Bengal Slow Loris, Nycticebus bengalensis
BENGAL SLOW LORIS
Nycticebus bengalensis
Geographic Distribution and Habitat
The Bengal slow loris, also known as the ashy slow loris, Bengal loris, or northern slow loris, is found in southeastern Asia, specifically the countries of Bangladesh, Cambodia (west of the Mekong River), China (southern and western Yunnan Province and possibly southwestern Guangxi Province), northeastern India, Laos, Myanmar, Thailand, and Vietnam. The Bengal slow loris has the largest geographic range of any slow loris species!
Within these countries, the Bengal slow loris can be found in both evergreen forests, and deciduous forests (those whose trees lose their leaves during the colder months), and have also been found in bamboo groves. They have a particular preference for rainforests with dense canopies, and their presence in their native habitat indicates a healthy ecosystem.
The Bengal slow loris was previously classified as a subspecies of the Sunda slow loris (N. coucang) until 2001. Both are now recognized as separate species.
Size, Weight, and Lifespan
The Bengal slow loris measures 10-15 inches (26-38 cm) from head to (stubby) tail, and weighs between 2.2-4.6 lbs (1-2.1 kg). They are, in fact, the largest slow loris species!
Males and females are relatively the same size and look alike. The only way to tell them apart is by their genitalia. Otherwise, the Bengal slow loris exhibits no sexual dimorphism (noticeable physical differences between genders).
Their lifespan in the wild is 15-20 years.
Appearance
The Bengal slow loris is covered in dense, wooly gray-brown fur on the back and white fur on the underside. A dark stripe runs from the center of the back up to the top of the head. Arched “eyebrows” in a brown fur pattern encircle each eye, evoking an expression of mild surprise. A narrow white strip of fur separates the eyes and extends to a short, faintly pink snout. The forearms and hands are almost white in color. The hindlimbs vary in coloration, ranging from brown to nearly white, while like the hands, the feet are pale. Molting may cause seasonal color variations along the backside.
The Bengal slow loris shares a number of characteristics with other loris species. These include: a wet nose (more on that in the Fun Facts section), a barely-visible vestigial (no longer of use, but still present) tail, a round head with a broad, flat face, short ears, needle-like teeth, and large, saucer-like eyes which reflect a bright orange eye shine.
The big toe on each hind foot faces opposite the other toes, enhancing the “handy-dandy” (haha) gripping power. In addition, the second toe on each hind foot has a curved “toilet claw” which is used for scratching and grooming, while the other toenails are straight.
Diet
An omnivore, the Bengal slow loris’s diet primarily consists of fruit, but also includes insects and their larvae, tree gum, snails, spiders, bird eggs, nectar, honey, and small vertebrates such as birds and lizards.
They are especially dependent on sap and tree gum during the winter months.
Behavior and Lifestyle
The Bengal slow loris is nocturnal (active at night) and arboreal, spending most of the time in the trees. They move through the forest quadrupedally (on all fours) with deliberate movements as they forage. That is, they change direction or move between supports without much noise or change in speed. Their hands are supremely adapted to climbing, thanks to their fingers and opposable thumbs which, when clenched together, allow them a pincer-like grip (like a crab or lobster!)
Daytime hours are spent sleeping curled up in a ball in dense vegetation or tree holes. Starting at sunset, nighttime is dedicated to foraging for food. This is where those amazing eyes come in: forward-facing, each eye has stereoscopic vision; in other words, each eye sees a slightly different image of a single perception. Their specialized eyesight gives them an increased awareness of objects through greater depth perception and enhanced “night vision.”
Predators of the Bengal slow loris include pythons, monitor lizards, birds of prey such as hawk-eagles, large cats, and even orangutans! To avoid predators, the Bengal slow loris moves quietly through the trees and can hide within the surroundings. They can also curl into a defensive position, mimic a cobra by swaying back and forth with the arms above the head, or use the markings around the eyes to appear like an even bigger nocturnal predator. If all else fails, Bengal slow lorises use… venom! Yes, that’s right! VENOM! All slow lorises have a special brachial gland near their elbow which produces a toxic oil. When mixed with their saliva, the result is venom, which, while primarily used for communication, can also be used to deliver a dangerous, deadly bite!
Wet nose, warm heart: Along with all other lorises, as well as lemurs and galagos, the Bengal slow loris is part of a unique clade (group) of primates known as Strepsirrhini. What does THAT mean!? Among a number of interesting traits, this clade is defined as such for having wet noses! (Just like your furry, friendly neighborhood dog!)
