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 long and has a very short tail. It weighs about . 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
The pygmy slow loris has a head and body length (measured from the top of the head to the base of the tail) of ; there is no significant difference in size between the sexes. The length of the skull is less than . The tail is short, averaging in length. The bodyweight ranges between , with an average mass of for males and for females. There are, however, large seasonal variations in bodyweight, and individuals up to have been recorded. The animal tends to have significantly higher bodyweights during the winter months, about 50 percent higher than the lowest values in the summer. The weight gains, achieved largely by increasing food intake, are triggered by changes in the length of the day and night. This seasonal change in bodyweight occurs in both sexes, in both pregnant and non-pregnant females—an adaptation thought to help ensure survival during winter when food resources become scarce. The species has distinctive teeth morphology: its third molar is triangular in outline and only slightly smaller than the first molar; its second molar is the largest. The incisors and canines on its lower jaw are procumbent (tilt forward) and together form a toothcomb that is used in grooming and feeding.
Like other strepsirrhine primates, the pygmy slow loris has tapeta lucida in its eyes to assist with night vision. In adults, the rings circling the eyes are seal brown; they are darker in young individuals. There is a white stripe extending from the nose to the forehead, and the sides of the head and upper lip are silvery gray, while the rest of the face and top of the head is rufous. It has small black ears, typically about long, which do not have fur on the tips. On the dorsal side of the animal, a rufous to brownish-black stripe runs from the nape to the middle of the lower back. The upper parts, including the shoulders and upper back, are russet to reddish-buff to brownish, and are sometimes "frosted" with silvery gray white hairs. The presence or absence of a dorsal stripe and silvery hair tips appear to be a seasonal variation and have led some to postulate the existence of an additional species, N. intermedius, although DNA analysis has since confirmed this to be an adult version of the pygmy slow loris. The pygmy slow loris has buffy flanks, paler than the back. The upper sides of the arms are ochraceous, and have silvery hairs mingled with the darker ones. The buff legs are also tipped with silvery white hairs. The underparts are plumbeous (lead-colored) at the base, with ochraceous apical portions. The hands and feet are silvery white, with yellowish-white nails. Foot length is relatively consistent, averaging about .
The pygmy slow loris has a diploid chromosome number of 2n=50. Although the banding patterns on the chromosomes of all slow lorises are similar, this species may be distinguished from the Bengal slow loris (N. bengalensis) by distinct differences in the number and location of nucleolus organizer regions.
Behavior The pygmy slow loris is nocturnal, although it is least active on cold, moonlit nights and is generally active on dark nights, regardless of temperature. In the wild, it is normally encountered alone, or in small groups of two to four individuals. Males use scent marking to defend territories and mark their boundaries. Females prefer to mate with males whose scent is familiar. Males will also countermark—mark over or adjacent to another individual's mark deposited earlier—to advertise competitive ability to females. Females actively prefer countermarking males to males whose odors have been countermarked.
The pygmy slow loris produces an apocrine secretion on scent glands near their elbow (brachial glands). This clear liquid, when mixed with its saliva, creates a volatile, noxious toxin. When startled, the slow loris licks its brachial glands and applies the secretion to its head. The oily secretion contains a complex mixture of volatile and semi-volatile components; one chemical analysis indicated over 200 components were present. One of the components is a member of the secretoglobin family of proteins, and similar to an allergenic protein found in cat dander. The similarity between the brachial gland secretions and domestic cat allergens may account for anaphylaxis in susceptible individuals.
Vocalizations of the pygmy slow loris include a short whistle, mother-infant contact calls, and a whistling sound produced during estrus.
Reproduction The female is mildly aggressive to her suitors during estrus, and will often lunge at males, usually after a long period of being approached and followed. Vocalizations during mating include a whistling sound, most commonly by the female, usually during June and August, coinciding with female estrus. Other vocalizations recorded during estrus include chittering and growling. The testosterone levels of the males are seasonal, with peaks coinciding with female estrogen peaks.
The pygmy slow loris can conceive by 18 months and give birth to its first offspring by two years of age. Studbook records show that the youngest male to sire offspring was around 18 months of age, and the youngest female conceived at 16 months. Gestation length is 184–200 days, and the lactation period lasts 123–146 days. Offspring are weaned at about 24 weeks of age. The pygmy slow loris is monoestrous, experiencing a single four- to five-day period of reproductive activity between late July and early October in captivity, with births occurring from early February to mid-March. As a result, opportunities for mating are rare, and females rely heavily on scent to assess mate quality. Females show a strong preference for familiar-smelling males over novel-smelling males. Research on the process of sexual selection in primates suggests that the exclusive presence of one male's scent in the area is a reliable cue that he is capable of defending the area and/or preventing rival males from marking. Pygmy slow lorises usually have a litter size of one or two; separate studies have reported frequencies of twinning as either 50% or 100% of births. Data collected from a seven-year captive breeding program indicates that they have a skewed birth sex ratio of 1 female to 1.68 males. Because they must divide time equally between offspring, mothers of twins spend less time engaging in social grooming and play with their young, which may lead to a lower infant survival rate. Mothers will "park" their young at one week of age while foraging, and the young begin following their mothers at about two weeks. The life span of the pygmy slow loris is about 20 years.
Diet