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Rhabdophis siamensis (MELL, 1931)

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Higher TaxaColubridae (Natricinae), Colubroidea, Caenophidia, Alethinophidia, Serpentes, Squamata (snakes)
Common Names 
SynonymNatrix subminiata siamensis MELL 1931
Rhabdophis himalayanus laobaoensis BOURRET 1934: 169
Rhabdophis subminiatus subminiatus siamensis [sic] — DEUVE 1961: 14
Rhabdophis siamensis — DAVID & VOGEL 2021 
DistributionThailand, Vietnam, China (Yunnan)

Type locality: “Siam”

laobaoensis: Vietnam; Type locality: “Lao Bao (Chaîne Annamitique altitude 300 m)”, now Lao Bao, Truong Son Range, Quang Tri Province, Vietnam.  
TypesLectotype: ZMB 30222, adult female, designated by David & Vogel 2021.
Holotype: MNHN 0121 [laobaoensis] 
DiagnosisDiagnosis (genus): parallel and keeled dorsal scales; 15, 17, 19 or 21 rows of dorsal scales at midbody; large cephalic scales; large eyes with round pupil; divided anal plate and paired subcaudal scales; maxillary teeth separated from a pair of fangs [DORIA et al. 2013].

Diagnosis. A moderately-sized species of the genus Rhabdophis characterized by the combination of (1) 19(rarely 17, 18 or 21) – 19 – 17(rarely 16 or 18) dorsal scale rows; (2) dorsal scales narrowly but strongly keeled, scales of 1st DSR smooth; (3) nuchal groove always present, moderate to strong; (4) 3–8 enlarged, paired nuchal scales; (5) VEN 137–156, SC 65–89, paired; (6) dorsum yellowish-grey, greyish-brown or pale brown, distinctly spotted with black and cream blotches; (7) upper surface and sides of the neck bright vermilion-red, sometimes subdued in larger specimens or in long-preserved specimens; (8) a dark brown or black subocular streak always present; (9) dark subocular streak shaped as a broad triangular streak (rarely faint), directed backwards; (10) venter pale, i.e., cream or creamish-yellow, without dark dots on the tips of ventrals, or only on the anterior part of the venter.
Rhabdophis siamensis comb. nov. differs from Rhabdophis subminiatus by the following characters:
(1) the number of ventral plates (weak sexual dimorphism, sexes combined), 137–156 (x = 146.0; sd = 5.4) in R. siamensis vs. 132–145 (x = 136.8; sd = 2.6) in R. subminiatus. These values suggest a large overlap in the numbers of ventrals but a Mann-Whitney U-test (one-tailed) gives a level of difference between the samples significant at p < 0.001 (U = 455.5; z-score = 6.67568). If the difference is tested on the number of ventral plates in males only, the values are 134–143 in R. subminiatus vs. 137– 156 in R. siamensis. These values also suggest a large overlap in the numbers of ventral plates but a Mann-Whitney U-test (one-tailed) gives a level of difference between the samples significant at p < 0.001 (U = 93.0; z-score = 4.96524). Result in taking into account females only is similar and is not detailed here.
(2) the number of subcaudals in males, 72– 89 (x = 79,9; sd = 5.4) in R. siamensis vs. 65–78 in R. subminiatus. A Mann-Whitney U-test (one-tailed) gives a level of difference between the samples significant at p < 0.001 (U = 38.0; z-score = 4.45761).
(3) the nuchal groove is always present, moderate to strong in R. siamensis vs. invisible (weakly visible in one specimen);
(4) 3–8 enlarged, aligned paired nuchal scales vs. 0 aligned and enlarged scale (2 aligned pairs in only one specimen);
(5) shape of the dark subocular streak: streak shaped as a conspicuous (rarely faint or incomplete), broad, triangle (looking like a shark fin), its apex directed both downwards and distinctly backwards in R. siamensis vs. a streak shaped as a narrow, vertical bar, its apex directed downwards, or as a curved streak directed downwards or downwards and forwards when the streak looks like a comma, or also shaped as a broad streak initially directed backwards then curved downwards in R. subminiatus (never a triangle directed backwards);
(6) venter cream or yellowish-cream without a dark dot on the tip of each ventral plate, or present only on the anterior part of the venter, vs. venter with a dark dot on the tip of each ventral (rarely only on the anterior part of the venter);
The general dorsal pattern is quite similar in these two species, although the background colour is paler and the dorsal pattern more contrasted in R. subminiatus than in R. siamensis. The pale dorsolateral stripe or series of aligned blotches edging the darker upper dorsal scale rows, most usually present in R. subminiatus, are absent in R. siamensis. Lastly, the red area of the neck is somewhat longer in R. siamensis.
Rhabdophis siamensis comb. nov. differs from Rhabdophis helleri by the following main characters:
(1) the difference in total size, about 80 cm in R. siamensis vs. up to 130 cm in R. helleri;
(2) the number of ventrals: 137–156 (x = 146.3, sd = 5.1) in R. siamensis vs. 157–178 (x = 165.3, sd = 3.6) in R. helleri;
(3) the number of subcaudals (both sexes altogether): 65–89 (x = 77.5, sd = 6.0) in R. siamensis vs. 75–97 (x = 84.5, sd = 3.4). These values suggest a large overlap in the numbers of subcaudals but a Mann-Whitney U-test (one-tailed) gives a level of difference between the samples significant at p < 0.001 (U = 368.5; z-score = -4.46731);
(4) the position of the dorsal scale rows reduction, expressed in number of ventral plates: 75–85 (x = 80.0, sd = 2.6) in R. siamensis vs. 82–93 (x = 87.4, sd = 2.5);
(5) nuchal groove usually moderate or even poorly visible, rarely strong in R. siamensis vs. strong and well-visible.
(6) dark subocular streak always present and shaped as a broad, triangle (rarely faint), directed backwards in R. siamensis vs. either totally absent or reduced to a few dots, or incomplete, or, more rarely, well-defined as a vertical bar, an incomplete triangle directed backwards, more or less faint and looking like being ―washed out‖ in its centre, or as a broad comma, rarely solid black and well-visible.
(7) dorsum yellowish-grey, greyish-brown or pale brown, distinctly spotted with black and cream blotches in R. siamensis vs. dorsum rather dark, dark olive green or dark grey, nearly uniform or chequered with black anteriorly.
(8) venter entirely cream or creamish-yellow, without dark dots on the tips of ventrals, or only on the anterior part of the venter in R. siamensis vs. venter cream or pale creamish-yellow on a short distance behind the head becoming heavily dotted or speckled with dark grey posteriorly.
The differences between R. siamensis and the new species described here are given under the diagnosis of this latter species (David & Vogel 2021).

