Nycticebus bengalensis (Lacépède, 1800)

Common Name & Classification

The Bengal slow loris (Nycticebus bengalensis), also called the northern slow loris, is a strepsirrhine primate and a slow loris species native to the Indian subcontinent and Indochina. It has a larger geographic range than any other slow loris species.

Taxonomic History

Until 2001, it was classified as a subspecies of the Sunda slow loris (N. coucang), and phylogenetic analysis shows it is most closely related to the Sunda slow loris. Introgresive hybridization has caused some individuals from both species to carry mitochondrial DNA sequences that match the other species.

Size Dimensions

It is the largest slow loris species, with a head-to-tail length of 26 to 38 cm (10 to 15 in) and a weight of 1 to 2.1 kg (2.2 to 4.6 lb).

Physical Characteristics

Like other slow lorises, it has a wet rhinarium (nose), round head, flat face, large eyes, small ears, a vestigial tail, and dense, woolly fur.

Brachial Gland Toxin

The toxin it secretes from its brachial gland, a scent gland on its arm, has a different chemical composition than the toxins of other slow loris species, and may be used to communicate information about sex, age, health, and social status.

Activity Pattern & Forest Habitat

The Bengal slow loris is nocturnal and arboreal, and lives in both evergreen and deciduous forests. It prefers rainforests with dense canopies, and its presence in its native habitat is an indicator of a healthy ecosystem.

Ecological Role

It acts as a seed disperser and pollinator, and is prey for carnivores.

General Diet

Its diet is primarily made up of fruit, and also includes insects, tree gum, snails, and small vertebrates.

Winter Diet

In winter, it depends on plant exudates such as sap and tree gum for food.

Social Behavior & Roosting

The species lives in small family groups, marks its territory with urine, and sleeps during the day curled up in dense vegetation or tree holes.

Reproduction Basics

It does not breed seasonally, reproducing once every 12 to 18 months and usually giving birth to a single offspring.

Offspring Development

Mothers carry their offspring for the first three months, and offspring reach sexual maturity at around 20 months of age. The Bengal slow loris can live up to 20 years.

Conservation Status & Main Threats

The species is listed as Endangered on the IUCN Red List, and is threatened with extinction due to rising demand from the exotic pet trade and traditional medicine. It is one of the most common animals sold in local animal markets.

Traditional Medicine Uses

In traditional medicine, it is primarily used by wealthy to middle-class urban women after childbirth, and is also used to treat stomach problems, broken bones, and sexually transmitted diseases.

Additional Threats

It is also hunted for food and affected by habitat loss. Wild populations have declined severely, and the species is already locally extinct in several regions.

Protected Area Limitations

Though it occurs in many protected areas across its range, this does not protect it from rampant poaching and illegal logging.

Conservation Priorities

Key conservation priorities for the species include strengthening protection measures, stricter enforcement of wildlife protection laws, and improving connectivity between fragmented protected areas.

Native Range Overview

This species has the largest geographic range of all slow loris species, and is native to Bangladesh, Northeast India, and Indochina, which includes Cambodia, Laos, Burma, Vietnam, southern China, and Thailand.

Distribution in Northeast India

It is the only nocturnal primate found in the northeast Indian states of Assam, Mizoram, Nagaland, Meghalaya, Manipur, and Tripura.

Distribution in China & Bangladesh

In China, it occurs in parts of Yunnan and southwest Guangxi, and has been recorded in the Chittagong Hill Tracts of Bangladesh.

Distribution in Vietnam & Thailand

It is known from 24 protected areas in Vietnam, and is distributed across most of Thailand.

Distribution in Burma & Laos

In Burma, it has been reported from Bhamo, Sumprabum, Kindat, Chin Hills, Pathein, Thaungdaung, and Pegu; populations in Laos have been recorded in the north, central, and southern portions of the country.

Sympatry with Pygmy Slow Loris

The Bengal slow loris is sympatric, meaning it shares its range, with the pygmy slow loris in southeast China, Vietnam, and Laos.

Sympatry with Sunda Slow Loris

It is also sympatric with the Sunda slow loris on the southern peninsula of Thailand. In 2001, Groves reported that hybrids exist between these two species in this region.

Range-wide Population Decline

Across its entire geographic range, Bengal slow loris populations are in severe decline. Their habitat has been heavily degraded, and growing human populations will increase pressure on the species.

Bangladesh Forest Loss

As of 2000, only 9% of Bangladesh's original forest cover remained intact.

Cambodia & Myanmar Forest Loss

In northeastern Cambodia, deforestation is accelerating, with 6% of natural forest lost between 1999 and 2000. Over the same period, Myanmar lost 14% of its natural forest, and Thailand lost 26%.

Vietnam Forest Loss

In Vietnam, only 30% of original forest cover remains, due to deforestation caused by the Vietnam War, and only 10% of that remaining forest is closed-canopy. Habitat destruction remains widespread, and all slow loris populations within Vietnam's borders are significantly depleted.

