Geographic Distribution and Habitat
The banded surili, also known as the banded langur or the banded leaf monkey, and sometimes known as Raffles’ banded langur, once roamed the tropical jungles from the southern peninsulas of Myanmar and Thailand to the islands of Singapore and Sumatra. Changes in their environment caused by humans have not only reduced the size of their range considerably but have also fragmented it severely.
Today, three subspecies of banded surilis inhabit pockets of their former range in small isolated bands. These are Raffles’ banded langur (P. f. femoralis), the East-Sumatran banded langur (P. f. percura), and Robinsons’ banded langur (P. f. robinsoni).
North, along the Malay Peninsula and into Myanmar, Thailand, and Burma, Robinsons’ banded langurs are believed to be the most abundant population of banded surilis. In Indonesia, on the island of Sumatra, East-Sumatran banded langurs dwell in the jungles between the Rokan and Siak rivers. Members of Raffles’ banded langur live in the peat forests of the Malay Peninsula and among the dwindling swampy rainforests of Singapore. Even among the three subspecies, groups tend to be isolated from one another by the ever-increasing encroachment of humans, which frequently causes gene pools to stagnate wherever the species manages to find refuge from the logging and urban development that goes on all around them.
While the banded surili is generally considered Near Threatened by the International Union for Conservation of Nature (IUCN), the Singapore population of Raffles’ banded langur is considered Critically Endangered nationally. Though they have been governmentally protected there since 1947, their numbers have dwindled considerably since that time. Forty years ago, only 10-15 individuals existed. Today, in the wake of a more regimented conservation project, their population has risen to somewhere between 40 and 60 individuals. They are confined to the Nee Soon Swamp Forest in Central Catchmet Nature Reserve (CCNR), which is the country’s largest nature reserve at the heart of the island.
Raffles’ banded langur was once common throughout Singapore, however. Unfortunately, the population that once inhabited the Bukit Timah Nature Reserve (BTNR), a smaller reserve just west of CCNR, was wiped out by 1987. The two reserves had been connected to one another prior to the construction of the Bukit Timah Expressway, completed in 1983. Its construction is considered a major factor in the decimation of the BTNR population.
Between the three subspecies, there are only minor differences, and researchers are still in the process of confirming if the three subspecies are more closely related to each other than they are to other monkey species living in similar areas. Based on genetic research, some even argue that the Raffles’ banded langur may be its own species altogether. If this were so, then it would make their conservation all the more significant and time-sensitive. Current research for the banded surili is focused on clearing up its uniquely ambiguous taxonomy.
Size, Weight, and Lifespan
A banded surili weighs between 13 and 18 pounds (5.9–8.2 kg) and stands 17–24 inches (43–61 cm) tall. Its long tail adds an extra 24–33 inches (61–84 cm) to its stature. There are no defining differences between the sexes.
Lanky limbs perfect for life in the trees and a lengthy tail stick out of the banded surili’s slender body. The hair on its head tends to stick up in a triangular tuft or mohawk configuration. Dark eyes gaze out at the canopy in search of fruit through two white circles.
There are only minor physical differences between the three subspecies of banded surilis. In general, members of the species sport dark fur coats that are strikingly lighter across their bellies, varying from bright white to duller gray streaks. Raffles’ langur has particularly dark hair, further accentuating the white bands they wear along their inner thighs.
Like other colobine monkeys whose infants typically sport brighter coats than adults, infant banded surilis have white or beige hair until it begins to darken around the age of six months. Raffles’ banded langur infants, in particular, are born with a black line running down their back along their spine, while another runs across their shoulders forming a cross pattern.
Banded surilis are frugivorous, meaning they primarily eat fruit, but they also eat seeds. Thanks to special bacteria in their gut, they are also able to digest young but fibrous leaves as well as fruit that has not fully ripened. Though this ability makes them hardy animals capable of surviving in heavily logged areas of former rainforest (they are even known to eat the leaves of rubber plants if desperate), they are picky eaters and happily travel significant distances to places where their preferred snacks are growing in lieu of less tasty (but more accessible) treats.
