Presbytis thomasinorth Sumatran leaf monkey

Geographic Range

Presbytis thomasi is one of several langurs found in the Oriental islands. However, its geographic range is fairly limited. Its native home consists of North Sumatra of Indonesia—more specifically, north of Sungai Wampu and Sungai Simpang Kiri. (Colijn and Muchtar, 1996; Nowak, 1999)


When searching through tropical rainforests, rubber plantations, and both primary and secondary forests, one is able to encounter Thomas’s langur. Since it is arboreal, one usually needs to search high in the trees to find this unique species. However, its positioning in the trees depends on what time of day it is. While it is taking a nap during the day, it selects a tree that tends to have lots of twigs and leaves for protection from the harsh sunlight. However, when it is sleeping at night, it sleeps in the top of a tall tree that faces the open areas. (Colijn and Muchtar, 1996; Flannery, 2004; Gurmaya, 1986)

Physical Description

Presbytis thomasi has a very distinct appearance. Due to their unique facial coloration, it is easy to distinguish North Sumatran leaf monkeys from other primates. The white fur on the underside and arms (which contrasts with the black fur surrounding the rest of the body) continues up around the neck. Two other white stripes, starting from the top of the head, run down the sides, come together in a V-shape at the eyes and encircle them. A purple-silver colored inner layer forms rings around the orange-brown eyes. Inside of the dark tint, one can see the pinkish skin of Thomas's langur. This same pinkish skin covers the muzzle.

The average size of Thomas's langur is 6.69 kilograms for adult females and 6.67 kilograms for adult males. The tail length is between 500 and 850 mm, and the head and body length ranges between a mere 420 and 620 mm. (Colijn and Muchtar, 1996; Flannery, 2004; Nowak, 1999)

  • Sexual Dimorphism
  • sexes alike
  • Range mass
    5 to 8.1 kg
    11.01 to 17.84 lb
  • Range length
    92 to 147 cm
    36.22 to 57.87 in


The mating system of P. thomasi is debated. In the Encyclopedia of Mammals, it is noted that the species is a monogamous primate. The female initiates the mating by performing various acts to persuade her male counterpart, such as releasing certain smells and displaying genitalia. ("Colobus and Leaf Monkeys", 2001)

However, others dispute the monogamy of the species. Steenbeck, et al. (1999) states that within groups, there are often several females and one breeding male. A possible resolution between the two observations is that only one of the females in the group breeds with the male while the other females help raise the young. (Steenbeek, et al., 1999)

There is no specified breeding interval for Thomas’s langurs. However, even though the breeding season is not restricted, there tends to be an increase in mating when the following weaning period is expected to correspond with an abundance of food. The gestation period lasts 5 to 6 months. At the end of gestation, female P. thomasi produce one young one young at a time. Females rarely produce more than one offsrping at a time, and never more than two. Weaning occurs at 12 to 15 months, after which an offspring is supposed to become independent. Although they are fully independent, juveniles do not reach sexual maturity until 4 or 5 years of age. ("Colobus and Leaf Monkeys", 2001; Eimerl and DeVore, 1965)

  • Breeding interval
    Breeding can occur every 1.5 to 2 years.
  • Breeding season
    The breeding season of these monkeys is not restricted.
  • Range number of offspring
    1 (low)
  • Average number of offspring
  • Range gestation period
    5 to 6 months
  • Range weaning age
    12 to 15 months
  • Range time to independence
    15 to 18 months
  • Average age at sexual or reproductive maturity (female)
    4 years
  • Range age at sexual or reproductive maturity (male)
    4 to 5 years

When caring for her young, the mother removes herself from the dominance structure. Other females of the group are often attracted to the young due to the distinct coloring, and so they care for and protect the young whenever possible. As soon as the infant becomes upset or distraught, another female quickly tries to comfort it. The male infant has no contact with a male adult until he is 10 months old. During pre-weaning, the young has to learn what to eat, what to avoid, and other behavioral tactics in order to survive. ("Colobus and Leaf Monkeys", 2001; Eimerl and DeVore, 1965)

The male infant has no contact with a male adult until he is 10 months old. A female infant, however, has no contact with an adult male until she is 3.5 to 4 years old. Often during weaning, a male within the group or an outside group commits infanticide, killing an infant so that the mother can regain her normal cycle of fertility faster than she would if her child were still alive. This may explain the delay in contact between young and adult males. ("Colobus and Leaf Monkeys", 2001; Eimerl and DeVore, 1965)

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


The average lifespan of P. thomasi is 20 years. In captivity, the average lifespan is 29 years. The nine-year difference may be due to numerous factors such as the destruction of habitat, hunting by humans, the presence of natural predators, and attacks between neighboring groups. ("Colobus and Leaf Monkeys", 2001)

  • Average lifespan
    Status: wild
    20 years
  • Average lifespan
    Status: captivity
    29 years


These langurs spend most of the day in groups either resting, feeding, or moving. These groups usually consist of five females and one male, but there can also be small groups of males or individual males living alone.

