High incidence of melanism in Masticophis taeniatus girardi (Reptilia: Colubridae), from the Cuatro Cienegas Basin of Coahuila, Mexico.
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.
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JEFFREY D. CAMPER AND JAMES R. DIXON
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|
|Date:||May 1, 1990|
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