The slender lorises (Loris) are a genus of loris native to India and Sri Lanka. The genus comprises two species, the red slender loris found in Sri Lanka and the gray slender loris from Sri Lanka and India. Slender lorises spend most of their life in trees, traveling along the tops of branches with slow and precise movements. They are found in tropical rainforests, scrub forests, semi-deciduous forests, and swamps. The primates have lifespans of approximately 15 years and are nocturnal. Slender lorises generally feed on insects, reptiles, plant shoots, and fruit.

Etymology

The Comte de Buffon was the first to identify the slender lorises with the term "loris" in the 13th volume of the Histoire Naturelle, which he adopted from the Dutch language name for the animals, . The Dutch term refers to clowns, which could be descriptive of the slender loris' apparent acrobatic capabilities or of the markings on its face. Two variants of the term "loris" were produced in the English translation of German traveler Christoph Schewitzer's 1700 publication, those being and . These renditions of "loris" would later be used to name pottos in the form of , meaning "sluggard", which was subsequently reapplied to slender lorises as . Another Dutch name for the slender loris, , meaning "slow beast", was also mentioned by Schewitzer. In English, the slender lorises are alternatively known as the "Ceylon sloths".

In India, slender lorises are called or in Telugu, in Kannada, in Tulu, and in Marathi. In Tamil they are known as , meaning "baby of the forest", or , meaning "the slender-bodied one". In Sri Lanka, they are known as in Sinhala, which is also a term for an emaciated person. Alternatively, they are called in Sinhala, which compares the mark on the slender lorises' foreheads to those worn by worshipers of Vishnu called .

Taxonomy

There are two valid species of slow loris:

Changes in taxonomy

Both slender and slow lorises were introduced to European scientists by Dutch voyagers returning from Sri Lanka and the East Indies, which resulted in early references to such animals being unclear as to which genus was the subject of any given description. The earliest verifiable account of the slender lorises was produced by Jean de Thévenot during his visit to India, which he based on two live specimens that were procured from Sri Lanka. Carl Linnaeus described slender lorises as Lemurtardigradus in the 1758 10th edition of Systema Naturae. Although the red slender loris is the type species of Linnaeus' Lemur, the term "lemur" has since been applied to lemuroids to the exclusion of other strepsirrhines. The trivial name tardigradus was adopted from Albertus Seba's description and illustrations of the red slender loris, so named because it reminded him of sloths. Arnout Vosmaer echoed this sentiment in 1770, which provoked a rebuttal from the Comte de Buffon, who disproved the sloth hypothesis by presenting the slender lorises' primate traits in the posthumous 36th volume of the HistoireNaturelle.

The first author to elevate Loris to generic status, thereby separating the slow and slender lorises into different species, was Pieter Boddaert in 1784, although his generic name, Tardigradus, was preoccupied. As such, taxonomic priority belongs to Étienne Geoffroy Saint-Hilaire's 1796 description of the Lorisgracilis in the . Geoffroy described Loris on the mistaken assumption that Linnaeus' Lemurtardigradus was a slow loris. Johann Karl Wilhelm Illiger authored a competing description of slender lorises, Stenops, in 1811. Expanding on Geoffroy's taxonomy, Gotthelf Fischer von Waldheim described a yellowish-brown loris as Lorisceylonicus in 1804. René Lesson later authored his own description of the red slender loris, Arachnocebuslori, in 1840. Richard Lydekker established the first definitive taxonomic distinction between the red and gray slender lorises in 1905. Believing that two gray slender loris specimens from Chennai were representatives of the typical L.gracilis form described by Geoffroy in 1796, Lydekker described the red slender loris as L.g.zeylanicus. Ángel Cabrera attempted to correct Lydekker's error in 1908 by describing the gray slender loris as L.lorilydekkerianus, with the red slender loris remaining as the type of the genus.