Description. Body rather robust, stouter in large females, cylindrical; nuchal groove always present, moderate to strong; head elongate, rather thick, distinct from the neck; snout elongate, slightly depressed, obtuse as seen from above, oblique seen in profile, 1.6–1.8 times longer than the diameter of eye; nostrils lateral and directed laterally, small, crescentic, piercing in the middle of the nasal; eye rather large, its diameter 1.6–1.8 times greater than the distance between its lower margin and the margin of the lip, with a round pupil; tail long, rather thick at its base, cylindrical and tapering.
The maximal total length in our sample is 772 mm (SVL 569 mm; TaL 203 mm; specimen MNHN-RA 1974.1290, female). The longest known male in our sample is 737 mm long (SVL 528 mm, TaL 209 mm; ZMB 263010). This species may reach greater total lengths as we have examined a specimen with a SVL > 600 mm. In our sample of 79 specimens, 18 have a total length over 600 mm and only 5 are over 700 mm long.
Ratio TaL/TL: 0.238–0.303.
22–25 maxillary teeth on each jaw, gradually enlarging, the last two abruptly and very strongly enlarged, without diastema.
DSR: (17, 18)19(21) – 19 – (16)17(18) rows; scales strongly keeled with a narrow keel throughout the body; scales of 1st DSR smooth. In our sample of 79 specimens, four have only 17 scale rows around the neck, three specimens have 18 scale rows whereas three others have 21 scale rows. Near vent, only one specimen has 16 DSR whereas only one has 18 rows.
Number of aligned, paired enlarged scales on the nape: 3–8 (usually 5–7).
VEN: 137–156 (plus, most usually, 2 preventrals, rarely 1); SC: 65–89, paired, without sexual dimorphism; cloacal plate divided. Ratio VEN/SC 1.62–2.14 (x = 1.89, sd = 0.12).
Position of the dorsal scale rows reduction from19to17DSR:VEN75–85(x=80.0,sd= 2.6).
Arrangement of upper head scales complete including 2 internasals, 2 prefrontals, 2 supraoculars, 1 frontal, and 2 parietals. Rostral wider than high, visible from above; nasals pentagonal, elongate, much longer than high, vertically divided above and below the nostril, with the posterior part larger than anterior one; internasals subtriangular, in broad contact with each other, longer than wide, moderately narrowing anteriorly and abruptly truncated; 2 prefrontals, distinctly broader than long, 1.0–1.2 times longer than internasals; frontal large, shield-like, longer than wide and 2.0–2.3 times longer than prefrontal; 1 supraocular on each side, subtriangular, 2.0–2.2 times longer than wide, narrower than internasals; parietals large and broad, 1.4–1.5 times longer than the frontal or suture between parietals 1.0–1.2 times longer than frontal; 1/1 loreal, subrectangular or pentagonal, barely longer than high, in broad contact with the nasal; 1/1 preocular in all examined specimens but one, in which the preocular is divided into 2 scales; 3/3 elongate postoculars (exceptionally 2 or 4); usually 8/8 but also 8/9 or 9/8, or 9/9 supralabials (8/9 or 9/8 in 14 specimens and 9/9 in only two specimens), the first five as long as high or longer than high, 1st and 2nd SL in contact with the nasal, 2nd or 2nd–3rd SL in contact with the loreal, usually 3rd– 5th SL touching orbit, also 4th–5th (in 12 specimens) or 4th–6th (in 17 specimens); 6th–7th SL or 6th–8th SL (in specimens with 9 SL) distinctly the largest; 2 anterior temporals (1 in only 2 specimens), much elongate, narrowing anteriorly, lower one largest, followed by 2 or 3 (rarely 1) posterior temporals, the most common total formula being 2+2 or 2+3 temporals; usually 10 infralabials, rarely 8 (in 2 specimens), 9 (in five specimens) or 11 (in 11 specimens), first pair in contact, 1st–4th or, most usually, 1st– 5th IL in contact with anterior chin shields (rarely 1st–6th), 5th, 6th and 7th IL largest; posterior chin shields narrower and longer than anterior ones (David & Vogel 2021). 
CommentSynonymy: Pope Pope 1935: 132, Smith 1943 and others considered this species a synonym of Rhabdophis subminiatus until it was revalidated by David & Vogel 2021.

