The pygmy slow loris (Xanthonycticebus pygmaeus) is a species of slow loris found east of the Mekong River in Vietnam, Laos, eastern Cambodia, and China. It occurs in a variety of forest habitats, including tropical dry forests, semi-evergreen, and evergreen forests. It was originally classified within Nycticebus until it was transferred to the genus Xanthonycticebus in 2022. Two species are recognised, the northern pygmy loris X. intermedius from northern Vietnam, Laos and China and the southern pygmy loris X. pygmaeus from southern Vietnam, Laos and Cambodia. The animal is nocturnal and arboreal, crawling along branches using slow movements in search of prey. Unlike other primates, it does not leap. It lives together in small groups usually with one or two offspring. An adult can grow to around 19 to 23 cm (7.5 to 9.1 in) long and has a very short tail. It weighs about 450 g (1.0 lb). Its diet consists of fruits, insects, small fauna, tree sap, and floral nectar. The animal has a toxic bite, which it gets by licking a toxic secretion from glands on the inside of its elbows. The teeth in its lower jaw form a comb-like structure called a toothcomb that is used for scraping resin from tree bark. The pygmy slow loris mates once every 12–18 months and has one or two offspring after an average gestation period of six months. For the first few days, the young loris clings to the belly of its mother. The offspring will be nursed for an average of 4.5 months, but weaning can sometimes take up to 8 months. The female reaches sexual maturity at about 9 months, while the male reaches maturity by about 18–20 months. The pygmy slow loris is seasonally fertile during the months of July and October. Chemical signals play a role in the reproductive behavior of female pygmy slow lorises. Urine scent markings have a strong characteristic odor and are used to communicate information about social relationships. The habitat of the pygmy slow loris in Vietnam was greatly reduced due to extensive burning, clearing, and defoliating of forests during the Vietnam War. Extensive hunting for traditional medicines is currently putting severe pressure on Cambodian populations. The pygmy slow loris is seriously threatened by hunting, trade, and habitat destruction; consequently, it is listed in Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), and in 2020 the International Union for Conservation of Nature (IUCN) classified it as endangered.
History, taxonomy, and phylogeny
The pygmy slow loris was first described scientifically by J. Lewis Bonhote in 1907. The description was based on a male specimen sent to him by J. Vassal, a French physician who had collected the specimen from Nha Trang, Vietnam (then called Annam, a French Protectorate) in 1905. In 1939, Reginald Innes Pocock combined all slow lorises into a single species, Nycticebus coucang. In an influential 1953 publication, primatologist William Charles Osman Hill also consolidated all the slow lorises in one species, Nycticebus coucang, and considered other forms distinct at the subspecies level. Osman Hill thus listed Nycticebus coucang pygmaeus, while acknowledging that "it may be deemed necessary to accede this form specific rank." In 1960, Dao Van Tien reported a species from Hòa Bình Province, Vietnam, that he called N. intermedius, but it turned out that his specimens were merely adults of the pygmy slow loris, which had originally been described on the basis of a juvenile. After studying slow lorises from Indochina, primatologist Colin Groves proposed that the pygmy slow loris was morphologically unique enough to be considered a distinct species. The validity of this opinion was later corroborated by studies of chromosomal structure, genetic distance determined by protein variation at polymorphic loci, and mitochondrial DNA restriction enzyme analysis. Nekaris and Nijman (2022) combined morphological, behavioural, karyotypical and genetic data and suggested that pygmy lorises are best placed in their own genus, Xanthonycticebus. The phylogenetic relationships within the genus Nycticebus have been studied with modern molecular techniques, using DNA sequences derived from the mitochondrial DNA markers D-loop and cytochrome b from 22 slow loris individuals. In this analysis, most of the recognized lineages of Nycticebus, including the pygmy slow loris, were shown to be genetically distinct, and the species was shown to have diverged earlier than the other slow loris species, beginning perhaps 2.7 million years ago. Analysis of nucleotide sequence diversity from individuals taken from the boundary areas between southern China and Vietnam (a region of sympatry between the pygmy slow loris and the Bengal slow loris) show that the pygmy slow loris is not subject to the same introgressive hybridization as the Bengal slow loris (N. bengalensis). The authors of the study suggest that the low polymorphism of pygmy slow lorises may be due to a founder effect, and that the individuals they used in the study originate from an ancestor that lived in middle or southern Vietnam between 1860 and 7350 years ago.
Anatomy and physiology
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