In 1913, Kathleen V. Ryley published a revision of the genus wherein she merged all then-extant Sri Lankan taxa into a single species, L.tardigradus, thereby returning to the Linnaean trivial name. She also elevated Cabrera's L.l.lydekkerianus subspecies to a species in its own right, which she named the Mysore slender loris (L.lydekkerianus). Robert Charles Wroughton later examined a female specimen from Kodagu and described it as the Malabar slender loris (L.malabaricus) on account of its small size and unique pelage. In 1932, the slender lorises of Sri Lanka were divided into two subspecies by William Charles Osman Hill and William Watt Addison Phillips, those being L.t.tardigradus and the novel Ceylon highland slender loris (L.t.grandis). The following year, Hill described the northern form of the Sri Lankan gray slender lorises as the northern Ceylonese slender loris (L.t.nordicus). Hill found it necessary to revise the entire genus, as L.t.nordicus appeared to occupy an intermediate position between all then-known forms of slender lorises. As such, he described the slender lorises as comprising a single species, L.tardigradus, with five different subspecies. He described a sixth subspecies, L.t.nyctibceboides, in 1942, which was based on a female specimen acquired from the Horton Plains by A. C. Tutein-Nolthenius in 1938. Hill's monospecific taxonomy was revised in 1998 by Colin Groves, who separated the red and gray slender lorises into two different species. A final subspecies, the northwestern red slender loris (L.t.parvus), was described by Saman Gamageetal. in 2017, who speculated that its morphological differences from the southwestern red slender loris may qualify it as an independent species, given further examination.

Description

The slender lorises are small primates that are characterised by their extreme gracility. They can be distinguished from other lorisids by their lithe habitus, enlarged orbits, and short, pointed snouts. Individual body weights range from , and females are larger than males. The head is round, although the top of the skull is flat when viewed from the side. Unlike some other strepsirrhines such as galagos, there is no supraorbital notch. The snout is elongated beyond the tooth row due to the large size of the premaxillae. The skull exhibits post-orbital constriction as a result of a reduced interorbital distance. The dental formula is ×2=36, and the dentition is characterised by the presence of a toothcomb and a caniform lower anterior premolar(P1). In the toothcomb, the lower incisors(I1–2) are tilted forward and are flattened laterally, with the angles of the crownss further accentuating this procumbent shape. The lower canine(C−) is incisiform and also participates in the composition of the toothcomb. Because the canine is occupied by the toothcomb, the lower anterior premolar adopts the shape and function of the lower canine in the dentition. The upper incisors(I1–2) are reduced in size due to a lack of collision with the lower incisors, and the inner pair are separated by a diastema. Compared to the slow lorises, the foramen magnum is positioned further forward in the skull.

The limbs are long and narrow, with the arms having large olecranons. The shoulders and hips permit considerable flexibility in the arms and legs. The hindlimbs are capable of a greater range of motion in all directions than that which is seen in simians, such that almost the entire body is accessible by a slender loris' own toe. As is the case with other lorises and angwantibos, slender lorises do not have visible tails. The hands and feet are robust. Both the manual and pedal second digits are significantly reduced in size, which allow for efficient grasping of thin branches. This locomotor behaviour is further supported by the presence of retia mirabilia in the blood vessels of the limbs, which allow a slender loris to sustain a continuous grasp for hours at a time. This grip is so strong that it can be difficult to dislodge a slender loris from its position once it has grasped a branch. Manual digit4 is the longest in the hand, which increases the breadth of a slender loris' grasp by being positioned opposite the thumb (manual digit1). Pedal digit1 diverges from digits2–5 at nearly a 180°angle. Pedal digit2 possesses a grooming claw, which is typical of strepsirrhines. This is not a true claw, as it is instead a broad, compressed nail that is used for grooming and scratching, which is especially important for toilet purposes because the other nails do not extend beyond the tips of the digits. The papillary ridges are prominent, especially on the hands of the forelimbs. The slender lorises have elongated spinal columns that allow their spines to sway from side-to-side in a serpentine manner during arboreal locomotion. The spine is composed of seven cervical vertebrae, 14–15 thoracic vertebrae, eight or nine lumbar vertebrae, three sacral vertebrae, and five caudal vertebrae. The lumbar vertebrae are elongated about the central axis of the spine, and the final five or six vertebrae are fused to form the coccyx. 14–15 pairs of ribs are present.

