Presbytis thomasi
north Sumatran leaf monkey

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 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. (Andromeda Oxford Ltd, 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. (Andromeda Oxford Ltd, 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. (Andromeda Oxford Ltd, 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)

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)

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. (Andromeda Oxford Ltd, 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. (Andromeda Oxford Ltd, 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)

Due to their primary diet of fruits, leaves, and seeds, Thomas's langurs are known to disrupt the crops of neighboring humans. (Andromeda Oxford Ltd, 2001)

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

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. (Andromeda Oxford Ltd, 2001; Steenbeek et al., 1999)