Geographic Distribution and Habitat
White-thighed surilis (Presbytis siamensis), also known as pale-thighed langurs (a species of leaf monkey), are endemic to Southeast Asia. Isolated populations reside in the countries of Indonesia on the island of Sumatra and east of Sumatra on the granite islands of the Riau Archipelago within the Strait of Malacca; in the Peninsular Malaysia (also known as West Malaysia, a 13-state federation occupying the southern half of the Malay Peninsula); and in extreme southern Thailand.
The species dwells in submontane forests, favoring primary and secondary rainforests, swamp forests, mangrove forests, and rubber tree plantations. Researchers largely base these habitat conclusions on that of other more closely studied species within the same genus, including Robinson’s banded surili (Presbytis robinsoni).
In Sumatra, white-thighed surilis live in small sections of eastern forests: between the Siak and Inderagiri rivers; between the Rokan and Barimun rivers; within the region of Lake Toba; and, researchers speculate, within the Jambi district. Their range is further delineated by that of the black-crested Sumatran langur (P. melalophos), also called the mitred leaf monkey.
Home in the Riau Archipelago is on the islands of Kundar, Bintang, and, researchers speculate, Batam and Galang. The monkeys are found throughout the Malay Peninsula, except for the south and northwest. In Thailand, the white-thighed surili’s range is delineated by that of Raffles’ banded langur (P. femoralis), also called the banded surili.
TAXONOMY IN TRANSITION
Before being elevated to full species status, the white-thighed surili had been considered a subspecies of the black-crested Sumatran langur. An even earlier classification had assigned it subspecies status to the Raffles’ banded langur, also known as the banded leaf monkey or banded surili. But the full-species crown worn today by the white-thighed surili is shaky; researchers claim an “urgent need” to further clarify this species, along with its subspecies, with respect to taxonomy and distribution.
For now, though, the white-thighed surili can wear its full-species crown and be a “parent” to four “children,” or subspecies.
As with the parent species, each of the four subspecies is interchangeably referred to either as surili or langur.
- Malaysian white-thighed surili (P. siamensis siamensis), the nominate subspecies, resides throughout Peninsular Malaysia, except for the south and northwest, with concentrations in the states of Perak (north to the Piah Valley), Selangor, Negeri Sembilan, Pahang, and Malacca.
In southernmost Thailand, the subspecies shares the range of Robinson’s banded surili.
- Bintan Island white-thighed surili, or more simply, Bintan white-thighed surili (P. siamensis rhionis), resides on Indonesia’s Bintan Island in the Riau Archipelago within lowland wet forest, swamp forest, secondary growth, and so-called “rubber gardens,” i.e., forests that have been razed by slash-and-burn agriculture, plowed, and replanted with rubber trees. The subspecies is thought to occupy Batam and Galang islands as well.
- Riau-Coast white-thighed surili, or more simply, Riau white-thighed surili (P. siamensis cana), resides in the eastern forests of Sumatra, between the Siak and Indragiri Rivers, and on Kundur Island in the Riau Archipelago.
- Mantled white-thighed surili (P. siamensis paenulata) resides in a small section of coastal forest in eastern central Sumatra, geographically separated from the East Sumatran banded surili (P. femoralis percura) by the Rokan River. Researchers have reported an isolated population near Lake Toba in northern Sumatra.
Some researchers posit that the Natuna Island surili (P. natunae), once considered a subspecies of the banded surili (as the white-thighed surili had been), is a fifth subspecies. This hypothesis is only the tip of ongoing debate and research, almost guaranteeing further taxonomic updates.
Size, Weight, and Lifespan
Head-to-body length for these small, slimly built Old World monkeys ranges from 1.35 to 2.26 ft (41–69 cm). A nonprehensile tail that is longer than the body adds another 1.9–2.8 ft (58–85 cm) to their tiny frame.
Weight for adult white-thighed surilis ranges from 11 to 14.8 lb (5–6.7 kg). The species exhibits no sexual dimorphism (i.e., defining physical characteristics or attributes between the sexes, apart from their reproductive organs.)
Lifespan is unknown for white-thighed surilis. However, the lifespan for another species in the genus, the black-crested Sumatran langur, endemic to Sumatra, is documented at greater than 18 years in captivity.
