Trachypithecus obscurusdusky leaf monkey

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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)

Habitat

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)

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. (Grzimek, 1990; Medway, 1969)

  • Range mass
    5 to 9 kg
    11.01 to 19.82 lb
  • Range length
    42 to 61 cm
    16.54 to 24.02 in

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)

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. (Grzimek, 1990; Lekagul and McNeely, 1977; Nowak, 1999)

  • Breeding interval
    These monkeys reproduce every two years.
  • Breeding season
    These monkeys breed intermittantly throughout the year.
  • Range number of offspring
    1 to 2
  • Average number of offspring
    1
  • Average number of offspring
    1
    AnAge
  • Average gestation period
    145 days
  • Average gestation period
    145 days
    AnAge
  • Range age at sexual or reproductive maturity (female)
    3 to 4 years
  • Range age at sexual or reproductive maturity (male)
    3 to 4 years

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)

  • Parental Investment
  • 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. (Nowak, 1999)

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. (Grzimek, 1990; Lekagul and McNeely, 1977; Medway, 1969)

  • Range territory size
    5000 to 12,000 m^2

Home Range

Group territories between 5 and 12 ha have been reported for animals inhabiting the Malay penninsula. (Nowak, 1999)

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. (Nowak, 1999)

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. (Lekagul and McNeely, 1977; Medway, 1969)

  • 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. (Grzimek, 1990)

  • Positive Impacts
  • 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. (Grzimek, 1990)

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. (Grzimek, 1990)

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.

Glossary

acoustic

uses sound to communicate

arboreal

Referring to an animal that lives in trees; tree-climbing.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

colonial

used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.

diurnal
  1. active during the day, 2. lasting for one day.
dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

fertilization

union of egg and spermatozoan

folivore

an animal that mainly eats leaves.

food

A substance that provides both nutrients and energy to a living thing.

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

herbivore

An animal that eats mainly plants or parts of plants.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

World Map

polygynous

having more than one female as a mate at one time

rainforest

rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate

taiga

Coniferous or boreal forest, located in a band across northern North America, Europe, and Asia. This terrestrial biome also occurs at high elevations. Long, cold winters and short, wet summers. Few species of trees are present; these are primarily conifers that grow in dense stands with little undergrowth. Some deciduous trees also may be present.

territorial

defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

year-round breeding

breeding takes place throughout the year

young precocial

young are relatively well-developed when born

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.