A new genus name for pygmy lorises, Xanthonycticebus gen. nov. (Mammalia, primates)

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.

Molecular Phylogenetic Relationships and Unveiling Novel Genetic Diversity among Slow and Pygmy Lorises, including Resurrection of Xanthonycticebus intermedius

“…Our divergence date estimates also lend support to the findings of Pozzi et al [ 21 ] and Li et al [ 56 ], showing a wide gap (roughly 6 million years) between the divergence of Xanthonycticebus and Nycticebus (6.04, 8.39–4.02 HPD mya) and the argument of Nekaris and Nijman [ 18 ] proposing a new genus name for pygmy lorises. The relatively deep divergence between Xanthonycticebus and the slow lorises may relate to other major differences between them, e.g., Xanthonycticebus is known to live sympatrically with other Nycticebus spp., to have hairless ears, to give birth to twins regularly, and to exhibit a multi-male, multi-female rather than a uni-male, uni-female social system [ 9 , 18 , 71 ]. Further, the divergence between Xanthonycticebus and the slow lorises is as old (mid-Miocene) as the divergence between other lorisid genera such as Sciurocheirus and Otolemur [ 21 ].…”

“…Description and diagnosis: X. intermedius is distinctive in having a smaller skull length (consistently 50 mm or less) and mandible length (consistently 33 mm or less), resulting in a shorter muzzle ( Figure 4 , Figure 5 , Figure 6 and Figure 7 , Table S1 ) compared to X. pygmaeus . Both Xanthonycticebus taxa exhibit finely textured pelage that is reddish buff with a medium to dark brown dorsal stripe and head forks [ 18 , 72 ] with seasonal variation in pelage, including crown coloration change and almost complete loss of the dorsal stripe [ 25 ], but X. intermedius often appears ‘fluffier’ with slightly longer body hair length in general ( Figure 6 and Figure 7 ). The average head and body length of X. intermedius is 230 mm (range 214-248 mm; N = 4 specimens; Table S1 ).…”

“…Distribution: X. intermedius occurs in mainland Southeast Asia east of the Mekong River, but also historically in northern Laos PDR as far west as Phôngsali (21.59 N, 102.25 E) [ 18 ]. In Vietnam, it is found north of the Hai Van Pass in Quang Nam Province (16.200 N, 108.125 E, 2846 m a.s.l.)…”

“…Within the slow and pygmy lorises, there are currently nine species recognized across South and Southeast Asia [ 4 , 7 , 15 , 16 , 17 , 18 , 19 , 20 ], including the greater slow loris ( Nycticebus coucang ), the Bengal slow loris ( Nycticebus bengalensis ), the Javan slow loris ( Nycticebus javanicus ), the Bornean slow loris ( Nycticebus borneanus ), the Philippine slow loris ( Nycticebus menagensis ), the Kayan slow loris ( Nycticebus kayan ), the Sumatran slow loris ( Nycticebus hilleri ), and Sody’s slow loris ( Nycticebus bancanus ). A new genus name was recently proposed for the ninth species, the pygmy loris ( Xanthonycticebus pygmaeus ; [ 18 ]).…”

“…Within the slow and pygmy lorises, there are currently nine species recognized across South and Southeast Asia [ 4 , 7 , 15 , 16 , 17 , 18 , 19 , 20 ], including the greater slow loris ( Nycticebus coucang ), the Bengal slow loris ( Nycticebus bengalensis ), the Javan slow loris ( Nycticebus javanicus ), the Bornean slow loris ( Nycticebus borneanus ), the Philippine slow loris ( Nycticebus menagensis ), the Kayan slow loris ( Nycticebus kayan ), the Sumatran slow loris ( Nycticebus hilleri ), and Sody’s slow loris ( Nycticebus bancanus ). A new genus name was recently proposed for the ninth species, the pygmy loris ( Xanthonycticebus pygmaeus ; [ 18 ]). Most molecular analyses to date consistently support five of the species, X. pygmaeus, N. coucang, N. menagensis, N. javanicus and N. bengalensis , as reciprocally monophyletic (e.g., [ 17 , 21 ]), but the geographic sampling of molecular studies within each of these taxa has not been sufficient to date to validate the morphologically based findings of Munds et al [ 16 ] demonstrating the distinction of N. bancanus, N. borneanus and N. kayan , or Nekaris and Jaffe [ 20 ]’s support for N. hilleri.…”