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.

Biogeographic Regions: oriental (Native )

Other Geographic Terms: island endemic

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.

Range mass: 5 to 8.1 kg.

Range length: 92 to 147 cm.

Other Physical Features: endothermic ; homoiothermic; bilateral symmetry

Sexual Dimorphism: sexes alike

• Terrestrial

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.

Habitat Regions: tropical ; terrestrial

Terrestrial Biomes: forest ; rainforest

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.

Animal Foods: birds; eggs; insects; mollusks

Plant Foods: leaves; wood, bark, or stems; seeds, grains, and nuts; fruit; flowers; algae

Other Foods: fungus

Primary Diet: herbivore (Folivore , Frugivore , Granivore )

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.

Ecosystem Impact: disperses seeds; pollinates

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.

Known Predators:

• reticulated pythons (Python reticulatus)
• clouded leopards (Neofelis nebulosa)
• tigers (Panthera tigris)
• Asiatic golden cats (Catopuma temminckii)

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.

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.

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.

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.

Communication Channels: visual ; tactile ; acoustic ; chemical

Other Communication Modes: choruses ; pheromones

Perception Channels: visual ; acoustic

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.

Average lifespan

Status: wild: 20 years.

Average lifespan

Status: captivity: 29 years.

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.

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.

Mating System: monogamous ; cooperative breeder

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.

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: 1.

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.

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

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.

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.

Parental Investment: precocial ; pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Provisioning: Female, Protecting: Female); extended period of juvenile learning

• 2000
  Lower Risk/near threatened
• 1996
  Lower Risk/near threatened

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.

US Federal List: no special status

CITES: appendix ii

IUCN Red List of Threatened Species: vulnerable

Due to their primary diet of fruits, leaves, and seeds, Thomas's langurs are known to disrupt the crops of neighboring humans.

Negative Impacts: crop pest

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