Within these groups, there is a dominance hierarchy that everyone, both male and female, is subjected to. It has been suggested that an individual’s rank within the hierarchy may depend upon its age or its ability to defend itself against others in the group. ("Colobus and Leaf Monkeys", 2001; Eimerl and DeVore, 1965; Gurmaya, 1986; Steenbeek, et al., 1999)

Even though this langur is known for having a calmer demeanor in gestures and responses to conspecifics than other primates, competition has been observed within groups. Competition increases with the size of the group, and femlaes show a preference for smaller groups due to the decrease in risk of infanticide. Group sizes are often dependent upon female dispersal. ("Colobus and Leaf Monkeys", 2001; Steenbeck and van Schaik, 2001)

Many factors impact a female’s decision to leave her group and join another. These include competition for food, the risk of predation, and avoidance of inbreeding. More important is the avoidance of infanticide. Often a female leaves her group to protect her young from infanticide. (Sterck, 1997)

Infanticide appears to be a very important behavior for P. thomasi. Infanticide usually occurs when males attack. Females do not typically attack unless defending their young from outside males. Females are much more aggressive when they have an infant than they are when they have no dependent offspring. When the group male is within 5 meters of a mother and her child, she is significantly less vigilant, apparently because the presence of the male so close by is an assurance of protection. (Steenbeek, et al., 1999)

  • Range territory size
    500 to 800 m^2

Home Range

Like most langurs, P. thomasi is both diurnal and arboreal. It occassionaly retrieves mineral resources from the ground. Groups relocate to different areas within the forests by migrating through the trees. This social species is also very territorial, often defending its area by barking or even attacking outsiders. (Nowak, 1999)

Communication and Perception

Vocal communication is at its most intense and frequent at dawn. It is utilized in a variety of situations such as relocation, attacking, establishing sleeping positions, defending territory and mates. Vocalizations are accompanied by olfactory communication when mating is intitiated. ("Colobus and Leaf Monkeys", 2001; Eimerl and DeVore, 1965)

Thomas' langurs use numerous types of vocal calls. For example, an alpha male tends to make a series of “choom” sounds when he is excited; however, when he is involved in an inter-troop or group-troop encounter, he makes a series of “kak” and “ngkung” sounds. Similarly, when threatened, juveniles make an alternating series of “kek”s and “wek”s. Aggressive females make “kuk” sounds. (Gurmaya, 1986)

Vocal and visual communication develop as these monkeys mature. In infant P. thomasi, communication is restricted to whining and squealing. Once an individual becomes a juvenile, its abilities have broadened to screaming, grimacing, slapping the ground, present, alarm barking, staring, and threat bobbing. Finally, as an adult, it no longer squeals or screams, but barks and partakes in dominance fighting. ("Colobus and Leaf Monkeys", 2001; Eimerl and DeVore, 1965)

As in all primates, tactile communication, between mates, rivals, as well as between mothers and their offspring is important. Tactile communication includes grooming, reassurance, and aggression.

Food Habits

This leaf-monkey’s diet is primarily centered around fruits, leaves, and seeds. However, it will eat other things such as flowers, bark, twigs, stalks of coconuts, leaf stalks, birds, bird eggs, algae, and some insects. Water is made available in tree holes. Occasional visits to the ground are made in order to obtain ants, toadstools, soil, and snails. Large peaks in foraging occur three times per day, accompanied by resting in the lower portions of trees. Foraging behavior is highly influenced by the risk of predation. (Flannery, 2004; Gurmaya, 1986; Nowak, 1999; Sterck, 2002; Swindler, 1998)

  • Animal Foods
  • birds
  • eggs
  • insects
  • mollusks
  • Plant Foods
  • leaves
  • wood, bark, or stems
  • seeds, grains, and nuts
  • fruit
  • flowers
  • algae
  • Other Foods
  • fungus


A characteristic shared among most of P. thomasi's predators is the ability to climb trees. Predators such as reticulated pythons, clouded leopards, tigers, and golden cats are often successful in capturing Thomas’s langurs. Because these langurs are able to move swiftly through branches, their predators have been found to attack only when the distance between them and their prey is very short. Predators are more effective when attacking P. thomasi on the ground rather than in trees. Because of this, the most dangerous area for these monkeys is in the lower strata of the forest, which is 0 to 10m.

Thomas' langurs rely on two valuable forms of protection: their arboreal habits and the alarm calls that are produced when groups are around. Thomas’s langurs climb down from trees more often when they are surrounded by neighbors.

