The silvery lutung (Trachypithecus cristatus), also known as the silvered leaf monkey or the silvery langur, is an Old World monkey. It is arboreal, living in coastal, mangrove, and riverine forests in Sumatra, Borneo, Java, as well as in parts of the southwestern Malay Peninsula, the Natuna Islands, and other nearby islands. It is the type species of its species group.
Description The silvery lutung is a medium-sized monkey with a long, non-prehensile tail. It has grey-tipped, dark brown or black fur, giving it a uniform silvery appearance. Unlike some related species, there are no paler markings on the face or body, except for a patch of whitish hair on the groin of females. A crest of fur runs along the top of the head, and the hair on the cheeks is long, often obscuring the ears. The hands and feet are hairless, with dark-coloured skin, and have opposable thumbs and toes. Females range from 46 to 51 centimetres (18 to 20 in) in head-body length, with an average weight of 5.7 kilograms (13 lb) and a tail length of 67 to 75 centimetres (26 to 30 in). Males are slightly larger, from 50 to 58 centimetres (20 to 23 in) in length, with an average weight of 6.6 kilograms (15 lb) and a tail length of 67 to 75 centimetres (26 to 30 in). Like other langurs, the silvery lutung has a large three-chambered stomach to digest the cellulose found in its herbivorous diet. This allows for fermentation of food, and has some similarities with the stomach of ruminants. The intestine is unusually long, even compared to those of other langurs, and has several pouches along its length, which carry out further fermentation of plant matter. The teeth have grinding ridges and other modifications to allow the more efficient processing of tough leaves.
Distribution and habitat The silvery lutung is found across Borneo, Sumatra, and Java, as well as in parts of the southwestern Malay Peninsula, the Natuna Islands, and other nearby islands. It inhabits mangrove swamps and nearby forest regions and generally avoids travelling far from coasts or rivers.
Taxonomy The number and identity of subspecies of the silvery lutung is currently debated. A 2008 analysis confirms the presence of only two subspecies:
Trachypithecus cristatus cristatus - Borneo, Sumatra, Natuna Islands Trachypithecus cristatus selangorensis - Malay Peninsula The Malay Peninsula form has been subsequently elevated to a separate species, the Selangor silvered langur T. selangorensis However, some sources distinguish the silvery lutungs of the Natuna Islands as a separate subspecies, T. c. vigilans.
Evolution Genetic analysis has shown that the silvery lutung probably first evolved during a rapid speciation event that occurred between 0.95 and 1.25 million years ago, during which all the living species of the T. cristatus species group evolved. Because of the relative speed and diversity of this event, the species of the group are difficult to distinguish genetically, and there is some uncertainty as to which represents genuinely distinct species. However, the closest living relative of the silvery lutung may be the Javan lutung, although silvery lutungs have also been reported to produce hybrids with Phayre's leaf monkey, generally considered to belong to a different species group. Fossils of the species are known from the late Pleistocene onwards and occupy the same geographic range as today. Some of these fossils had significantly larger cheek teeth than living animals, although they have not been assigned to a distinct subspecies.
Hybridization In Sabah, Malaysia, silvery lutungs have been observed in mixed-species groups with proboscis monkeys, and interspecific mating and a possible hybrid has been observed. The suspected hybrid was first sighted when it was a juvenile near the Kinabatangan River in Malaysian Borneo. Researchers began monitoring the animal and analyzing photos of it as it matured which led to the conclusion it was an extremely rare phenomenon. Researchers say hybridization between closely related species is not unheard of, however, the offspring of two distantly related species is rarely observed in the wild, going on to say this particular hybrid is only the second time intergeneric hybridization (offspring resulting from the matings between different genera) has been recorded in wild primates. Hybrid animals are often infertile, particularly with distantly related species, because it causes malfunctions in the chromosomes. However, a photo of the animal as an adult showed her holding an infant that she appeared to be nursing. Researchers believe this may be a result of the two species being confined to a small patch of riverine forest due to deforestation to plant oil palm trees, a form of habitat fragmentation, that created a segregated population of both species.
Behaviour and ecology
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