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Stegonotus caligocephalus KAISER, LAPIN, O’SHEA & KAISER, 2020

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Higher TaxaColubridae, Colubrinae, Colubroidea, Caenophidia, Alethinophidia, Serpentes, Squamata (snakes)
Subspecies 
Common NamesE: Dark-headed Sabah Groundsnake 
SynonymStegonotus caligocephalus KAISER, LAPIN, O’SHEA & KAISER 2020 
DistributionMalaysia (Borneo: Sabah)

Type locality: near the Gunung Emas [Gunung Alab] Telekom tower, Crocker Range National Park, Tambunan District, Sabah State, Malaysia, ca. 5.83°N, 116.34°E (Fig. 1), elevation ca. 1950 m  
Reproduction 
TypesHolotype. FMNH 251054 (field number RFI 49331), an adult female, collected on 19 January 1993 by the American herpetologist Robert F. Inger (1920–2019), Curator of Amphibians and Reptiles at the Field Museum, Chicago, Illinois, USA, from 1954–1994 (Matsui 2019).
Paratype. USNM 130244, an adult female, from “Bundu Tuhan, Mount Kina Balu, Ranau District, Sabah” [Malaysia], ca. 6.05°N, 116.53°E, elevation 1370 m, collected on 1 August 1951 by the American mammalogist David H. Johnson (1912–1996), Curator of Mammals at the National Museum of Natural History, Smithsonian Institution, Washington, D.C., USA, from 1941–1965 (Perry 2007).
Other specimens: ZMB 47866 
DiagnosisDiagnosis. Stegonotus caligocephalus is a medium-sized (maximum SVL = 1071 mm) snake with a relatively short tail (SCR♀ = 0.21), the shortest in the genus Stegonotus. It can be distinguished from all other known Stegonotus species by the following combination of characters: (1) rostral extending onto the dorsal part of the head intruding into the internasal area but not reaching the level of the nasals (character state: gull wing +; Fig. 4A, A’); (2) area of prefrontals three times that of the internasals, internasal suture one-fourth to one-third the length of the prefrontal suture (SSR 0.25–0.33, mean = 0.28); (3) frontal clearly pentagonal with well-formed corners and a slightly concave anterior suture; (4) AE lies at the same level as the anterior end of the frontal; (5) length of frontal three-fourths that of the parietal suture (FPR = 0.75); (6) PfS one-fourth to one-third the length of external head-scale length (HSR = 0.25–0.31). (7) PF< ≥ 90° (102°); (8) AP< = 122°, with its lateral ray directed posterolaterally (Fig. 4A, A’); (9) two temporal scales touching the parietals; (10) three neck scales contacting parietals; (11) loreal as long as wide to one-third longer than wide (LSR = 0.6–1.0); (12) two preoculars, upper preocular 25% larger than lower one (Fig. 4B, B’); (13) nine supralabials, two (SL4–5) touching the eye; SL5 larger than SL4 (Figs. 4B, B’); (14) ten infralabials, five (IL1–5) touching the anterior genial (Fig. 4C’); (15) three gulars separating the posterior genials and the anterior-most gastrostege; (16) 17-17-15 dorsals; 211–218 (mean = 214 ± 2.9) ventrals, 56–65 (mean = 61 ± 3.7) paired subcaudals; (17) cloacal plate entire; (18) color in preservative (27 years post-collection): dorsum Tawny Olive (17) with some dark spots on lateral edge of a few rows of dorsal scales, head Grayish Olive (273) with some scattered lighter areas in Olive Gray (265). The supralabials and infralabials are Pale Buff (11) with mottled areas of Drab Gray (256) (Figs. 3A, 4A, B). The venter is Pale Horn Color (11) with dark areas on the anterior edge of the ventral scales and a dark “dot” on the lateral edge (Burnt Sienna, 38); the anterior one-third of the subcaudals are Burnt Sienna (38), creating a dark tail (Fig. 3B in Kaiser et al. 2020).

Coloration in life. Based on a color analysis of the images in Fig. 6, which have different lighting, we esti- mated the coloration in life as follows. The dorsal area of the head is Dark Grayish Brown (284) with some scattered Smoky White (261) spots. The color intensity fades laterally, and a lighter brown with elements of Cinnamon-Drab (50) and Beige (254) dominates the supralabial area. The dorsal coloration of the body is in tones of Mars Brown (25) and much lighter than the head, fading in the area where dorsal and ventral scales meet to a light shade of Light Russet Vinaceous (246). This faded color is also visible on the anterolateral margins of each ventral scale (Kaiser et al. 2020).

