Thomas’s Leaf Monkey, Presbytis thomasi
THOMAS'S LEAF MONKEY
Presbytis thomasi
Geographic Distribution and Habitat
The Thomas’s leaf monkey (Presbytis thomasi), also known as the Thomas’s langur, the North Sumatran leaf monkey, or the Sumatran grizzled langur, is an Asian primate native to the northern regions of Sumatra, Indonesia.
This species inhabits the evergreen tropical rainforests of Sumatra, where temperatures remain stable throughout the year. They are typically found in the inner areas of forests northwest of the Wampu River and north of the Alas River. Some populations have been spotted south of the Alas River, although they are generally absent in the regions south and east of Lake Toba and the Wampu and Alas rivers.
Thomas’s leaf monkeys occupy a variety of habitats, including swamp forests, lowland forests, and lowland alluvial forests—areas shaped by abundant rainfall and flowing rivers. They can also be found scattered in mosaics of rubber plantations, complicating estimates of their home range. Population density decreases with elevation, although they can be found at altitudes as high as 11,155 feet (3,400 meters).
Size, Weight, and Lifespan
Thomas’s leaf monkeys are medium-sized primates with long tails. Their head-to-body length is approximately 21.65 inches (55 cm), while their tails add 27–30 inches (69–76 cm). They typically weigh 15.43–17.64 pounds (7–8 kg). In the wild, their lifespan is up to 20 years, while in captivity they can live up to 29 years.
There are no significant differences between the sexes, except for their canines, which are more pronounced in males.
Appearance
What is sure to catch the eye of the observer is definitely the black mohawk that crowns their heads. This eye-catching hairstyle is even more striking because it contrasts with the white fur covering the rest of their heads.
They have piercing brown eyes that are framed by short, silvery-gray hair that resembles a mask highlighted by patches of white fur. Their ears are black, and their lips are brownish. Soft, silvery-gray fur covers most of their bodies, while their bellies and chests are covered by creamy white fur. Their long, slender limbs are covered in dark gray fur, while their hands and feet are jet black.
The young look like adults but are much lighter in color. The characteristic mohawk is absent in infants, and the fur on their heads and the inner part of their tails is completely white.
Diet
The diet of Thomas’s leaf monkeys is varied and influenced by local resources. They are primarily folivorous, which means that they feed on plant leaves. Their specialized multi-chambered stomachs and unique gut flora allow them to efficiently digest fibrous plant matter, including unripe fruit and leaves. These are hard-to-digest substances, but these animals are perfectly suited to this task.
Despite their love for greens, they do not turn up their noses at flowers, ripe fruit, and buds. Studies of populations in Ketambe reveal they consume 218 plant species and occasionally small mollusks like snails, exposed algae during drier seasons, and dirt from termite mounds. In contrast, populations in more agricultural areas, such as Bahorok, often feed on farmed plants like bananas and rubber.
Behavior and Lifestyle
Thomas’s leaf monkeys are largely arboreal, that is they spend most of their time in trees. They move through the forest canopy by leaping and moving on four limbs. They also forage on the ground, looking for delicacies such as snails and algae.
These monkeys love to take it easy. Their day includes a lot of resting, likely due to the energy demands of digesting unripe fruit and leaves. Eating and socializing occupy a smaller portion of their day. Despite their resting habits, they are highly social and gregarious animals.
The tropical forests are home to a myriad of animals. Thomas’s leaf monkeys must be on the lookout for predators such as clouded leopards, Neofelis nebulosa, Sumatran tigers, Panthera tigris sumatrae, bay cats, Pardofelis badia, and reticulated pythons. Malayopython reticulatus. They seek refuge in trees and alert group members with loud alarm calls whenever they spot them.
Thomas’s leaf monkeys live in groups ranging from 2 to 16 individuals. They are typically male-dominated, with an alpha male leading multi-female groups. However, all-male bands and groups with two adult males have also been recorded. Groups with more than one adult male are usually larger, containing up to 21 individuals. Their home range varies and it is estimated to be around 66 acres (27 ha).
Group composition changes frequently as both males and females disperse. Truth be told, males leave out of necessity, while females leave for reasons not yet fully understood.
