Taxonomy and Phylogeny
The Sunda slow loris was first described (in part) in 1785 by the Dutch physician and naturalist Pieter Boddaert under the name Tardigradus coucang. However, its discovery dates to 1770, when the Dutchman Arnout Vosmaer (1720–1799) described a specimen of it as a type of sloth. Vosmaer gave it the French name "le paresseux pentadactyle du Bengale" ("the five-fingered sloth of Bengal"), but Boddaert later argued that it was more closely aligned with the lorises of Ceylon (now Sri Lanka) and Bengal.
Between 1800 and 1907, several other slow loris species were described, but in 1953 the primatologist William Charles Osman Hill, in his influential book, Primates: Comparative Anatomy and Taxonomy, consolidated all the slow lorises into a single species, N. coucang. In 1971 Colin Groves recognized the pygmy slow loris (N. pygmaeus) as a separate species, and divided N. coucang into four subspecies. In 2001 Groves opined that there were three species (N. coucang, N. pygmaeus, and N. bengalensis), and that N. coucang itself had three subspecies (Nycticebus coucang coucang, N. c. menagensis, and N. c. javanicus). These three subspecies were promoted in 2010 to species status—the Sunda slow loris, the Javan slow loris (N. javanicus) and Bornean slow loris (N. menagensis). Species differentiation was based largely on differences in morphology, such as size, fur color, and head markings. (At the end of 2012, the Bornean slow loris was itself divided into four distinct species.)
When Étienne Geoffroy Saint-Hilaire defined the genus Nycticebus in 1812, he made the Sunda slow loris the type species. This was questioned in 1921 by British zoologist Oldfield Thomas, who noted that there was some confusion over which specimen was used as the type specimen. Instead, he suggested that the type specimen was actually the Bengal slow loris, Lori bengalensis Lacépède, 1800. There was further confusion during the 1800s when Boddaert's Tardigradus coucang was routinely mistaken for Carl Linnaeus' Lemur tardigradus – a species he had described in the 10th edition of Systema Naturæ (1758) The fact that Lemur tardigradus was actually a slender loris remained obscured until 1902, when mammalogists Witmer Stone and James A. G. Rehn finally cleared the air.
The species has 50 chromosomes (2n=50), and it genome size is 3.58 pg. Of its chromosomes, 22 are metacentric, 26 are submetacentric, and none are acrocentric. Its X chromosome is submetacentric, and its Y chromosome is metacentric.
To help clarify species and subspecies boundaries, and to establish whether morphology-based classifications were consistent with evolutionary relationships, the phylogenetic relationships within the genus Nycticebus have been investigated using DNA sequences derived from the mitochondrial markers D-loop and cytochrome b. Although most of the recognized lineages of Nycticebus (including the pygmy slow loris (N. pygmaeus), Bornean slow loris (N. menagensis) and the Javan slow loris (N. javanicus)) were shown to be genetically distinct, the analysis suggested that DNA sequences from selected individuals of Sunda slow loris (N. coucang) and Bengal slow loris (N. bengalensis) shared a closer evolutionary relationship with each other than with other members of their own respective species. The authors suggest that this result may be explained by introgressive hybridization, as the tested individuals of these two taxa originated from a region of sympatry in southern Thailand; the precise origin of one of the N. coucang individuals was not known. This hypothesis was corroborated by a 2007 study that compared the variations in mitochondrial DNA sequences between N. bengalensis and N. coucang, and suggested that there has indeed been gene flow between the two species.
Read more about this topic: Sunda Slow Loris