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High incidence of melanism in Masticophis taeniatus girardi (Reptilia: Colubridae), from the Cuatro Cienegas Basin of Coahuila, Mexico.

The Cuatro Cienegas Basin of central Coahuila, Mexico, is of considerable faunisitc interest for three reasons: 1) it lies at the interface between the Chihuahuan Desert and the Tamaulipan Biotic Province (Blair, 1949); 2) it contains much permanent water, in both lentic and lotic habitats; and 3) it is surrounded by mountain ranges that have apparently acted as barriers to terrestrial animal movements (Minckley, 1969). This last factor has resulted in a high frequency of endemic and relictual animal populations. Endemic taxa of aquatic snails (Taylor, 1966), isopod crustaceans (Cole and Minckley, 1966), scorpions (Williams, 1968), fishes (Minckley, 1962, 1969; Miller and Minckley, 1963; Hubbs and Miller, 1965; Miller, 1968), turtles (Schmidt and Owens, 1944; Legler 1960; Webb and Legler, 1960), and lizards (McCoy, 1970; Walker, 1981) are known from the Cuatro Cienegas Basin. Relictual populations of the toad, Bufo valliceps, and the reptiles, Eumeces tetragrammus, Scincella laterale, Coluber constrictor, Nerodia erythrogaster, and Nerodia rhombifera, also are known to inhabit the basin (McCoy and Minckley, 1969). The zoogeography and ecology of the Cuatro Cienegas herpetofauna were reviewed by McCoy (1984). In this note, we describe the first incidence of possible endemism of a snake inhabiting the Cuatro Cienegas Basin.

In conducting a systematic revision of Masticophis taeniatus, five specimens from the Cuatro Cienegas Basin were examined. Three of these, all females, were melanistic dorsally. An adult male (AMNH 77315), 983 mm in snout-vent length (SVL), had more dark pigment dorsally than is typical, lacking the pale nuchal collar diagnostic of the subspecies M. t. girardi. It was not completely melanistic, however, because five pale transverse bands diagnostic of M. t. girardi were present. A female (TNHC 33012), 623 mm SVL, displayed a color pattern similar to Chihuahuan Desert M. t. girardi but lacked the typical transverse banding. The melanistic females ranged from 823 to 885 mm SVL. These specimens were not completely melanistic ventrally. Black pigment was absent from small areas, but they had more extensive dark ventral mottling than is typical for M. t. girardi.

Dorsal scale formulae--15-15-11 (two specimens), 15-15-12 (two), and 15-15-13 (one)--and the head scutellation of the Cuatro Cienegas material was typical for the subspecies. The ventrals of the females ranged from 200 to 214 (mean, 206); the mean was identical to the average for 187 female M. t. girardi from throughout the range of that race. The male had 193 ventrals, well within the range of 183 to 236 (mean, 205) ventrals for 257 males from throughout the range of M. t. girardi. Subcaudals varied from 129 to 166 (mean, 155), for the three females with complete tails. This is higher than the average of 145 subcaudals for 123 female M. t. girardi from throughout the range of the subspecies. The male had 155 subcaudals, which is greater than the average of 149 for 188 males of the subspecies, but within the known range of 122 to 175 subcaudals. Maxillary teeth ranged from 16 to 19, with a mean of 17.25, in the Cuatro Cienegas females, fewer than the mean of 18.7 for all 194 female M. t. girardi examined. The male had 20 maxillary teeth, well within the range of 15 to 23 (mean, 19.1) from all 258 males of the subspecies examined.

The three melanistic females were collected on the floor of the basin north or east of the Sierra San Marcos, presumably in mesic habitats. The pale-colored female that lacked transverse banding was taken from the west side of the Sierra San Marcos in Chihuahuan Desert habitat. The male was obtained along the southern base of the Sierra de la Madera, in the northwestern portion of the basin, in habitat unknown to the authors. Even though some of the Cuatro Cienegas specimens have atypical color patterns, scutellation and dentition character states fall within the known variation of M. t. girardi. The different color patterns of the five specimens available suggests the possibility of isolation and divergence. Because of similarity in morphology to M. t. girardi, the observed variation in color pattern, and the small sample size, no taxonomic conclusions are drawn at this time.