Females are more prone to predation due to their preference for snails and their smaller canines. Males are often found nearby in trees looking out for predators when females venture down to the ground by themselves. (Flannery, 2004; Sterck, 2002)

Ecosystem Roles

Because of its preference for fruits, flowers, and seeds, one can infer that P. thomasi may disperse seeds and help pollinate plants while feeding. Like many primates, P. thomasi may also be a host to parasites such as fleas and ticks. To the extent that these monkeys serve as prey for other animals, they may impact predator populations. (Flannery, 2004)

  • Ecosystem Impact
  • disperses seeds
  • pollinates

Economic Importance for Humans: Positive

These langurs are not known to have any particular benefit to humans.

Economic Importance for Humans: Negative

Due to their primary diet of fruits, leaves, and seeds, Thomas's langurs are known to disrupt the crops of neighboring humans. ("Colobus and Leaf Monkeys", 2001)

  • Negative Impacts
  • crop pest

Conservation Status

Due to an increase in the availability of firearms and the destruction of forests in Sumatra, P. thomasi has to face losing its habitat and being hunted by neighboring humans. Another factor which may be responsible for declining populations is habitual infanticide that persists in this species. This behavior has been known to have increased in recent years; however, an increase in awareness has indicated that this behavior is very important to the species. ("Colobus and Leaf Monkeys", 2001; "Colobus and Leaf Monkeys", 2001; Steenbeek, et al., 1999)


Nancy Shefferly (editor), Animal Diversity Web.

Matthew Wund (editor), University of Michigan-Ann Arbor.

Mika Matthews (author), University of Michigan-Ann Arbor, Phil Myers (editor, instructor), Museum of Zoology, University of Michigan-Ann Arbor.



uses sound to communicate


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.


uses smells or other chemicals to communicate


to jointly display, usually with sounds, at the same time as two or more other individuals of the same or different species

cooperative breeder

helpers provide assistance in raising young that are not their own

  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


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.


union of egg and spermatozoan


an animal that mainly eats leaves.


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


an animal that mainly eats fruit


an animal that mainly eats seeds


An animal that eats mainly plants or parts of plants.

island endemic

animals that live only on an island or set of islands.


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).


Having one mate at a time.


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.


generally wanders from place to place, usually within a well-defined range.


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

World Map


chemicals released into air or water that are detected by and responded to by other animals of the same species


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.


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


associates with others of its species; forms social groups.


uses touch to communicate


Living on the ground.


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


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.


uses sight to communicate


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


Andromeda Oxford Ltd. 2001. Colobus and Leaf Monkeys. Pp. 380-393 in D Macdonald, ed. The Encyclopedia of Mammals, Vol. II: Primates and Large Herbivores, 2nd Edition. 132 West 31st Street, New York NY 10001: Facts on File, Inc..

Colijn, E., M. Muchtar. 1996. "Primates of Indonesia--Presbytis Thomasi (Collet, 1893)" (On-line). The Indonesian Nature Conservation Database. Accessed February 08, 2004 at

Eimerl, S., I. DeVore. 1965. Life Nature Library: The Primates. New York: Time-Life Books.

Flannery, S. 2004. "Thomas's Leaf-monkey (Presbytis thomasi)" (On-line). The Primata. Accessed April 10, 2004 at

Gurmaya, K. 1986. Ecology and Behavior of Presbytis thomasi in Northern Sumatra. Primates, 27(2): 151-172.

Nowak, R. 1999. Primates; Family CERCOPITHECIDAE: Old World Monkeys. Pp. 599-600 in Walker's Mammals of the World, Vol. I, 6th Edition. Baltimore and London: The Johns Hopkins University Press.

Steenbeck, R., C. van Schaik. 2001. Competition and group sive in Thomas's langurs (Presbytis thomasi): the folivore paradox revisited. Behavioral Ecology and Sociobiology, 49(2-3): 100-110. Accessed March 07, 2004 at

Steenbeek, R., R. Piek, M. van Buul, J. van Hooff. 1999. Vigilance in wild Thomas's langurs (Presbytis thomasi): the importance of infanticide risk. Behavioral Ecology and Sociobiology, 45: 137-150. Accessed February 11, 2004 at

Sterck, E. 1997. Determinants of female dispersal in Thomas Langurs. American Journal of Primatology, 42(3): 179-198.

Sterck, E. 2002. Predator sensitive foraging in Thomas langurs. Pp. 74-91 in L Miller, ed. Eat or be eaten: Predator sensitive foraging among primates. Cambridge, UK: Cambridge University Press.

Swindler, D. 1998. Introduction to the Primates. Seattle and London: University of Washington Press.