on an analysis of cranial morphology and characteristics of pelage. Species differentiation was based largely on differences in morphology, such as size, fur color, and head markings. 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 were investigated by Chen and colleagues using DNA sequences derived from the mitochondrial markers D-loop and cytochrome b. Previous molecular analyses using karyotypes, restriction enzymes, and DNA sequences were focused on understanding the relationships between a few species, not the phylogeny of the entire genus. The analyses published in 2006 by Chen and colleagues' proved inconclusive, although one test suggested that N. coucang and N. bengalensis apparently share a closer evolutionary relationship with each other than with members of their own species, possibly due to introgressive hybridization since the tested individuals of these two taxa originated from a region of sympatry in southern Thailand. 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 been gene flow between the two species. In 2012, two taxonomic synonyms (formerly recognized as subspecies) of N. menagensis—N. bancanus and N. borneanus—were elevated to species status, and a new species—N. kayan—was also distinguished from the same. Rachel Munds, Anna Nekaris and Susan Ford based these taxonomic revisions on distinguishable facial markings. With that, the N. menagensis species complex that had been collectively known as the Bornean slow loris became four species: the Philippine slow loris (N. menagensis), the Bornean slow loris (N. borneanus), the Bangka slow loris (N. bancanus), and the Kayan River slow loris (N. kayan). Nekaris and Nijman (2022) combined morphological, behavioural, karyotypical and genetic data and suggested that the pygmy slow loris is best placed in its own genus, Xanthonycticebus. Anatomy and physiology Slow lorises have a round head because their skull is shorter than in other living strepsirrhine. Like other lorisids, their snout does not taper towards the front of the face as it does in lemurs, making the face appear less long and pointed. Compared with the slender lorises, the snout of the slow loris is even less pointed. As with other members of Lorisidae, its interorbital distance is shorter than in lemurs. The skull has prominent crests (ridges of bone). A distinguishing feature of the slow loris skull is that