Comparisons. In the following comparisons, characteristics of S. caligocephalus are listed in parentheses. The number of specimens used to determine ranges of continuous characters follows species names. Based on our previous analysis of Stegonotus (Kaiser et al. 2019) there does not appear to be any sexual dimorphism in the Bornean or Philippine species we compare in detail below. Given how few specimens of these populations are available in museum collections, combining scale count data for females and males in the absence of sexual dimorphism therefore increases our sample size and improves our analysis. Furthermore, given the geographic position of this Bornean population, most Stegonotus species can be considered extralimital to this comparison. It is highly unlikely that a New Guinean population disperses all the way across Wallacea into Borneo. After presenting an exhaustive comparative section in the description of S. ayamaru (Kaiser et al. 2019), we are confident in limiting our detailed, formal comparisons to Stegonotus from Borneo and the Philippines and eliminate other species based on different dorsal scale counts (S. admiraltiensis, S. diehli, S. dorsalis, S. florensis, S. guentheri, S. poechi, S. sutteri), significant differences in number of gastrosteges (S. admiraltiensis, S. aruensis, S. ayamaru, S. derooijae, S. diehli, S. guentheri, S. heterurus, S. keyensis, S. lividus, S. melanolabiatus, S. modestus, S. parvus, S. poechi), strongly diverging rostral morphology (S. aruensis, S. australis, S. batjanensis, S. cucullatus, S. dorsalis, S. iridis, S. parvus, S. poechi), the configuration of supralabials touching the eye (S. aruensis, S. ayamaru, S. batjanensis, S. derooijae, S. diehli, S. florensis, S. heterurus, S. lividus, S. melanolabiatus, S. modestus, S. parvus, S. poechi, S. sutteri), and differences in head and body color (S. batjanensis, S. florensis, S. iridis, S. modestus, S. reticulatus, S. sutteri). Where appropriate, gender is indicated throughout by subscripted male (♂) and female (♀) symbols for ease of reference; when sexes are combined there is no subscript. Given that the three extant specimens of S. caligocephalus are all females, we separately provide data for females of the other species. A listing of those characters most relevant for interspecies comparisons is provided in Table 2 in Kaiser et al. 2020.
Stegonotus borneensis (n = 1 male, 1 female) has a significantly (unpaired t-test, t = 6.97, P < 0.01) lower ventral scale count of 193–196 (211–218) and a significantly (t = 4.98, P < 0.05) higher subcaudal count of 78 (56–65). In the single female specimen of S. borneensis, the values for ventral and subcaudal count are 193 and 78, respectively. As a consequence, the SCR in S. borneensis is higher, at 0.28–0.29 (0.21–0.24). Additional differences include (1) SSR = 1/5 (1/4 or 1/3); (2) FPR = 4/5–7/8 (3/4); and (3) a uniform dark body coloration (head darker than body). The closest known locality for S. borneensis is in Sarawak State, Malaysia, a horizontal distance of ca. 560 km by air from the type locality of S. caligocephalus. Furthermore, whereas S. borneensis appears to be a lowland tropical forest species (highest known elevation ca. 56 m at the type locality; Kaiser et al. 2018a), S. caligocephalus occupies montane forest habitat at elevations well above 1000 m. The lowest elevation for which we have found a published sighting (Malkmus et al. 2002) is 1370 m, at the locality where the paratype was found.
Stegonotus muelleri (n = 13 females, 19 male) is from the south-central Philippine Islands and differs by scale counts of V = 217–241 (t = 5.24, P < 0.001), mean = 231 ± 5.5 (211–218, mean = 214 ± 2.9), V♀ = 217–234, mean = 227 ± 4.5 (211–218, mean = 214 ± 2.9), SC = 78–108 (t = 7.74, P < 0.001), mean = 96 ± 7.3 (56–65, mean = 61 ± 3.7), SC female = 81–103, mean = 95 ± 6.5 (56–65, mean = 61 ± 3.7), SCR = 0.29 (SCR = 0.22), SCR♀ = 0.30 (SCR♀ = 0.22), D = 19-17-15 (17-17-15), SL = 8 in 98.6% or 9 in 1.4% of specimens (9 in 100% of specimens). Additional differences include (1) SSR = 2/5 (1/4 or 1/3); (2) three temporal scales touching the parietals (two); and (3) AE lies behind the anterior end of frontal (same level). The closest known locality for S. muelleri is on the island of Mindanao, ca. 1000 km by air from the type locality of S. caligocephalus. 
CommentSimilar species. The species could, in principle, be confused with three similarly colored species (Stuebing et al. 2014). Lycodon albofuscus4 has keeled scales, is more slender, and its lowland habitat make it unlikely that this species occurs in sympatry with S. caligocephalus. On Borneo, older individuals of Lycodon subcinctus may have lost the characteristic light bands of younger snakes and may present an overall body coloration similar to S. caligocephalus. As is the case with L. albofuscus, the range of L. subcinctus may not extend into the higher elevations (above 1000 m) where S. caligocephalus is found (Kaiser et al. 2020). 
EtymologyThe species name caligocephalus is a compound word derived from the Latin caligo, with meanings including dark(ness), out of sight, fog, and the Greek kephalos (κεφαλοζ) meaning head. The species name first of all references the darker head color in this species. Secondly, the dark, looming shape of Mount Kinabalu, Southeast Asia’s tallest mountain and the montane home of S. caligocephalus, is a dramatic, head-like massif, and we wish to remind people that mountain regions are wonderful places to explore and preserve nature. 
References
  • KAISER, CHRISTINE M.; JACK LAPIN, MARK O’SHEA & HINRICH KAISER 2020. Carefully examining Bornean Stegonotus (Serpentes, Colubridae): the montane groundsnake population in Sabah is a new and distinct species. Zootaxa 4894 (1): 053–068 - get paper here
 
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