This lanky, little monkey is so stinking cute! From beneath a poofy brown or gray cap that hugs the forehead’s inconspicuous brow ridge in a triangular formation, copper-colored eyes shyly peek out. Mother Nature has outlined the skin around the eyes in pale pink and has used the same color palette to outline the monkey’s lower lip, adding further contrast to the dark facial skin. She then fitted the surili with a long, aquiline nose, demurely accenting it with unassuming nostrils. The overall effect is an expressive and beguiling face.
Of course, the species takes its name from the white (or pale) fur on its outer thighs. The belly is also covered by white fur, and long white fur flies out from the face nearly obscuring the monkey’s dark scalloped ears. Long, brownish-gray fur covers the back, arms, lower legs, and tail, giving contrast to the white furry profusion. Hands and feet are black.
As you would expect, each subspecies is characterized by white (or pale) patches of fur on its outer thighs, same as the parent species.
White-thighed surilis are herbivorous connoisseurs, which is just a fun way of saying that they eat only plants and have a discriminating palate, with a preference for young leaves, fruits, and seeds. Compared to other species in the genus Presbytis, white-thighed surilis eat greater amounts of fruits and seeds than leaves. The Malaysian white-thighed surili has been observed eating fronds (or “fiddleheads”) of the tree fern genus Cyathea, along with the leaves and flowers of morning glories.
A large, multichambered stomach aids in bacterial fermentation and neutralization of any toxins found in the leaves consumed. This digestive adaption is important in providing critical nutritional nourishment to the monkeys.
What Does It Mean?
A biological classification, or ranking, of living beings that includes a group(s) of species that are structurally similar or “related” to one another through evolution.
A coastal forest with trees adapted to growing in salty soil and water that protect coastlines from erosion. A natural barrier to strong coastal storms like hurricanes and one of the Earth’s most rapidly disappearing ecosystems.
*In zoological nomenclature, when a species is split into subspecies, the originally described population is retained as the “nominotypical subspecies” or “nominate subspecies,” which repeats the same name as the species.
*Denoting a race or subspecies that is given the same epithet (the part of a taxonomic name identifying a subordinate unit within a genus) as the species to which it belongs; for example, Presbytis siamensis siamensis.
Incapable of grasping or gripping (opposite of prehensile: capable of grasping).
Also termed old-growth forest, virgin forest, or primeval forest—a forest that has attained great age without significant disturbance and thereby exhibits unique ecological features and might be classified as a climax community, an ecological community in which populations of plants or animals remain stable and exist in balance with each other and their environment.
A forest that has regrown after a major disturbance, such as fire or timber harvest, but has not yet reached the mature state of primary forest.
A forest that is situated at the foothills or lower elevation slopes of a mountainous region.
A population usually restricted to a geographical area that differs from other populations of the same species, but not to the extent of being classified as a separate species.
Peat swamp forests are tropical moist forests that grow in permanently water-logged areas with mineral-deficient water; the waterlogged soil prevents dead leaves and wood from fully decomposing, over time creating a thick layer of acidic peat. Peat swamp forests are typically surrounded by lowland rainforests on better-drained soils, and by brackish or salt-water mangrove forests near the coast.
Occurring or living in the same area; overlapping in distribution.
Visit the Glossary for more definitions
Behavior and Lifestyle
These enchanting monkeys are active during daylight hours (making them diurnal) and spend most of their time in the trees (making them arboreal). They travel quadrupedally (on all fours), clambering, jumping, climbing upward, or sometimes swinging from tree branch to tree branch. With a long arm outstretched they are able to reach above and pluck a piece of desired fruit with a dexterous hand. While eating, they squat or sit splay-legged on a supporting branch within a tangle of leaves.
Unfortunately, we don’t know a lot of details about the species’ behavior and lifestyle. Researchers have not been successful in closely studying white-thighed surilis, due to the primates’ shy nature and aversion to humans. But who can blame these monkeys? Humans destroy their habitat, hunt them for food, and even kidnap them to become pets or to be sold into the entertainment industry.
However, if we look to the genus (Presbytis) and to closely related species within the genus (such as the white-thighed surili’s “cousin,” Robinson’s banded surili, a sympatric species), we can make some safe conjectures. Still not a lot of information to share, though, as other surilis are equally shy, and for the same reasons.
Daily Life and Group Dynamics
Small, unimale-multifemale family groups (or troops) are the norm for the species. A typical family group includes one adult male and two to four adult females, along with their young. White-thighed surilis are territorial, and they are always on alert for predators. The monkeys are particularly vulnerable to birds of prey (raptors) and snakes. Should a predator come near, the adult male sounds the alarm (known as a “loud call”) and performs a distraction to capture the predator’s attention, giving his family time to flee to safety. Researchers claim that this “predator avoidance strategy” is more effective with smaller family groups.