Distribution: For a map see David & Vogel 2021: 110 (Fig. 9). 
EtymologyNamed after the type locality. 
  • David, Patrick & Gernot Vogel 2021. TAXONOMIC COMPOSITION OF THE Rhabdophis subminiatus (SCHLEGEL, 1837) SPECIES COMPLEX (REPTILIA: NATRICIDAE) WITH THE DESCRIPTION OF A NEW SPECIES FROM CHINA. Taprobanica 10 (2): 89–120 - get paper here
  • Deuve, J. 1961. Liste annotee des Serpents du Laos. Bull. Soc. Sci. Nat. Laos 1:5-32.
  • Hauser, Sjon; Ton Smits, Johan van Rooijen 2022. Records of Body Bending Behavior (‘Liana Crypsis’) in Five Snake Species in Thailand and One in Spain Russian Journal of Herpetology 28 (2): 65-75
  • JANZEN, P. 2022. Amphibians and Reptiles of Thailand Part 4: The northern part of the Thai Peninsula and again the Namtok Samlan NP. Sauria 44 (1): 13-32 - get paper here
  • Mell,R. 1931. List of Chinese snakes. Lingnan Sci. Jour., Canton, 8 [1929]: 199-219.
  • Wang K, Lyu ZT, Wang J, Qi S, Che J 2022. Updated Checklist and Zoogeographic Division of the Reptilian Fauna of Yunnan Province, China. Biodiversity Science 30 (4): 21326, 1–31 - get paper here
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