Vietnam Local Extinctions

The species is already locally extinct in southern Quảng Nam Province and parts of the Vietnamese highlands, and local extinction is also expected in Song Thanh and Kon Cha Rang nature reserves.

India Forest Loss

In India, dense forest canopy has declined by as much as 55% in some areas and continues to disappear rapidly. As early as 1987, the Indo-China region was reported to have lost 75% of the natural habitat for slow lorises.

Historical Population Estimate

In 1992, total population size was estimated at between 16,000 and 17,000 individuals based on available habitat; however, recent publications note that far fewer individuals remain due to a contracted geographic range.

India Population Status

The Bengal slow loris may now be restricted to a small number of isolated populations, and faces serious risk of local extinction in parts of Assam and Meghalaya. Its population is declining and threatened in Arunachal Pradesh.

Assam Population Density Study

A 2006 study estimated population density in Assam, India, between 0.03 and 0.33 individuals per km².

Tripura Survey Results

A 2007 survey at Thrisna Wildlife Sanctuary and Sipahijola Wildlife Sanctuary in Tripura, India, recorded an encounter rate of 0.22 individuals per km, with seven out of nine sightings falling within a 1.71 km² (0.66 sq mi) area, and most animals found at a height of 8–15 m (26–49 ft) near the interior of wet deciduous forest.

Assam Abundance Study

In 2008, species abundance was measured at 0.18 individuals per km at Gibbon Wildlife Sanctuary in Assam.

China Forest Decline

Since the 1990s, China's forests have declined significantly. In Yunnan and Guangxi provinces, primary forests are scarce and isolated, and secondary forests have been severely degraded.

China Population Status

Yunnan has lost 42% of its forests, and has 2,000 or fewer slow lorises remaining. The Bengal slow loris is nearly extinct in Guangxi.

Guangxi Local Extirpation

It has been extirpated in Ningming County, and only a few individuals remain in Jingxi, Longzhou, and Pingxiang.

Preferred Habitat Types

The Bengal slow loris's preferred habitats are across tropical and subtropical regions, and include evergreen and semi-evergreen rainforests with forest edges and continuous, dense canopies. It can also be found in bamboo groves.

Tree Preference in Habitat

It prefers habitats with large-diameter tall trees that have a large crown depth, defined as the length along the main axis from the tree tip to the base of the crown; these areas typically have greater food availability and lower predation risk.

Ecosystem Indicator Role

Because of its preference for dense forest, it serves as a good indicator of ecosystem health. The species is an important seed disperser and pollinator, and is prey for several carnivores.

Plant Exudate Diet

It feeds on plant exudates including sap, gums, resins, and latexes, particularly from plants in the Fabaceae family.

Exudate Foraging Behavior

Although the species does not have keeled nails, it scrapes plants to actively break through their surface; this behavior is similar to that seen in marmosets and fork-marked lemurs. It also obtains exudates by gouging holes into bark.

Winter Exudate Dependence

Plant exudates make up almost all of its winter food supply.

Exudate Plant Sources

Terminalia belerica, the bastard myrobala, a deciduous tree common in Southeast Asia, is a preferred source of exudates, but the species has also been observed feeding on plant exudates from a number of other plant families: Moraceae (Artocarpus), Magnoliaceae (Manglietia), Fabaceae (Acacia, Bauhinia), Lecythidaceae (Careya arborea), and Sterculiaceae (Pterospermum).

Primary Diet Type

While it will feed on large insects such as katydids and crickets, gum, snails, small birds, and reptiles, it is primarily frugivorous. Lianas from the flowering plant genus Bauhinia are a common food source.

Night Vision Adaptation

As a nocturnal animal, the Bengal slow loris has excellent night vision, enhanced by a tapetum lucidum—a layer of eye tissue that reflects visible light back through the retina.

Roosting & Territorial Marking

It sleeps during the day curled up in a ball in dense vegetation or tree holes. Both males and females mark their territory with urine.

Social Behavior

The species lives in small family groups, and individuals may engage in social grooming.

Non-seasonal Breeding

Unlike the pygmy slow loris, the Bengal slow loris is not a seasonal breeder. Females in estrus attract males with a loud whistle.

Reproduction Frequency & Gestation

Females reproduce every 12–18 months and have a six-month gestation period. Because they do not breed seasonally, females can become pregnant when their existing offspring are approximately 6 months old, making it possible for them to produce two offspring per year.

Offspring Litter Size

Females typically give birth to a single offspring, with twins occurring rarely. This differs from the sympatric pygmy slow loris, which commonly produces twins.

Offspring Development Details

The mother carries her young for about three months before the young become independent, though offspring may be temporarily left on branches while the mother searches for food. Sexual maturity is reached at approximately 20 months of age. The species can live up to 20 years.