Neat freaks: Along with their toilet claw, like other members of the clade Strepsirrhini, the Bengal slow loris also has a toothcomb (or dental comb). These are tilted incisors shaped like a garden spade at the front of the mouth on the lower jaw and are used to aid in grooming.
Although generally solitary, Bengal slow lorises have been seen living in family groups. These family groups usually consist of a mother and her young, or sometimes a male-female pair when the female is receptive for breeding. Think of their lifestyle as living in a community of tiny homes. There is no dominance hierarchy; they live peacefully with one another and are tolerant of other loris species in their range. The only potential exception to this are males. Males of other species of slow loris are generally territorial and aggressive toward one another. It can be presumed this is the case with the Bengal slow loris as well. Home ranges overlap and vary in size, influenced by the number of competitors for food sources, and by habitat quality. Based on one study conducted in Bangladesh, individual home range sizes varied from 0.22-4.27 acres (0.09-1.73 ha). Territorial boundaries are marked with urine, and urine is the primary mode of communication for the Bengal slow loris.
Bengal slow lorises have been observed peacefully foraging within meters from pygmy slow lorises (N. pygmaeus / Xanthonycticebus pygmaeus) in the same tree. Bengal slow lorises are also sympatric (occurring in the same area) with the Sunda slow loris.
With regard to infants, they will often be “parked” on a tree limb by Mom while she goes off to forage. The infant is camouflaged by dense vegetation, ensuring their safety. Upon reaching maturity, individuals leave their natal (birth) group to establish a territory of their own. It is not known, however, how close to their natal territory newly-independent individuals establish themselves.
Bengal slow lorises use a variety of vocalizations to communicate with one another. Infants use “clicks” to get Mom’s attention or to call her to their parked location if she is out foraging. Upon hearing her baby’s call, the mother will immediately return.
High-pitched whistles are used to attract a mate. These whistles are primarily used by females in estrus (sexually receptivity), but may also be used by males. Females will also supplement their whistling by scent-marking a spot with urine to increase her chances of attracting a male.
Pants ending with a distinct growl precede an attack (that is, before biting the threatening individual). “Chitters” lasting one to two seconds or longer are used in defensive situations when encountering members of the same species (such as two rival males encountering one another).
Like most nocturnal primates, Bengal slow lorises rely on olfactory (scent) cues. Urine is the primary means of communication for the species. Additionally, scientists believe that the brachial gland secretions may be used to mark home ranges, deter predators, or warn other lorises of potential danger.
Bengal slow lorises use their dental combs to groom one another, a tactile (touch) activity that helps to strengthen social bonds. Playtime, particularly between mothers and their babies, also reinforces social bonds.
Bengal slow lorises are polygynandrous (promiscuous) meaning both males and females have multiple partners during the breeding season.
Females attract males by whistling and scent marking when in estrus, and reproduce every 12-18 months. They have a gestation (pregnancy) period of six months, eventually giving birth to one offspring, though rarely, twins may be born. Upon attracting a potential mate, the female will usually hang from a branch by all four limbs to solicit mating. The male positions himself around the female while hanging on the branch with his feet, and clings to the female’s waist with his hands. Upon mating with the female, his task is done, as he plays no role in rearing the offspring.
Bengal slow lorises are year-round breeders, and because of their non-seasonal breeding patterns, females can become pregnant when their offspring are around six months old. This makes it possible for them to produce two offspring per year (if they haven’t already produced twins, obviously).
Infants are born with their eyes open and are covered in fur. The mother carries her young for about three months before they become independent, although they may be temporarily left on branches while the mother forages for food. Weaning usually occurs at six months, and sexual maturity is reached at approximately 20 months of age. The bond between mother and infant is close, as they will bond by playing, and by Mom grooming her infant to reinforce their connection.
As frugivores (fruit-eaters), Bengal slow lorises aid in the regeneration of their forest habitats by dispersing seeds through their feces as they move around the habitat. As a prey species, they also play a role in feeding local predators within the habitat. They additionally play a role in pollination. Like bees and butterflies, they collect pollen from flowers while drinking nectar. They then deposit it on each flower they visit, thereby pollinating the plants. Finally, they can also be considered pest controllers, thanks to their diet consisting of insects and their larvae.
The International Union for Conservation of Nature lists the Bengal slow loris as Endangered (IUCN, 2020), appearing on the IUCN Red List of Threatened Species.