Alpha males have a reign lasting about 60 months, divided into three phases:
- Initial Tenure: Begins with the male joining a group and continues until the birth of his first offspring.
- Middle Tenure: A stable phase marked by relative group harmony.
- Late Tenure: The final 12 months, during which females with independent offspring often leave.
Female dispersal often involves taking young and dependent female offspring. Juvenile males are instead left behind due to common infanticide behavior of this species: new alpha males kill juvenile males to secure their dominance and preserve their gene pool.
On balance, groups change because females seek other groups after raising their children, while males are forced to form new groups since they are eventually left behind.
Thomas’s leaf monkeys are a highly vocal species. The vocal repertoire of this primate species consists of 13 identified vocalizations, most of which are produced by males. These calls play essential roles in aggression, social cohesion, and responses to external disturbances.
Aggressive and Warning Signals
- Kak-kak-kak…..ngkung-ngkung-ngkung: Combined calls that start loudly before lowering, with a nasal tone at the end. These are uttered by males during aggressive interactions or in response to human disturbances.
- Choom-Choom-Choom: Cough-like calls emitted by alpha males as a warning signal.
- Chup-Chup-Chup: Similar to coughs, these are used by beta males and females during aggressive interactions, especially directed toward alpha males.
Social Cohesion and Group Relations
- Wek-Wek-Wek/Kek-Kek-Kek: Duck-like squeaks produced by juveniles, likely used in playful or demonstrative group interactions.
- Kut-Kut-Kut: Hen-like clucks emitted by females when chasing others.
- Krr-Krr-Krr: Snore-like sounds made by alpha males to herd females within the group.
- Chaark-Chaark-Chaark: Husky, low-toned sounds produced by low-ranking males when approached by alpha males, signaling submission.
Submissive and Emotional Calls
- Chiit-Chiit-Chiit: Squeaky sounds emitted by juveniles to express submission toward alpha males.
- Shiit-Shiit-Shiit: Sharper and louder than “Chiit,” this sound is used by all age groups and may indicate frustration, particularly in infants.
- Chekok/Kok: Single abrupt calls emitted after surprising events, for example when an Owl flies by.
- Check-Check-Check: Low hacking sounds made by all monkeys except infants when passing near human observers, potentially expressing fear.
- Graaakk: Deep, singular calls produced by juveniles, alpha males, and subadult females during play-fighting, chasing, or actual fighting.
- Uuungk-Uuungk-Uuungk: Tonal, nasal calls produced by infant and juvenile males during tense situations, such as when group structure changes, possibly reflecting worry or uncertainty.
The loud calls of Thomas’s langur males serve multiple purposes, from intergroup defense to within-group communication. They are highly structured, lasting approximately 4 seconds, and incredibly powerful as they can be heard at considerable distances. These calls begin with a rapid buildup, reach a peak, and then decline in intensity.
When two Thomas’s leaf monkey groups meet one another, the resident male takes center stage and intimidates potential rivals with loud vocalizations and physical displays such as chasing, running, and jumping. In contrast, the females of the groups typically ignore one another and remain silent.
Different studies suggest that Thomas’s leaf monkeys can recognize calls from members of their own group, and of specific individuals. Males give alarm calls only when other group members are present. If a predator approaches a solitary male, he remains silent. It is clear that their calls are not merely the result of emotional turmoil, but rather a refined communicative signal that depends on context.
Like in most primate species, grooming plays an important role as a communicative gesture that functions as social glue.
Living in a multi-female/one-male society means that a single alpha male has the right to mate with a multitude of females who have joined his group. Males tend to be quite protective of the females, while territorial aggression is less common among this species. There is no specific mating season; nevertheless, Thomas’s leaf monkeys prefer to breed when food is abundant and they are free of any dependent offspring.
After a 5–6-month gestation period, Thomas’s leaf monkeys give birth to a single offspring, rarely to twins. Their babies are taken care of by their mothers for several months, and they are weaned after 12–16 months when the offspring are technically independent. Juvenile Thomas’s leaf monkeys, however, only reach sexual maturity around the age of 5. Parental care is almost exclusively provided by the mothers, as males rarely take care of their offspring. Despite this, they do offer protection to their group.
