Presbytis thomasi

Geographic Distribution and Habitat
Indicative of one of its aliases, Northern Sumatran leaf monkeys, Thomas’s leaf monkeys inhabit the island of Sumatra in Indonesia, residing in remote Aceh Province. The species’ range extends north of the Wampu and Simpangkiri Rivers and stretches to the southern bank of the Simpangkiri River. Home range is between 30.39 to 38.79 ac (12.3-15.7 ha).

Thomas’s leaf monkeys reside in tropical primary and secondary forests, from sea level up to elevations of 4,921 ft (1,500 m). Rubber tree plantations provide the monkeys with alternative habitat, much to the chagrin of local farmers.

Sumatran grizzled langur and Thomas’s langur are two additional aliases for this species, whose future in our world is in jeopardy because of habitat destruction.

Thomas's leaf monkey geographic range. Map: IUCN, 2008

Size, Weight, and Lifespan
Male and female Thomas’s leaf monkeys are similar in size, weighing between 11 and 17.6 lb (5-8 kg).

The average weight for an adult male is 14.70 lb (6.67 kg); the average weight for an adult female is slightly more than her male counterpart, 14.75 lb (6.69 kg).

Head-to-body length in the species is between 16.5 to 24 in (42-61 cm). Thomas’s leaf monkey’s extremely long tail adds another 19.68 to 33.46 in (50-85 cm) to its body.

In the wild, Thomas’s leaf monkeys live about 20 years. Captive Thomas’s leaf monkeys are reported to live up to 29 years. Wildlife biologists speculate that the shorter lifespan in the wild is likely due to destruction of the species’ natural habitat, hunting by humans, natural predators, and attacks from neighboring primate groups.

Mother Nature appears to take exceptional delight in creating certain species. Certainly, Thomas’s leaf monkey is one of her more striking primate creations.

After topping the crown of Thomas’s leaf monkey’s head with an impressive black Mohawk, she accented it with bushy white tufts of fur on either side. A white ring, like the outline of a set of goggles, distinguishes the monkey’s expressive face. It is met by gray fur that extends dramatically, like wings, from either side of the monkey’s cheeks. Silvery-purple hues tint the skin beneath amber-colored eyes, and the monkey’s hairless muzzle, except for a thin, black moustache, is powdered pink. A furry white beard tickles the monkey’s chin.

The torso of Thomas’s leaf monkey is a contrast of light and dark coloring. Creamy, white fur covers the chest, stomach, and underside of arms and legs. The fur on its back and topside of limbs are dark gray; its long tail is pale on the underside and gray on top. Hands and feet are black.

Juvenile monkeys are cloaked in creamy white fur.

As you might guess, Thomas’s leaf monkeys love to eat leaves! Although they are primarily “folivorous” (leaf eaters), they also eat fruits, flowers, and the occasional toadstool or coconut stalks, and sometimes crunch a few ground snails with their sharp teeth. Their sharp teeth enable them to pierce through the husk or rinds of hard fruits (for which they have a penchant) and through thick bunches of leaves. To supplement the water they receive from their diet, the monkeys drink from pools of water collected in tree holes.

A specially adapted stomach allows Thomas’s leaf monkeys to digest leaf cellulose; microbes in the forestomach break down the cellulose, converting it to nutrients that the monkeys use for their daily energy.

Behavior and Lifestyle
Thomas’s leaf monkeys are active during daylight hours, making them a diurnal species. Resting is a favored pastime; they spend just over 60 percent of each day in repose. Just over 30 percent of each day is spent foraging, and time spent traveling and in social activity (such as grooming) make up less than 10 percent, collectively, of each day.

The species is largely arboreal; that is, Thomas’s leaf monkeys spend a lot of their time in trees, moving through the lower level of the canopy and the understory on all fours (“quadrupedally”) or by leaping from branch to branch. But they also spend a part of each day on the ground. To search for snails or toad stools, they must descend to the forest floor. Early morning, noon, and late afternoon are primary feeding times.

Foraging expeditions are tailored to avoid known predators, who include tigers, reticulated pythons, and clouded leopards. Females are more likely to fall victim to a predator, due to their more frequent visits to the ground to satisfy their dietary preference for snails. Males do their best to keep a lookout, from the relative safety of a nearby tree, when females are on the ground by themselves. Thomas’s leaf monkeys risk additional predator peril from humans whenever they venture close to human settlements.

For their daytime naps, Thomas’s leaf monkeys select the lower level of a tree with lots of twigs and leaves that block harsh sunlight. For overnight sleeping, they sleep in the upper level of a tall tree that faces open areas.

Known for a calm demeanor in interactions with other species who share the same habitat, Thomas’s leaf monkeys exhibit territorial behavior when food resources are scarce or when their habitat is threatened. Competition for resources between individuals of larger groups has also been reported. Interactions with other Thomas’s leaf monkeys occur when ranges overlap. Aggressive encounters sometimes occur between Thomas’s leaf monkeys and silvered leaf-monkeys (Trachypithecus cristatus). However, play between juveniles of different groups is also known to occur.

Fun Facts

Thomas’s leaf monkey has no subspecies, making it “monotypic.”

Daily Life and Group Dynamics 
Groups as small as three individuals and as large as 21 individuals, occurrences of all-male groups, and solitary males have been reported. However, unimale societies—that is, groups consisting of one male and several females—are the norm for Thomas’s leaf monkeys.

The male of a unimale society typically holds his position as leader (or alpha male) for about 6 years. His tenure relates to the number of females in a group. Females often stay or leave a group to protect their young; availability of food resources is another factor.

