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Thamnophis unilabialis TANNER, 1985

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Higher TaxaColubridae (Natricinae), Colubroidea, Caenophidia, Alethinophidia, Serpentes, Squamata (snakes)
Common NamesE: Madrean Narrowheaded Garter Snake
S: Jarretera Cabeza-angosta 
SynonymThamnophis rufipunctatus unilabialis TANNER 1985: 648
Thamnophis unilabialis — WOOD et al. 2011
Thamnophis unilabialis — WALLACH et al. 2014: 727 
DistributionMexico (E Sonora, Chihuahua, Durango)

Type locality: 0.5 mi SW of Bocoyna, Chihuahua, Mexico.  
TypesHolotype: BYU 14217, adul female; paratypes: BYU, ASU, UAZ, NMMZ, AMNH, UTEP 
DiagnosisDiagnosis: A subspecies of rufipunctatus that differs from both r. rufipunctatus and r. nigronuchalis in having only the 4th supralabial contacting the eye by reason of the lower postocular having a firm contact with the 4th labial; ventral reduced and ventral-caudal averages 13-15 scales fewer than in r. rufipunctatus. Ventral color pattern with anterior margins of scales usually pigmented, forming alternating dark and light transverse bands. (Tanner 1985)

DESCRIPTION OF TYPE. Head and body length 412 mm; tail at 96 mm, 21.8% of total length; head elongate and compressed laterally, dorsal head scales normal for the species; no azygous preinternasal scale, loreal single, preoculars 2-2, postoculars 3-3, temporals 1-3, scale rows 21-21-17, ventrals 156, caudals 67, anal single, lower postocular with a firm contact on 4th supralabial (Fig. 5C). A longitudinal series in six rows of light rust or dark brown spots (rust spots faded to light spots in preservative) on a dark grey ground color; ventrals and caudals with dark pigmentation on anterior margins, forming dark and light cross bars on ventrals; caudals with reduced pigment forming a dark speckling on a light grey gound color; dorsal head plates dark greyish brown, grading to lighter shades on labials; latter with dark stripes crossing upper third or half of each scale and forming a dark, posterior margin on all but the last 2 supralabials; lower labials with posterior margins edged in dark brown; gulars light grey and without spots. (Tanner 1985)

VARIATION. In this subspecies, there is little variation among scale and color pattern in the specimens examined. The 67 specimens seen show a small variation of only 8 to 14 scales difference in ventral plus caudal counts, with the greatest variation occurring in the ventrals of males. The one noticeable difference is in the presence of the preinternasal azygous scale, which is seemingly present in most, if not all individuals in the headwaters of the Río Bavispe. Five specimens from Chuhuichupa and its environs all have this scale in several variations (Fig. 6), whereas few specimens on the east side of the Continental Divide and those in the Río Papagochic and Río El Fuerte drainage do not (3 of 45 specimens). The most noticeable variation in the color pattern is the light, rust-colored spots in some specimens, whereas in others the spots are a dark brown; this difference persists in preserved specimens as faded light spots. Conant (1963), referring to the field notes of R. G. Zweifel, indicates that the live snakes at Miñaca had "reddish spots." The population at San Pedro also had individuals with bright spots, but we determined them to be a bright yellowish rust, quite in contrast to the red colors seen in kingsnakes or bicolored Sonora. That this color pattern was not observed in all specimens collected or seen may suggest an expression of a seasonal or sex-induced character. In a few specimens, the two dorsal rows of spots anteriorly contact each other dorsally, giving the appearance of only 5 rows. The rufipunctatus in northern and eastern Durango, Chihuahua, and Arizona have a divided nuchal blotch. This is best seen in young or juvenile specimens in which the nape has a narrow (usually one or a fraction of a scale) light stripe extending from the parietals to divide the dark, irregular blotches. In older specimens, there is a fading or perhaps a blending of the nape color pattern, making it more difficult to identify the divided blotches, especially in preserved specimens. In the young, the spots on the body are dark and readily noted. Posterior to the nuchal blotches in the young is a series of 1-3 dark, middorsal spots. The spots posterior to this series usually divide, forming two rows of dorsolateral spots and thus leaving the normal number of six rows of spots at midbody. In most specimens, the anterior margins of the ventral are heavily pigmented, but with the posterior much lighter. This produces an even or an irregular cross-barring pattern of dark and light pigment. (Tanner 1985) 
CommentSynonymy: The subspecific distinctness of T. r. unilabialis was based on its having only one supralabial contacting the orbit (usually two in T. r. rufipunctatus), fewer ventrals and subcaudals than T. r. rufipunctatus, and a different ventral pattern than T. r. rufipunctatus. Rossman (1995) showed that only the differences in ventral and subcaudal numbers are reasonably consistent, and he concluded that this seemed an insufficient basis for taxonomic recognition of the Mexican populations. (Rossman et al. 1996: 39, 245)

Distribution: see map in Wood et al. 2011: 3838. 
EtymologyNot explicitly given by Tanner but apparently named after the 4th supralabial which is the only supralabial contacting the eye. 
  • Heimes, P. 2016. Snakes of Mexico. Chimaira, Frankfurt, 572 pp
  • Lemos-Espinal JA, Smith GR, Gadsden-Esparza H, Valdez-Lares R, Woolrich-Piña GA 2018. Amphibians and reptiles of the state of Durango, Mexico, with comparisons with adjoining states. ZooKeys 748: 65-87 - get paper here
  • Lemos-Espinal JA, Smith GR, Rorabaugh JC 2019. A conservation checklist of the amphibians and reptiles of Sonora, Mexico, with updated species lists. ZooKeys 829: 131-160 - get paper here
  • Rorabaugh, J C; Turner, D; van Devender, T R; Hugo-Cabrera, V; Maynard, R J; Van Devender, R W; Villa, R A; Hamilton, P; Hale, S F; Aguilar-Morales, C; Blanco-Gutiérrez, A; Wallace, E & Hedgcock, C; 2019. Herpetofauna of the Mesa Tres Ríos area in the Northern Sierra Madre Occidental of Sonora, Mexico. Herpetological Review 50 (2): 251-259 - get paper here
  • Tanner, Wilmer W. 1985. Snakes of Western Chihuahua. Great Basin Naturalist 45 (4): 615-676 - get paper here
  • Ukuwela, K.D.B., de Silva, A., Sivaruban, A. et al. 2022. Diversity, distribution, and natural history of the marine snakes of Sri Lanka. Marine Biodiversity 52, 24 (2022) - get paper here
  • Wallach, Van; Kenneth L. Williams , Jeff Boundy 2014. Snakes of the World: A Catalogue of Living and Extinct Species. [type catalogue] Taylor and Francis, CRC Press, 1237 pp.
  • Wood, Dustin A.; A. G. Vandergast, J . A. Lemos Espinal, R. N. Fisher & A. T. Holycross 2011. REFUGIAL ISOLATION AND DIVERGENCE IN THE NARROWHEAD GARTER SNAKE SPECIES COMPLEX (THAMNOPHIS RUFIPUNCTATUS) AS REVEALED BY MULTILOCUS DNA SEQUENCE DATA. Molecular Ecology 20: 3856–3878 - get paper here
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