Specimens examined. -- Mexico Coahuila: AMNH 77315, 3 mi. W Cuatro Cienegas de Carranza; CM 58192, 14 km. S Cuatro Cienegas, Tio Candido; KU 80282, 1 mi. E tip San Marcos Mtns; LACM 116256, 11 km. SW Cuatro Cienegas de Carranza, N tip Sierra de San Marcos; TNHC 33012, 9 mi. SSW Cuatro Cienegas, on San Pedro Rd.


Blair, W. F. 1949. The biotic provinces of Texas. Texas J. Sci., 2:93-117.

Cole, G. A., and W. L. Minckley. 1966. Speocirolana thermydronis, a new species of cirolanid isopod crustacean from central Coahuila, Mexico. Tulane Stud. Zool., 13:17-22.

Hubbs, C. L., and R. R. Miller. 1965. Studies of cyprinodont fishes. XXII. Variation in Lucania parva, its establishment in western United States, and description of a new species from an interior basin in Coahuila, Mexico. Misc. Publ. Mus. Zool., Univ. Michigan, 127:1-104.

Legler, J. M. 1960. A new subspecies of slider turtle (Pseudemys scripta) from Coahuila, Mexico. Univ. Kansas Publ., Mus. Nat. Hist., 13:73-84.

McCoy, C. J. 1970. A new alligator lizard (genus Gerrhonotus) from the Cuatro Cienegas Basin, Coahuila, Mexico. Southwestern Nat., 15:37-44.

______. 1984. Ecological and zoogeographic relationships of amphibians and reptiles of the Cuatro Cienegas Basin. J. Arizona-Nevada Acad. Sci., 19:49-59.

McCoy, C. J., and W. L. Minckley. 1969. Sistrurus catenatus (Reptilia: Crotalidae) from the Cuatro Cienegas Basin, Coahuila, Mexico. Herpetologica, 25:152-153.

Miller, R. R. 1968. Two new fishes of the genus Cyprinodon from the Cuatro Cienegas Basin, Coahuila, Mexico. Occas. Papers Mus. Zool., Univ. Michigan, 659:1-15.

Miller, R. R., and W. L. Minckley. 1963. Xiphophorus gordoni, a new species of platyfish from Coahuila, Mexico. Copeia, 1963:538-546.

Minckley, W. L. 1962. Two new species of fishes of the genus Gambusia (Poeciliidae) from northwestern Mexico. Copeia, 1962:391-396.

______. 1969. Environments of the Bolson of Cuatro Cienegas, Coahuila, Mexico, with special reference to the aquatic biota. Univ. Texas El Paso Publ., Sci. Ser., 2:1-165.

Schmidt, K. P., and D. W. Owens. 1944. Amphibians and reptiles of northern Coahuila, Mexico. Zool. Ser., Field Mus. Nat. Hist., 29:97-115.

Taylor, D. W. 1966. A remarkable snail fauna from Coahuila, Mexico. Veliger, 9:152-228.

Walker, J. M. 1981. Systematics of Cnemidophorus gularis I. Reallocation of populations currently allocated to Cnemidophorus gularis and Cnemidophorus scalaris in Coahuila, Mexico. Copeia, 1981:826-849.

Webb, R. G., and J. M. Legler. 1960. A new softshell turtle (genus Trionyx) from Coahuila, Mexico. Univ. Kansas Sci. Bull., 40:21-30.

Williams, S. C. 1968. Scorpions from northern Mexico: five new species of Vejovis from Coahuila, Mexico. Occas. Papers California Acad. Sci., 68:1-24.


Department of Wildlife and Fisheries Sciences, Texas A & M University, College Station, Texas 77843-2258
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Title Annotation:GENERAL NOTES
Author:Camper, Jeffrey D.; Dixon, James R.
Publication:The Texas Journal of Science
Geographic Code:1U7TX
Date:May 1, 1990
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