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.

• Biogeographic Regions
• oriental oriental
  • native native

Habitat

Dusky leaf monkeys can be found in a wide range of habitats. Being arboreal forest dwellers, they prefer dense forests with tall trees.

• Terrestrial Biomes
• taiga taiga
• forest forest
• rainforest rainforest

Physical Description

Dusky leaf monkeys are widely variable in color. 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 nonprehensile 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 color, and have a pink face; the fur changes to a greyish color within six months.

• Other Physical Features
• endothermic endothermic
• bilateral symmetry bilateral symmetry

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.

• Mating System
• polygynous polygynous

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 menstual 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.

• Key Reproductive Features
• iteroparous iteroparous
• year-round breeding year-round breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
• viviparous viviparous

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.

• Parental Investment
• precocial precocial
• pre-fertilization
  • provisioning
  • protecting
    • female
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • female
• pre-weaning/fledging
  • provisioning
    • female
  • protecting
    • female
• pre-independence
  • protecting
    • female
• extended period of juvenile learning

Lifespan/Longevity

Another monkey in the same genus, Trachypithecus cristatus , is reported to have lived over 31 years in captivity. It is likely that T. obscurus is similar. Lifespan in the wild is probably lower than in captivity.

Behavior

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 quadripedally 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.

• Key Behaviors
• arboreal arboreal
• scansorial
• diurnal diurnal
• motile motile
• sedentary sedentary
• territorial territorial
• colonial colonial
• dominance hierarchies dominance hierarchies

Home Range

Group territories between 5 and 12 ha have been reported for animals inhabiting the Malay penninsula.

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 inmportant in these monkeys.

• Communication Channels
• visual visual
• tactile tactile
• acoustic acoustic

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

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, grean beans, maize, carrots, and soft fruits. Meat was refused, but certain insects were occasionally accepted.

• Primary Diet
• herbivore herbivore
  • folivore folivore

• Plant Foods
• leaves
• seeds, grains, and nuts
• fruit
• flowers

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.

Ecosystem Roles

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

• Ecosystem Impact
• disperses seeds

Economic Importance for Humans: Positive

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.

• Positive Impacts
• food food

Economic Importance for Humans: Negative

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.

Conservation Status

There is very little information about the status of dusky leaf monkeys. I found no published estimates of how many monkeys currently exist.

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.

Additional Links

Encyclopedia of Life

Contributors

Nancy Shefferly (editor), Animal Diversity Web.

Lata Viswanathan (author), University of Michigan-Ann Arbor, Phil Myers (editor), Museum of Zoology, University of Michigan-Ann Arbor.

References

Grzimek, B. 1990. Grzimek's Encyclopedia of Mammals; Volume 2 . New York: McGraw-Hill Publishing Company.

Lekagul, B., J. McNeely. 1977. Mammals of Thailand . Kurusapha Ladprao:

Medway, L. 1969. The Wild Mammals of Malaya (Peninsular Malaysia) and Singapore . New York: Oxford University Press.

Nowak, R. 1999. Walker's Mammals of the World, Sixth Edition . Baltimore and London: The Johns Hopkins University Press.

Wisconsin Regional Primate Research Center, University of Wisconsin, Madison, 2000. "Primate Info Net, Dusky Leaf Monkey (Trachypithecus obscurus)" (On-line). Accessed February 12, 2002 at http://www.primate.wisc.edu/pin/factsheets/trachypithecus_obscurus.html .