The slender lorises (Loris) are two species of loris endemic to India and Sri Lanka that comprise the genus Loris. They are small, gracile strepsirrhines that weigh between , with the gray slender loris (L. lydekkerianus) being larger than the insular red slender loris (L. tardigradus). Slender lorises are arboreal, nocturnal primates that forage solitarily at night and congregate into gregarious groups to sleep during the day. Mothers will leave their infants alone during the night, a behaviour known as "parking", while they forage. Slender lorises maintain predominantly carnivorous diets consisting of invertebrates such as spiders, orthopterans, snails, and ants, as well as vertebrates such as lizards, frogs, and small birds. Between the two slender loris species are seven recognised subspecies, each of which has unique morphological traits such as varying pelage patterns and body proportions. The slender lorises have undergone numerous taxonomic revisions since they were first described in the 18th century.
The head of a slender loris is distinguished by the presence of large, tapetum-covered eyes that lie close together, exhibiting the lowest interocular distance of all primates. The dentition possesses the typical strepsirrhine toothcomb, which a slender loris can use for grooming itself and others. Accomplishing a similarly hygienic function is a toilet claw of the second pedal digit, which can be used to clean almost any part of the body via the slender loris' remarkable flexibility. Slender lorises have small, pincer-like hands and feet with reduced second digits that allow an individual to powerfully grip a branch for hours at a time, with the assistance of specialised blood vessels in the limbs known as retia mirabilia. The spine is characterised by an elongated shape, and sways from side-to-side as a slender loris travels across its preferred substrates of twigs, branches, and vines. Though otherwise variable, pelage patterns are unified by the presence of a thin, vertical white stripe between the eyes, as well as dark circumocular patches.
The gray slender loris is classified as a near threatened species by the International Union for Conservation of Nature, while the red slender loris is classified as endangered. Both species are listed under Appendix II by CITES. Slender lorises are threatened by poaching for the acquisition of body parts for use in traditional medicine, as well as habitat loss.
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 "" refers to clowns, which could be descriptive of the slender loris' apparent acrobatic capabilities or of the markings on their faces. Two variants of the term "loris" were produced in the English translation of German traveller 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 lorises, , 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 vertical interocular mark on slender lorises' foreheads to those worn by worshippers 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. Carl Linnaeus described slender lorises as Lemur tardigradus 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 theory by presenting the slender lorises' primate traits in the posthumous 36th volume of the Histoire Naturelle.
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 Loris gracilis in the . Geoffroy described Loris on the mistaken assumption that Linnaeus' Lemur tardigradus 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 Loris ceylonicus in 1804. René Lesson later authored his own description of the red slender loris, Arachnocebus lori, 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. lori lydekkerianus, 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 described 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). At that time, 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. 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 Gamage et al. in 2017, who speculated that its morphological differences from the southwestern red slender loris may qualify it as an independent species, given further examination.
Anatomy and physiology
The slender lorises are small primates that are characterised by their extreme gracility and their suite of highly derived traits. They can be distinguished from other lorisids by their lithe habitus, enlarged orbits, and short, pointed snouts. Females are generally larger than males. The gray slender loris is much larger than the red slender loris, as infant gray slender loris are about the same size as adult red slender lorises. The head appears spherical and the coronal suture is often asymmetrical. 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, which is the lowest among all primates. The resultant overlapping visual fields of the two forward-pointed eyes creates a sophisticated stereoscopic vision. The dental formula is × 2 = 36. The dentition is characterised by the presence of a toothcomb and a caniform lower anterior premolar (P2). In the toothcomb, the lower incisors (I1–2) are tilted forward and are flattened laterally, with the angles of the crowns further accentuating this procumbent orientation. 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 that allows the upper lip to attach to the gum. Compared to the slow lorises, the foramen magnum is positioned further forward in the skull. Slender lorises differ from all other lorisids in that their heads are not pointed in a downward direction.
The limbs are long and narrow, with the arms having large olecranons. The intermembral index is 93. The shoulders and hips permit great 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 reduced in size, which allows 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 digit 4 is the longest in the hand, which increases the breadth of a slender loris' grasp by being positioned opposite the thumb (manual digit 1). The thumb is not truly opposable, as the palmar pad at the base of the digit is stationary. Pedal digit 2 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 hygienic purposes because the other nails do not extend beyond the tips of the digits. The digital formula is 4>3>5>2>1. 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.