Individuals belonging to unimale societies perform activities as a group. But members of all-male societies separate into subgroups for certain activities, such as foraging. Like unimale societies, they engage in social grooming. Individuals in all-male groups are known to mount and embrace one another.

Infanticide—the killing of young offspring by adult individuals—is an extreme, but not uncommon, behavior in the species. It occurs more often when outside males attack a group; however, infanticide can also occur within a group’s membership. To lessen the risk of their young from becoming victims, females prefer belonging to a smaller group, where there is less drama, there are fewer aggressive interactions, and the group’s alpha male provides safety from attacks by outsider males.  

Females are not the initiators of aggressive attacks. However, when the alpha male is absent, they will defend their infants from attacks by males. Females might also decide to leave their birth group, with their infants in tow, if the situation calls for an exit strategy. Conversely, if a new adult male assumes leadership as alpha and a mother feels that he is a good protector, she may leave her offspring behind should she choose to leave a group for her own self-interests.

Males disperse from their birth groups as juveniles; rather, they are nudged out by adult male(s). Occasionally, the alpha male might permit his son to remain with the group. Although the son assumes a subordinate position to his father, he benefits from certain perks by remaining with his birth group. The son avoids being a target of aggressive actions, which are common in all-male groups, and he learns how to fight from his father.

Thomas’s leaf monkeys use a wide variety of vocalizations to communicate with one another. Their calls carry over long distances and are distinct from Sumatra’s other langur species. Calls convey information about mood, relocation, predator threat, attack, sleeping sites, territory, and mating. The species is most vocal at dawn.

All members of a group emit certain calls, but most calls are specific to an individual’s age or status. For example, infants whine or squeal. As they become older, as with human primates, their vocabulary broadens. Juveniles scream and learn to “bark.” In addition to “barking,” adult males emit a series of calls to convey warning or aggression in their encounters with group members or with members of outside groups. Adult females emit a hen-like call.

Calls are often accompanied by physical behavior, which includes grimacing, slapping the ground, play fighting, “threat bobbing,” staring, and actual fighting.

Grooming is an important tactile communication that helps to instill social bonds with one another. Between a mother and her infant, grooming conveys reassurance.

Olfactory communication accompanies the act of copulation, where the male receives a strong, enticing “whiff” from his female paramour. 

Reproduction and Family
Female Thomas’s leaf monkeys reach sexual (reproductive) maturity at 4 years old; males lag slightly behind, reaching sexual maturity between 4 and 5 years old.

The female appears to be the initiator when it comes to copulation. By releasing certain sex pheromones and flashing her genitalia, she charms the male into mating with her.

After a gestation period of 5 to 6 months, a female gives birth to a single offspring (twins are rare). Although the species breeds year-round, with a breeding interval between 1-1/2 and 2 years, births typically coincide when an abundance of food is available following an infant’s weaning period. During pre-weaning, a mother teaches her young which foods are safe to eat and which to avoid. Infants are considered weaned between 12 and 15 months of age. Juveniles are considered independent between 15 and 18 months old, though they are not yet reproductively mature.

Earlier researchers had believed that Thomas’s leaf monkeys practiced monogamy, having posited that only one female in a group breeds with the alpha male. Later researchers contested this claim, arguing that the species practices polygyny; that is, the alpha male mates with multiple females in his group. A hierarchy among females, likely defined by age, is a point on which researchers agree.

A new mother removes herself from the dominance structure to focus exclusively on her infant. Should her baby become upset or agitated, other females in the group quickly offer their comfort to the little one.

It’s possible that these other females act as a buffer between an infant and a group’s adult male(s). A male infant has no contact with a group’s adult male(s) until he is 10 months old. Females infants are kept from an adult male(s) even longer; they have no contact with adult male(s) until they are 3-1/2 to 4 years old.

The possibility of infanticide might be reason for this delayed introduction between offspring and the males who sired them. These males are known to kill infants before they are full weaned so that a mother’s “biological clock” will reset, and she can regain her normal fertility cycle quicker.

Ecological Role
Thanks to their diet, Thomas’s leaf monkeys act as plant pollinators; they help to regenerate their forest environment by dispersing seeds of the fruits they eat through their feces.

Conservation Status and Threats
Thomas’s leaf monkey is listed as Vulnerable (IUCN, 2015) by the International Union for Conservation of Nature, appearing on the IUCN Red List of Threatened Species. A Vulnerable status means that a species is likely to become endangered unless the circumstances threatening its survival and reproduction improve.

Like many of the animals who live on the island of Sumatra—including tigers, elephants, rhinos, and orangutans—Thomas’s leaf monkeys are imperiled by severe habitat loss. Logging, pulp, and paper industries have chewed up rainforest. Palm oil plantations, which provide a nefarious source to a lucrative commercial industry, have further eradicated lush habitat. The species has declined more than 30 percent, losing more than 3 generations, in the past decades.

Forced to venture onto local farmland, Thomas’s leaf monkeys have earned the reputation, from local farmers, as “crop pests.” An increase in firearm ownership among farmers has led to an unofficial culling of the species.

Thomas’s leaf monkeys are also victims of the illegal primate trade; they are captured, killed, dismembered, and their body parts sold for use in medicinal remedies.

Conservation Efforts
Thomas’s leaf monkey is 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. However, as is often the case with endangered species, laws created to protect them are largely ignored and difficult to enforce.

Thomas’s leaf monkeys are, in theory, protected by Indonesian law. They occur in Gunung Leuser National Park, a protected area. A quick Internet search of YouTube videos show that the species is becoming habituated (accustomed) to humans, even taking food from the hands of visitors to the park. Unfortunately, habituation can lead to negative consequences in wild species.



Written by Kathleen Downey, May 2018