The Spectacled Leaf Langur or Dusky Leaf Monkey (Trachypithecus obscurus) is known as Lotong bercelak in Malaysia. It belongs to Family Cercopithecidae (Old World monkeys) and is further classified under subfamily Colobinae (which are mainly arboreal, lack cheek pouches, have a long tail and a large, sacculated stomach with many chambers) (Groves, 2001; Francis, 2008). Trachypithecus obscurus is commonly called the Spectacled Leaf Langur or Dusky Leaf Monkey because of its appearance; it has incomplete white rings around its eyes resembling spectacles (Md-Zain & Ch'ng, 2011), and a dietary habit of eating leaves. According to range and hair coloration, T. obscurus is divided into 11 subspecies; these are T. o. obscurus, T. o. carbo, T. o. corax, T. o. flavicauda, T. o. halonifer, T. o. phayrei, T. o. sanctorum, T. o. seimundi, T. o. shanicus, T. o. smithi and T. o. styx (Brandon-Jones et al., 2004). These subspecies are distributed from India to Bangladesh, Burma, Thailand and Peninsular Malaysia (Brandon-Jones et al., 2004). There are at least five subspecies native to Peninsular Malaysia. These are T. o. obscurus (Reid's dusky leaf monkey) which is the major subspecies in Peninsular Malaysia except along the north coast, T. o. flavicauda (Blond-tailed dusky leaf monkey) which is found in northern Peninsular Malaysia, T. o. styx (Perhentian dusky leaf monkey) which occurs on Perhentian Island and probably adjacent coastal areas, T. o. halonifer (Cantor's dusky leaf monkey) which is mostly restricted to Penang Island, and T. o. carbo (Tarutao dusky leaf monkey) which is found on Dayang Bunting Island and Langkawi Island (Brandonn-Jones et al., 2004).

Trachypithecus obscurus can be easily distinguished from other primates due to its unique appearance. Apart from the white-ringed eyes, it also has contrasting bare pink patches on its upper and lower lips. The upper part of its body is greyish-brown to dark grey, while the underparts, outside of hind legs, tail and crest on top of the head are a paler grey. The new-born Spectacled Leaf Langur is yellow to pale orange with a pink face (Francis, 2008). The young's fur colour will change to grey or brownish within 6 months. The average lifespan of a female Spectacled Leaf Langur under captivity was found to be around 15.3 years (About Animals, 2017).

The Spectacled Leaf Langur is mainly arboreal and inhabits a variety of forest types, from lowland to hill forests. It can also be found in other habitats such as coastal, riverine and urban forests, botanical gardens, and parks. This species is primarily folivorous (herbivore specialized in eating leaves). It mainly feeds on leaves and shoots but also eats some fruits, especially unripe ones (Francis, 2008). Some fruits consumed by this species include Ficus delosyce, F. sumatrana and F. stricta (Lambert, 1990). Recently, Baker & Graeme (2017) reported that this species also feeds on dry pods of Acacia auriculiformis where the adults would extract part of the pod using their teeth, but the infant was seen extracting only the seeds with its fingers. This langur is able to feed on leaves and unripe fruits due to the presence of bacteria in its gut which can break down cellulose and help to detoxify poisonous leaves or unripe fruit. It consumes 2 kg of food everyday while weighing only 6.5 kg to 7.5 kg.

The Spectacled Leaf Langur is diurnal and mostly active during the day. It spends most of its time feeding, followed by resting and moving (Md-Zain & Ch'ng, 2011). It prefers foraging and staying up in canopies of tall trees, moving around, running, climbing and leaping from the branches of the trees. It is known to be territorial and lives in groups of 5 to 20 individuals, consisting of juveniles, two or more adult females and one or more adult males, with only one dominant male. The dominant adult male is responsible for maintaining the group together, alerting the group members of predators, and defending its territorial boundaries (Curtin, 1980). The non-dominant males tend to leave their birth group and form "bachelor" groups which consist of other solitary males (Downey, 2016). Sometimes, males or females can also be found living solitarily. Each group can occupy a home range of 5 to 20 ha (Francis, 2008).

Unlike other primates, the Spectacled Leaf Langur lives peacefully with one another. It shows low aggression, focusing more on reconciliation. During reconciliation and consolation, it often uses tactile communication which involves social grooming and embracing. This species also often uses visual communication such as tongue flicks, and lunges to show domination and also to send signals when threats are observed. It also uses vocal communication which involves alternate soft warning and coughing calls during detection of threats while honking is used for demarcation of the group's territory. During aggression, this langur uses tactile communication which involves jump kicking, grappling, wrestling, pulling, and grabbing each other. If a dispute occurs within the group, they tend to reconcile by ventro-ventro (belly to belly) hugging.

Trachypithecus obscurus is a polygynous breeder which means that only the large, aggressive and powerful male fertilizes the females. During mounting, the dominant male will stand behind the female and raise its rump into the air with or without grasping the female's ankles, while the female has its four limbs on the ground. Its gestation period is 145 days on average. Usually, the female gives birth during the months of January, February and March. It is known to give birth to one or two offspring but usually only one will survive. Both sexes reach maturity at between 3 and 4 years of age. The majority of the parental care is provided by the mother.

Unfortunately, the Spectacled Leaf Langur population is currently decreasing and it is listed as Near Threatened on the International Union for Conservation of Nature (IUCN) Red List. The main reason for the noticeable decline of this species is hunting by humans for food (Boonratana et al., 2008). The species is also threatened by habitat loss and degradation due to expanding oil palm plantations, agriculture and urbanization (Boonratana et al., 2008). In Peninsular Malaysia, this species is frequently the victim of road-kill (Boonratana et al., 2008). This species is currently protected internationally by the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) as it is listed under Appendix II, and in Malaysia it is protected by the Wildlife Conservation Act (WCA) 2010 as a Protected Species.

It is very important that this species be protected because it plays an important role in seed dispersal and serves as prey for predators. Let us work together to protect the Spectacled Leaf Langur from extinction and to sustain this species for future generations.