The Bengal slow loris faces numerous threats to the species’ survival.
Despite being found in numerous protected areas throughout their range, poaching and illegal logging are rampant in these areas, while conservation measures are not species-specific. The most severe threats facing the Bengal slow loris are the wildlife trade (trapping for exotic pets, sport, and use in traditional medicine) and deforestation. Habitat destruction has also occurred through slash-and-burn agriculture (a technique involving the cutting and burning of plants in a forest to create agricultural space). Additional factors in their decline include road construction, dams, soil loss and erosion, and power lines. Hunting has been found to be most severe when nearby human urban populations increase.
One unique threat facing slow lorises is due to a combination of their adorable appearance and the “photo prop trade” marketed toward tourists. The use of slow lorises as “adorable accessories” for photo opportunities among tourists causes unnecessary stress for the poor animals. This trade occurs in some of the Bengal slow loris’s range (especially Thailand), but it is also extending into Europe, in countries such as Turkey, which import the Bengal slow loris for this purpose.
The Bengal slow loris is listed in Appendix I 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.
Specific wildlife protections have been enacted on behalf of the Bengal slow loris within all of the countries where they reside. However, as is often the case with endangered species, laws created to protect them are largely ignored and difficult to enforce.
While they are found in numerous protected areas, exact population numbers are unknown. Actual surveys, rather than anecdotal reports, are necessary to determine their true abundance in the wild.
Researchers have attempted to draw attention to and reduce the photo prop trade involving Bengal slow lorises. Ongoing campaigns combat the number of illegal social media videos, in which the lorises are heavily exploited. Although conservation education programs exist across parts of their range, in most areas these are sporadic or even non-existent. A more coordinated effort among conservation organizations within their range would be immensely helpful. Currently, no education programs are known in Laos or Myanmar.
Rescue centers across their range maintain facilities for confiscated animals, particularly in Cambodia, Thailand, Vietnam, and Laos. Attempts have been made to release these rescued lorises, although they are often unmonitored, and therefore, it is difficult to assess the success of these releases, and their contribution to Bengal slow loris conservation. Following the protocols set forth by the IUCN Reintroduction Specialists Group is critical to assess the impact of these reintroductions. Furthermore, considering that the Bengal slow loris can be found near human settlements, translocating lorises should not be done without a proper understanding of their distribution and ecology in an area.
Ultimately, more strictly enforced laws and protection of (and within) the lands in which the Bengal slow loris resides, along with increased public awareness, education, and communications are critically needed. These actions, alongside additional research regarding the Bengal slow loris’s population size, distribution, and trends, and monitoring of population trends are needed to save the species from the threat of extinction.
- https://www.alltheworldsprimates.org/Members/Home/MasterPrimate.aspx?tid=85
- https://animalia.bio/bengal-slow-loris
- https://a-z-animals.com/blog/meet-the-slow-loris-the-nocturnal-slow-climbing-primate/
- http://bioweb.uwlax.edu/bio203/s2013/gustafso_alli/adaptation.htm
- https://en.wikipedia.org/wiki/Bengal_slow_loris
- https://en.wikipedia.org/wiki/Slow_loris
- https://en.wikipedia.org/wiki/Toothcomb
- https://www.int-res.com/articles/esr_oa/n023p099.pdf
- https://www.iucnredlist.org/species/39758/179045340
- https://www.ourendangeredworld.com/bengal-slow-loris/
- https://primate.wisc.edu/primate-info-net/pin-factsheets/pin-factsheet-slow-loris/
- https://www.researchgate.net/profile/Jennifer-Daltry/publication/274410288_Cambodian_Journal_of_Natural_History_Volume_2011_Issue_2/links/551fcedb0cf2a2d9e1409c5b/Cambodian-Journal-of-Natural-History-Volume-2011-Issue-2.pdf#page=30
- https://www.researchgate.net/profile/Hassan-Al-Razi/publication/339777102_Home_Range_Activity_Budgets_and_Habitat_Use_in_the_Bengal_Slow_Loris_Nycticebus_bengalensis_in_Bangladesh/links/638251bec2cb154d292d43e4/Home-Range-Activity-Budgets-and-Habitat-Use-in-the-Bengal-Slow-Loris-Nycticebus-bengalensis-in-Bangladesh.pdf
- https://simple.wikipedia.org/wiki/Strepsirrhini
Written by Sienna Weinstein, May 2024