Behavior and Lifestyle
Banded surilis are generally shy and alert creatures that hardly ever leave the comfort of the canopy. Instead, they prefer the peacefulness and solitude of tall fruit-bearing trees, where they can eat out of view of potential predators. As a result, banded surilis rarely come into contact with human primate researchers who would wish to observe them. When and where the two species of primates do meet, the humans’ oohs and ahs are typically met with the banded surilis’ distinct alarm calls as they scurry to hide again in the dense jungle foliage. Their particular dislike of humans is not without precedence, given that they were desirable as pets and food amongst the Singaporean people only a few decades ago. Unfortunately, the surilis’ shy dispositions are one of the main reasons why so much remains unknown about them.
Banded surilis are frugivorous, but they are not a significant factor in dispersing seeds.
Researchers are still working to piece together their unique taxonomy.
Fragmentation of their habitat has caused gene flow to slow or halt between groups.
The late member of a banded surili troop in the Bukit Timah Nature Reserve in Singapore (wiped out by 1987) is kept on display at the Raffles Museum of Biodiversity Research.
Banded surilis are highly social. However, their refusal to be closely observed or studied leaves their daily habits and routines largely a mystery to humans. What few observations researchers have managed to make about them are not always consistent.
The species lives in groups of at least three, but no more than twenty individuals, with their average group size being around eleven. These are mixed-sex groups with an average of one male for every five females and more than one adult for every immature monkey. When they are not eating or busy searching for their next meal, groups spend their day lazing around on branches, grooming one another, and playing among themselves.
Banded surilis make a distinctive alarm call when they sense danger. Researchers have compared its sound to the rattle of a machine gun.
It is not clear to researchers if the banded surili has a specific mating season or if the species just breeds year-round Both have been hypothesized, but current data is inconsistent. Nothing about females seems to signal when they are in estrus and no noteworthy mating rituals have ever been observed.
Before they turn four months old, infant banded surilis are regularly held by adults other than Mom, a phenomenon common among colobine species known as alloparenting. Infants’ brighter pelage, or hair color, lets adults know exactly which individuals they need to look out for and protect. By nine months, shortly after their infants’ hair darkens, mothers begin the weaning process. Females remain with the same group throughout their lives, while males leave their natal groups before reaching full maturity, at about four years old.
The ecological role of the banded surili is another topic that researchers are only just beginning to investigate. Unlike other frugivorous monkeys, they are not a significant factor in dispersing seeds. This is because their stomachs actually destroy the many seeds they eat.
Raffles’ banded langurs, in Singapore, eat at least 27 species of plants, the majority of which are locally threatened. Data regarding the other two subspecies’ diets are deficient.
The banded surili is classified as Vulnerable by the International Union for Conservation of Nature (IUCN, 2015), appearing on the IUCN Red List of Threatened Species. But its status is more complicated than that. Between the three subspecies of banded surilis, population numbers differ greatly and, in some cases, are not well studied. Adding to the fact that banded surilis are easily mistaken for other species of monkeys living in similar areas—who themselves are sometimes confused for banded surilis—sightings of banded surilis are somewhat rare, making it difficult to determine their exact density. Even if an average population density per kilometer could be calculated, it would not reflect the significant variations between different areas. Furthermore, the species’ complicated and unclear taxonomy, still being worked out by researchers, makes many statistics possibly inaccurate or downright flawed. It may even be the case that the Raffles’ banded langur is its own species altogether! If this were so, it would make their already dwindling population that much closer to total extinction.
Of the three subspecies, researchers believe Robinsons’ banded langur makes up the largest percentage of the overall population. Currently, data about the population of the East-Sumatran banded langur is deficient. By far, Raffles’ banded langur makes up the smallest percentage of the overall population with only 40 to 60 individuals remaining.
Regardless of their taxonomy or location, the same issues threaten all banded surilis and efforts to protect them all must be taken if they are to thrive again as a species. Urban and residential development, agriculture, and aquaculture are trending in the countries where they live. Specifically, the rapid and expansive conversion of rainforest into palm oil plantations within the banded surili’s range is of particular concern. The more these forests are upended to make room for human expansion, the less habitat banded surilis have in which to roam.