The colour of pelages varies between species and subspecies. Climatic factors may influence the distribution of colourations, as taxa found in wetter climates tend to be more rufous, whereas those in drier climates are darker and are sometimes accompanied by a pale "frosting" atop the pelage. Dark circumocular patches are present in all individuals, as is a white stripe that runs vertically between the eyes. The male's scrotum changes colour during rut, wherein the pigmentation adopts a reticulated pattern.

Distribution and habitat

The red slender loris is found in Sri Lanka while the gray slender loris is found in Sri Lanka and India. Two of the subspecies of red slender loris differ in their habitat preference; the lowland loris, L. t. tardigradus, favors wet lowland forests (up to above sea level) in the south western wet-zone of Sri Lanka while the mountain loris, L. t. nycticeboides, prefers cloud, montane, and highland evergreen forests at elevations of . The gray slender loris can be found in tropical rainforests, primary and some secondary, coastal acacia scrub forests, semi-evergreen forests, swamps, and bamboo groves up to above sea level.

Behaviour

Female slender lorises generally have an exclusive home range while males may overlap with others. They often form small social groups to sleep, containing adults of both sexes as well as the young. The groups also undertake mutual grooming and play wrestling. The adults typically hunt separately during the night. Males will follow females while they are in oestrus and mating may occur after a bout of play fighting. Gray slender lorises will often bear twins but the survival rate is low. Newborn infants cling to the mother's front for a few weeks and after that will be kept on a tree while the mother goes off to feed. They make nests out of leaves or find hollows of trees or a similar secure place to live in.

During sleep and rest, a slender loris rolls into a ball with the head and hands tucked between the thighs while the legs grasp a branch. While the sleep and rest posture is maintained, the torso is stabilised within the fork of two branches. Before this posture is assumed, a leaf is often procured and placed in the fork to cushion the body from the hard surface of the branch, which possibly results from the lack of ischial callosities on the slender lorises' hindquarters. When frightened, a slender loris will stand up on a branch bipedally with its elbowss flexed at the height of the sternum.

Diet

The slender lorises are one of the most faunivorous primates; the red slender loris has only been observed eating animal prey while the gray slender loris is primarily carnivorous (mostly insects) but will also eat bird eggs, berries, leaves, buds and occasionally invertebrates as well as geckos and lizards. To maximize protein and nutrient uptake they consume every part of their prey, including the scales and bones. They are able to digest toxic prey such as ants and noxious beetles, urinating on their hands before entering ant colonies to mimic the chemical profile of their prey to avoid attack.

Threats

According to biologists, poaching activity has led to the steady decline of the species in Tamil Nadu. Native people have always believed that all parts of the slender loris have some medicinal or magical powers. This has contributed greatly to the decline of the slender loris. In addition, slender lorises are illegally smuggled to supply a growing exotic pet trade. Other threats include habitat loss, electrocution on live wires, and road accidents. Along the western region of Tamil Nadu, there is a vigorous clampdown on illegal poaching of slender lorises. Furthermore, development of major cities in the regions the slender lorises inhabit, including Colombo, Kandy, Jaffna, Trincomalee, Bengaluru, Thiruvananthapuram, Chennai, and Nellore, has led to destruction of its habitat; however, as shown in Bengaluru, urban populations of the gray slender loris can survive in urban areas provided there is enough tree and forest cover.

Destruction of tropical rain forest habitat is also contributing to declines in population.

IUCN lists the red slender loris as near threatened and the gray slender loris as endangered, whereas they are listed under the Schedule I of the Wildlife (Protection) Act of India, 1972, according them the highest level of legal protection. WWF-India is working to protect the habitats of the slender lorises through its wider conservation work in the Western Ghats - Nilgiris Landscape.

Kadavur Slender Loris Sanctuary

Kadavur Slender Loris Sanctuary was declared as India's first slender loris sanctuary. It is located in Karur and Dindigul districts of Tamilnadu. This wildlife sanctuary has an area of .

References

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