Dusky Leaf Monkey (Trachypithecus obscurus)

 

Geographic Range:

Dusky Leaf Monkeys are found primarily on the Malay Peninsula, including southern Burma and parts of Thailand. They also inhabit the islands of Langkawi, Penang, and Perhentian Besar. (Lekagul and McNeely, 1977; Medway, 1969) Dusky leaf monkeys can be found in a wide range of habitats. Being arboreal forest dwellers, they prefer dense forests with tall trees. (Medway, 1969)

Biology:

Dusky Leaf Monkeys are widely variable in colour. Their upper parts may be any shade of brown, grey, or black, whereas the under parts, hind legs, and tail are paler. The face is grey and is often marked with a patch of white fur located around the eyes and mouth. The hands and feet are capable of grasping and closely resemble those of humans. The palms and soles are hairless and usually black. The fingers of dusky leaf monkeys are well developed, but are distinct because of their opposable thumb. The no prehensile tail varies in length and fur coverage from short and hairless, to long and hairy.

Head and body length ranges from 42 to 61 cm, and tail length from 50 to 85 cm. There are no significant morphological differences between males and females except that males tend to be slightly larger and heavier than females. On average, a healthy adult male weighs 7.4 kg, whereas a healthy adult female weighs approximately 6.5 kg. Newly born dusky leaf monkeys are bright yellow or orange in colour, and have a pink face; the fur changes to a greyish colour within six months. (Grzimek, 1990; Medway, 1969). The average lifespan is  recorded as 15.3 years in captivity. Dusky Leaf Monkeys are diurnal. They are very active during the day, but return to their roosts in the trees by night. These monkeys are active in the tree canopy, and prefer to stay at heights of 35 meters or higher in trees. They move from tree to tree by climbing, leaping, and running quadrupedally along branches, When feeding, dusky leaf monkeys pluck leaves and shoots off by hand. They also pull down leafy branches and browse on them directly.

These monkeys travel in groups that consist of 5 to 20 individuals. Social groups usually have one or more adult males, and two or more adult females. The adult male has three main responsibilities, which include detecting predators, holding the group together, and patrolling the boundaries of the territories. The young monkeys play in groups near the vicinity of an adult female. Overall, Dusky Leaf Monkeys are quite social animals.

Dusky Leaf Monkeys have a wide range of calls that are considered to be quite complex. A variety of snorts, hoots, murmurs, and squeaks are used to communicate with other members of their social group. The motion of the tail plays a significant role in maintaining balance. (Grzimek, 1990; Lekagul and McNeely, 1977; Medway, 1969) The diet of these monkeys consists of young leaves, shoots, and seedlings. They feed from 87 different species of trees, ingesting both leaves and fruit. In general, a dusky leaf monkey eats up to 2 kg of food per day.  These monkeys can be maintained, in captivity, on sweet potato shoots, lettuce, cabbage, kangkong, green beans, maize, carrots, and soft fruits. Meat was refused, but certain insects were occasionally accepted. (Lekagul and McNeely, 1977; Medway, 1969)

Reproduction:

There is no information available regarding the mating system of dusky leaf monkeys. However, the social system typically involves groups with only one or two males. In other primate species, this social organization is typically associated with polygynous breeding. It is reasonable to assume that this species is like other similar primates in this respect. (Lekagul and McNeely, 1977; Medway, 1969; Nowak, 1999)They monkeys reproduce every two years. They monkeys breed intermittently throughout the year. Range number of offspring 1 to 2.The  average number of offspring is 1 with an average gestation period of 145 day with an average birth mass of 300 g (10.57 oz The age at sexual or reproductive maturity (female) 3 to 4 year. The age at sexual or reproductive maturity (male)3 to 4 years

Breeding in Dusky Leaf Monkeys is intermittent and not always seasonal. Births usually take place during the months of January, February, and March, but have been documented to occur during the summer months as well. Typically one young is born. The gestation period is, on average, 145 days. Females have a menstrual cycle lasting approximately three weeks. Oestrus is often accompanied by a swelling of the genitalia. The normal interbirth interval is about 2 years. Sexual maturity is reached between 3 and 4 years of age. (Grzimek, 1990; Lekagul and McNeely, 1977; Nowak, 1999) Research on the parental behavior of these monkeys is lacking. However, we may assume that they are like other primates in that the mother provides the bulk of the parental care. She grooms, protects, and feeds the newborn. The role of the father in parental care is not known. There is no information available regarding the weaning age of dusky leaf monkeys. It is known, however, that the newborn monkey is fully furred and active. (Grzimek, 1990)

Communication and Perception:

Details on communication in these monkeys are scant. However, we know that they use vocalizations to protect their territories from other members of the species. Like other primates, tactile communication (e.g. grooming, playing, mating, aggression) and visual communication (e.g. facial expressions and body postures) are probably both important in these monkeys. (Nowak, 1999)

Predation:

Information on predation of these monkeys is not available. Because they are arboreal, it is likely that they do not have many predators. Possible predators are large carnivores, snakes, and raptors. It is probable that Dusky Leaf Monkeys are hunted for food by the human population, since a large number of primates are hunted as sources of food throughout Asia. (Grzimek, 1990). Dusky Leaf Monkeys are generally restricted to primary forests. Therefore, it is unlikely that they contribute to any serious agricultural (or other) problems that would adversely affect humans. (Grzimek, 1990)


Ecosystem Roles:

These monkeys are likely to be important predators of foliage. They may help to disperse seeds. To the extent that they serve as prey for other species, these monkeys may affect local food webs.

Major Threat(s):

Hunting for food is a major threat, as is habitat loss and degradation due to expanding oil palm plantations, agriculture, and urbanization. In Peninsular Malaysia the animals are frequent victims of road-kill.

Conservation Actions:

This species is listed on CITES Appendix II. It is known to occur in a number of protected areas, including: Krau Wildlife Reserve, Taman Negara National Park (Malaysia); Kaeng Krachan National Park, Khao Sam Roi Yot National Park, Khao Sok National Park, Taratau National Park (Thailand). It is relatively common in captivity. There is a need for further survey work to determine the current population status of the insular forms. Dusky leaf monkeys are confined within a relatively small area of southeast Asia. It is probable that these forested areas are under threat of development or logging. Therefore, there is reason to believe that the species is threatened to some extent because of habitat loss, but there are no studies to support this. (Grzimek, 1990
IUCN Red List of Threatened Species
Near Threatened

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