Foraging and eating are key daily activities, but these little primates know how to “chill,” too. With their tummy and chest resting on a thick tree branch—arms and legs loosely hanging downward, along with their long, nonprehensile tail—they strike a pose of total relaxation.
White-thighed surilis belong to the family of Old World monkeys known as Cercopithecidae, and to the subfamily Colobinae.
“Surili” is the local name given to leaf monkeys of the genus Presbytis who live in Malaysia and western Indonesia. Those leaf monkeys outside this region are referred to as langurs.
The history and etymology of the word Presbytis comes from New Latin, taken from Greek, meaning “an elderly or aged man.” A secondary definition translates the word to “ambassador.” We should regard white-thighed surilis as ambassadors. Perhaps if we regard them with this dignity and distinction, their survival on earth will be better ensured.
Langurs, and hence, surilis, are known to be “chatterboxes.” Wildlife biologists have remarked upon the “chattering” heard in the trees above as these primates chat with one another. Of course, what sounds like mere chatter to us human primates is complex nonhuman primate language. Important information is being conveyed. Oftentimes a cackling call, in addition to a loud call, is sounded in alarm to warn others of a perceived threat. The white-thighed surili’s neighbor (and close kin), Robinson’s banded surili, is known to make “cackling calls” just before dawn.
Besides vocalizing, white-thighed surilis use postures and gestures to communicate.
Reproduction and Family
Between 4 and 5 years of age, white-thighed surilis are considered fully mature. (Details regarding female and male sexual maturity are not available.) The species is not monogamous.
After a six-month pregnancy, a female gives birth to a single infant. Mothers are protective of their babies but gratefully accept help with child care from other female troop members (a behavior known as “alloparenting”).
Newborns are covered in a white furry coat, contrasted by a black strip or cross-shaped marks along the spine and shoulders. They are considered fully weaned by 1 year of age.
As citizens who share our world, white-thighed surilis are significant in their own right, but particularly to Indonesia and Thailand, where they contribute to the diversity of those ecosystems.
By “pruning” the trees from which they pull and consume young leaves, these small primates help to encourage new forest growth. They further help to regenerate their dwindling forest habitat by dispersing (through their feces) the seeds of the fruits they consume.
Conservation Status and Threats
The white-thighed surili remains classified as Near Threatened by the International Union for Conservation of Nature (IUCN, November 2015), appearing on the IUCN Red List of Threatened Species.
Extensive habitat loss due to logging and deforestation has put the species in decline, nearly 30 percent over three generations (or 30 years). As an example, the peat swamp forests of coastal Peninsular Malaysia, an important ecoregion, are now logged and degraded; once pristine forests exist as remnant patches where the surilis must do their best to survive.
Within the eastern forests of central Sumatra, home to the Riau-Coast white-thighed surili, 60 percent of habitat has been lost to frequent burning, logging, and road construction—even in so-called “protected” areas.
On Bintan Island, home to the eponymously named Bintan Island white-thighed surili, 70 percent of pristine forest habitat has been razed and converted to rubber tree gardens, industrial areas, roads, and tourist resorts. An airport constructed in 2017 (Raja Haji Fisabilillah International Airport) brings a steady influx of tourists to the island along with further infrastructure development . . . and habitat loss.
The expansion of palm oil plantations has further decimated white-thighed surili habitat. Hunting has also taken a grim toll on population. Overall population for the species and four subspecies is unknown.
The white-thighed surili 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.
Researchers assert that genetic analysis is necessary to better clarify the taxonomy of the species, so as to better address the protection and conservation of the species and its subspecies. They also advocate for public education-conservation programs for the indigenous community to raise awareness and appreciation for the white-thighed surili and her “children” subspecies.
Two of the four subspecies are reported to live in protected areas. However, even “protected” areas are not always safe, as laws to protect habitat and species are not always respected, nor easily enforced.
The Riau-Coast white-thighed surili is known to occur in Tesso Nilo National Park, Kampar forest, and Kerumutan protected area in Sumatra. These habitat areas are under developmental siege, despite their protected designation.
The Malaysian white-thighed surili is known to occur in Taman Negara National Park and Krau Wildlife Reserve, Malaysia.
Whether the other two subspecies, Bintan Island white-thighed surili and the mantled white-thighed surili, occur in protected areas is uncertain.
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Written by Kathleen Downey, June 2021