This not only limits the resources available to them, but also fractures their population severely. Cut off from one another, groups now have little chance of intermingling. As a result, gene flow has stagnated or halted altogether. The inbreeding that typically results from this trend negatively effects the health of banded surili offspring. Over time, this will continue to make the banded surili more and more vulnerable to environmental changes, especially fire and disease outbreak.
The Raffles’ banded langur population in Singapore is a prime example of this phenomenon. At the beginning of the 20th century, the banded surili was still extremely common on the island of Singapore. Following Singapore’s substantial push to urbanize in the latter half of the century, few places remained where banded surilis could thrive. Prior to the construction of the Bukit Timah Expressway (BKE) in 1983, the Central Catchmet Nature Reserve and the Bukit Timah Nature Reserve (now considered the last major nature reserves, and the so-called “lungs” of Singapore), had been connected. Until 1987, only five years after the BKE split the two reserves down the middle, another troop of banded surili had been clinging to existence. Gone now, the BKE is considered a major factor in what led to their demise, not only because it ravaged their habitat but because it fenced them in and halted gene flow between the two areas. With only 40 to 60 individuals remaining in CCNR, cut off completely from all other groups of banded surilis in existence, researchers are currently trying out new tactics to prevent gene pools from stagnating in the same way as the BTNR group.
Though the Raffles’ banded langur has been governmentally protected in Singapore since 1947, conservation efforts during the last half of the 20th century were less than effective. In general, confusion about the banded surili’s taxonomy and the complication of studying the species in the wild have made it hard for researchers to prioritize their efforts effectively. Thankfully, efforts to preserve the banded surili have taken off in the last decade or so.
In 2013, Singaporean officials built an ecological corridor over the Bukit Timah Expressway, named the ECO-link@BKE. They hoped this project would reconnect the last two stretches of primary forest left in the country, the Central Catchmet and Bukit Timah Nature Reserves, and that animals, such as the banded surili, would regain access to their former ranges. The ECO-link’s effectiveness is controversial, however. For a banded surili troop, who basically never leave the canopy, a land bridge such as the ECO-link@BKE is pretty much useless. Andie Ang, a pioneer and champion of banded surili research since 2008, has expressed the need for canopies to be connected if the banded surili is to regain its former ranges.
In August of 2016, a meeting was held at the Singapore Zoo to develop a 50-year vision for the conservation of the banded surili. This marked not only the first genuine effort to conserve this particular species, but also the first time that two of the countries where they live (Singapore and Malaysia) agreed to work together in their conservation. Attendees of the conference laid out a detailed Species Action Plan, paying careful attention to the nuances of this primate’s particular situation. First, they acknowledged that a main priority of this work will involve replenishing and reconnecting habitat so that groups of banded surili can once more be connected and genes can flow generously between them. Second, the collaboration also stressed the need to establish more and longer studies that will gather the data needed to clarify its taxonomy, biology, and ecological role. Special attention was also given to the need to make the public more aware of the banded surili’s plight, to ensure government commitment and organizational funding for all future projects, and to establish long-term cross-country collaboration better at addressing the issues banded surili face everywhere they call home. Since 2016, many projects in favor of the banded surili’s survival are under way.
Andie Ang now heads the Raffles’ Banded Langur Working Group. This group, a direct result of the August 2016 meeting, has been using non-invasive tactics to complete the much-needed field research that proper conservation of the banded surili requires.
In 2016, the National Parks Board of Singapore (NParks) began work to expand the banded surili’s habitat into Thomson National Park, north of CCNR. This new park is being designed to act as a buffer between CCNR and the urban areas surrounding it. Though it is projected to open to human visitors this year (2019), its very existence no doubt already benefits the banded surili as well as all other wildlife.
If the Raffles’ banded langur is determined to be a subspecies of Presbytis femoralis and not its own species altogether, the Species Action Plan calls for the reestablishment of gene flow between Singapore and Malaysia. Thus, in the coming years, efforts to translocate individual banded surili, or to collect and transplant individuals’ semen, may also be in the works.
Written by Zachary Lussier, June 2019. Conservation Status update 2020.