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)
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)
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)
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)
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)
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)
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)
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.
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)
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)
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)
These langurs are not known to have any particular benefit to humans.
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)
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.
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
helpers provide assistance in raising young that are not their own
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.
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.
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.
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.
breeding takes place throughout the year
young are relatively well-developed when born
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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 http://www.springerlink.com/media/np330r1a8gdrrjcauq7p/Contributions/4/R/A/U/4RAUVQNAP5B6P0JR.pdf.
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