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Xanthonycticebus pygmaeus (Bonhote, 1907) Photograph of wild adult male X. pygmaeus from Mondulkiri District, Cambodia and skull from Li Chau, Vietnam (FMNH 32499), compared with Nycticebus javanicus from Garut Regency, Indonesia and skull (RMNH14563) from South Java, Indonesia; and with Loris lydekkerianus nordicus from Trincomalee District, Sri Lanka and skull (FMNH95029) from Jaffna District, Sri Lanka. Features distinctive to Xanthonycticebus include yellowish-orange colour, mid-broad snout with long premaxilla, M2 larger than M1 and ears hairless at the tips in Nekaris & Nijman, 2022. DOI: 10.3897/zse.98.81942 |
Abstract
Lorisiformes are nocturnal primates from Africa and Asia with four genera, with two (Arctocebus and Loris), three (Perodicticus) and nine (Nycticebus) recognised species. Their cryptic lifestyle and lack of study have resulted in an underappreciation of the variation at the species and genus level. There are marked differences between the pygmy slow loris Nycticebus pygmaeus and the other Nycticebus species and, in the past, several authors have suggested that these may warrant recognition at the generic level. We here combine morphological, behavioural, karyotypical and genetic data to show that these contrasts are, indeed, significantly large and consistent. We propose Xanthonycticebus gen. nov. as a new genus name for the pygmy slow lorises and suggest a common name of pygmy lorises. Based on analysis of complete mitochondrial DNA sequences, we calculate the divergence of pygmy from slow lorises at 9.9–10.0%. The median date, calculated for the divergence between Xanthonycticebus and Nycticebus, is 10.5 Mya (range 4.9–21.0 Mya). Xanthonycticebus differs from Nycticebus by showing sympatry with other slow loris species, by habitually giving birth to twins, by showing seasonal body mass and whole body coat colour changes (absent in other species living at similar latitudes) and a multi-male, multi-female social system. Pygmy lorises are easily recognisable by the absence of hair on their ears and more protruding premaxilla. Xanthonycticebus is threatened by habitat loss and illegal trade despite legal protection across their range and all slow lorises are listed on appendix 1 of CITES. The suggested nomenclatural changes should not affect their legal status.
Key Words: conservation, cytotaxonomy, Lorisidae, Lorisiformes, primate taxonomy, Strepsirrhini
Order Primates Linnaeus, 1758
Suborder Strepsirhini É. Geoffroy Saint-Hilaire, 1812
Family Lorisidae Gray, 1821
Xanthonycticebus gen. nov.
Xanthonycticebus pygmaeus (Bonhote, 1907)
syn: Nycticebus pygmaeus Bonhote, 1907
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Characteristics of pygmy loris Xanthonycticebus pygmaeus gen. nov. A. Photograph of wild adult male X. pygmaeus from Mondulkiri District, Cambodia and skull from Li Chau, Vietnam (FMNH 32499), compared with Nycticebus javanicus from Garut Regency, Indonesia and skull (RMNH14563) from South Java, Indonesia; and with Loris lydekkerianus nordicus from Trincomalee District, Sri Lanka and skull (FMNH95029) from Jaffna District, Sri Lanka. Features distinctive to Xanthonycticebus include yellowish-orange colour, mid-broad snout with long premaxilla, M2 larger than M1 and ears hairless at the tips; B. Neighbour-joining tree of 175 cytochrome b sequences (alignment 1,068 bp) of Nycticebus, Xanthonycticebus and Loris; C. Neighbour-joining tree of complete mtDNA sequences of Nycticebus, Xanthonycticebus and Loris, with Perodicticus as outgroup, showing considerable divergence of Xanthonycticebus from Nycticebus. All photographs courtesy of K.A.I. Nekaris. |
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| Photograph of wild adult male Xanthonycticebus pygmaeus from Mondulkiri District, Cambodia and skull from Li Chau, Vietnam (FMNH 32499), compared with Nycticebus javanicus from Garut Regency, Indonesia and skull (RMNH14563) from South Java, Indonesia; and with Loris lydekkerianus nordicus from Trincomalee District, Sri Lanka and skull (FMNH95029) from Jaffna District, Sri Lanka. Features distinctive to Xanthonycticebus include yellowish-orange colour, mid-broad snout with long premaxilla, M2 larger than M1 and ears hairless at the tips. |
Diagnosis: Morphological synapomorphies to Xanthonycticebus include: (i) skull length consistently less than 55 mm, (ii) diastema between P2 and P3, (iii) long black ears, hairless at the tips (iv) relatively narrow interorbital distance compared to Nycticebus and (v) full seasonal coat colour change including almost complete loss of dorsal stripe (Fig. 1). The species is furthermore distinguished from Nycticebus species by giving birth habitually to twins, frequent sympatry with N. bengalensis (sensu lato) and more rapid locomotion. Regarding multiple births, this trait occurs with varying frequency in primates. Most of the marmosets and tamarins are polyovulatory and twins are the dominant litter size in the wild and most twins are considered dizygotic (Ward et al. 2014; Wahab et al. 2015). Old World monkeys, apes and humans are monovulatory species and while single births are the rule, multiple births do occasionally occur in various species, typically at a rate at, or below one percent (Geissmann 1990). Around two-thirds to three-quarters of these twins are estimated to be monozygotic (Geissmann 1990). This contrasts with twinning in strepsirrhines, as here all, or practically all, are dizygotic (Pasztor and Van Horn 1979). No other species of slow lorises are known to be sympatric, with their distribution similar to gibbons Hylobatidae Gray, 1870 and langurs Presbytina Gray, 1825, which, even though having more recent evolutionary histories, contain multiple genera (Rowe and Meyers 2016). Where nocturnal primate genera or species are sympatric, different locomotor strategies have evolved, allowing reduced competition (Charles-Dominique 1977). Additional differences, as well as those from Loris, are summarised in Table 1.
Etymology: The genus name Xanthonycticebus, masculine, refers to the species orange/ish overall colouration and their nocturnal activity pattern; Xanto, Gr. Yellowish-orange; nykt-, Gr., night; kêbos, Gr., monkey (Gainsford 2020). Currently, the most frequently used common name of this genus is pygmy slow loris, followed by the rarely used lesser slow loris or intermediary slow loris. For the common English name, we suggest pygmy loris in order to differentiate the new genus from the two other loris genera (slow and slender lorises). We acknowledge, however, that with the recognition of N. menagensis Munds, Nekaris and Ford 2013, from Borneo, with a minimum adult body mass of 265 g, the small size is no longer a unique feature of the pygmy loris.
The most commonly-used name for pygmy lorises in Vietnamese is Cu li nhỏ, in Mnong, it is Tau kless, in Lao, it is Linh lom and in Chinese, it is 小懶猴 / Xiǎo lǎn hóu (Nijman and Nekaris 2016;Thạch et al. 2018).
K. Anne-Isola Nekaris and Vincent Nijman. 2022. A New Genus Name for Pygmy Lorises, Xanthonycticebus gen. nov. (Mammalia, Primates). Zoosystematics and Evolution. 98(1); 87-92. DOI: 10.3897/zse.98.81942