After giving birth, females tend to disperse and seek other groups or single males to join. One of the assumed reasons for this behavior is to discourage inbreeding. It has also been suggested that females seek males who are better at protecting them, especially when their original group is too small.
As a folivorous species, Thomas’s leaf monkeys help promote plant growth and maintain biodiversity by consuming leaves, allowing certain plants to take over the forest floor and dominate their ecosystem. They play a vital role in pruning leaves, which allows sunlight to penetrate the dense canopies of the tropical forests they inhabit, stimulating further vegetation growth. Additionally, they disperse seeds and assist with pollination—both essential factors aiding reforestation. Finally, their digested plant matter is returned to the forest floor via their excrement, recycling nutrients back into the soil.
The Thomas’s leaf monkey is classified as Vulnerable by the International Union for Conservation of Nature (IUCN, 2015), appearing on the IUCN Red List of Threatened Species. The Thomas’s leaf monkey is Vulnerable due to population declines, estimated at more than 30% over the past 40 years.
They are not significantly threatened by hunting due to local religious beliefs and taboos. However, hunting is still a problem to address in some areas. Logging and agriculture are the primary sources of concern as they have caused severe damage to their habitat. Palm oil plantations, for example, have caused population displacements. This species is also often sold at local markets as a pet.
Thomas’s leaf monkeys are listed in Appendix II of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival. A few populations live in protected areas, for example in the national park of Gunung Leuser.
1. Gurmaya, K. J. (1986). Ecology and behavior of Presbytis thomasi in Northern Sumatra. Primates, 27(2), 151–172.
2. Indonesian Primate Profile Presbytis thomasi. (2016). Jurnal Primatologi Indonesia, 13(2), 1–2. https://journal.ipb.ac.id/index.php/primata
3. IUCN SSC Primate Specialist Group. (2008). Presbytis thomasi. In IUCN Red List of Threatened Species 2008: e.T18132A17954139. International Union for Conservation of Nature. https://www.iucnredlist.org/species/18132/17954139
4. Wich, S. A., Schel, A. M., & de Vries, H. (2008). Geographic variation in Thomas langur (Presbytis thomasi) loud calls. American Journal of Primatology, 70(6), 566–574. https://doi.org/10.1002/ajp.20527
5. Wich, S. A., Steenbeek, R., Sterck, E. H. M., Korstjens, A. H., Willems, E. P., & van Schaik, C. P. (2007). Demography and life history of Thomas langurs (Presbytis thomasi). American Journal of Primatology, 69(6), 641–651. https://doi.org/10.1002/ajp.20386
6. Wich, S. A., & Sterck, E. H. M. (2010). Thomas langurs: Ecology, sexual conflict, and social dynamics. In S. Gursky & J. Supriatna (Eds.), Indonesian Primates (pp. 285–308). Springer New York. https://doi.org/10.1007/978-1-4419-1560-3_17
7. Wich, S. A., & de Vries, H. (2006). Male monkeys remember which group members have given alarm calls. Proceedings of the Royal Society B: Biological Sciences, 273(1587), 735–740. https://doi.org/[insert DOI if available]
8. Wich, S. A., Koski, S., De Vries, H., & Van Schaik, C. P. (2003). Individual and contextual variation in thomas langur male loud calls. Ethology, 109(1), 1–13. https://doi.org/10.1046/j.1439-0310.2003.00837.x
9. Wich, S. A., & Sterck, E. H. M. (2003). Possible audience effect in Thomas langurs (Presbytis thomasi): An experimental study on male loud calls in response to a tiger model. American Journal of Primatology, 60(2), 155–159. https://doi.org/[insert DOI if available]
10. Mittermeier, R. A., Rylands, A. B., & Wilson, D. E. (2013). Presbytis thomasi (Collett, 1893). In Handbook of the Mammals of the World – Volume 3 Primates (p. 723). Lynx Edicions. https://zenodo.org/record/6863412
Written by Mirko Cerrone, December 2024