://www.cites.org
4. The Primata (November, 2009)
   http://www.theprimata.com/presbytis_thomasi.html
5. Sumatra Forest (November, 2009)
   http://www.sumatranforest.org/northernSumatra
6. Macdonald, D.W. (2006) The Encyclopedia of Mammals. Oxford University Press, Oxford.
7. International Primate Protection League (November, 2009)
   http://www.ippl.org/primate-trade