San Luis Potosí Kingsnake — Lampropeltis mexicana Garman 1884
Other Common Names: Mexican Kingsnake; commonly referred to as “mex mex” in hobbyist circles.
Spanish Names: Culebra Real Escarlata
Original Description: Garman, S. 1884. The reptiles and batrachians of North America, Part I, Ophidia. Memoirs of the Museum of Comparative Zoology, Harvard 8(3):xxxi–185.
Holotype & Type Locality: Museum of Comparative Zoology (MCZ) 4652–53 (both females), collected by E. Palmer, August 1879, from “Mexico, near [Ciudad] San Luis Potosi.”
Etymology: The species name is in reference to the country of occurrence.
Scientific Describer: Samuel Walton Garman (1843–1927), born in Indiana County, Pennsylvania (USA), died at Plymouth, Massachusetts. Garman spent his professional career working in both herpetology and ichthyology at the Museum of Comparative Zoology at Harvard University, where he was a protégé of Louis Agassiz. A detailed herpetological biography is in Adler et al. (1989).
Distribution: Sierra Madre Oriental and satellite ranges of San Luis Potosí, Aguascalientes, Zacatecas, Guanajuato, Hidalgo, and Estado de México. However, other than for the area in and near the Valle de los Fantasmas in SLP, the few additional localities are represented by few specimens. Clearly, much fieldwork is needed in this part of Mexico to better understand the range of this species.
Habitat: This species occurs over an impressive range of environments, ranging from cool, moist mountains approaching 8,000 ft elevation to desert or desert scrub habitats at much lower elevations. Snakes in the higher elevation sites are closely associated with limestone outcroppings, and spend time under rocks or in crevices with morning sun exposure. Records at lower elevations are mainly from specimens found dead or alive on roads at night, thereby making connections to specific microhabitats tenuous.
Growth and Size: Typical of other members of this complex, although smaller than alterna. Adults are generally 24–36 in. (61–91 cm) TL, with hatchlings starting out at 8–10 in./20–25 cm TL. Little information exists concerning wild snakes. Our field observations suggest that adults are approximately 24-30 in. TL (61–76 cm), but this is based on a small series of snakes from one locality. Captives apparently grow larger than their wild counterparts, with the largest individuals approaching or exceeding 1 m TL. Sexual size dimorphism has not been established in the wild, but among captives, the largest snakes are males.
Pattern Variation: Ground color may be light gray, dark gray (to nearly black), or brown. Dorsal markings consist of red or orange rectangular blotches or saddles that taper and terminate before reaching the ventrals. These are outlined in black, and typically contrast against the ground color. A trait present in most San Luis Potosí Kingsnakes (at least those from in and around the Valle de los Fantasmas) is an elongated nuchal blotch, often with a central opening (sometimes referred to as a “nuchal window” by breeders). Configuration of the nuchal blotch, together with often-intricate head markings, combine to make each snake distinctive.
This is the only Mexicana kingsnake species in which sexual dichromatism occurs; males tend to be more brightly marked, a distinction evident even among hatchlings in our group. However, it’s unclear if this is consistent across the range. L. mexicana hatchlings start out fairly drab, brightening and coloring up within their first two years. This ontogenetic change does not occur in other Mexicana kings.
Pattern variants seen among captive animals include a striped morph, produced from typically-marked adults, and which seems to no longer exist (and indeed, these may have been the result of incubation temperatures rather than having a genetic basis). Snakes with various degrees of speckling—ranging from blotched individuals with a few speckles to animals whose pattern consists exclusively of speckles (“granite morph”)—are produced in low numbers by breeders. An additional variant is the “black phase,” in which snakes start out with a gray ground color that darkens considerably within two years, while the orange/red dorsal blotches brighten. The contrast is especially vivid in males.
Relationships: Phylogenetic relationships within the Mexicana Group are poorly resolved. The range of L. mexicana approaches that of L. leonis to the north and L. ruthveni to the south, but without evidence of contact. Within the small central Mexican state of Aguascalientes, the ranges of greeri and mexicana are only 40 km apart, but are separated by low-lying, unsuitable habitat. Natural barriers to contact are not obvious between southernmost mexicana and the population of ruthveni in northeastern Querétaro, but the ranges of both species are virtually unknown in this region.
Conservation Status: Montane localities (e.g., Valle de los Fantasmas) have been less affected by habitat loss than lower-elevation areas, as for example the areas surrounding the city of San Luis Potosí, where the landscape has been severely degraded and where kingsnakes likely persist only in upland, rocky areas, if at all. Extensive deforestation has occurred within the past 100 years within some parts of the range, though the effect on kingsnake populations is unclear. The overall distribution of this species is still so poorly known that offering general statements concerning conservation status is difficult.
Captive Stock: To the best of our knowledge, all captive stock is derived from specimens originally collected in or near the Valle de los Fantasmas (including nearby Alvarez) in San Luis Potosí. Some of these have been hybridized with other Lampropeltis (e.g., alterna, ruthveni); the resulting progeny are not always recognizable as hybrids and often are sold simply as L. mexicana or “mex-mex” without reference to their hybrid origin. Fortunately, there are several breeders who maintain straight L. mexicana animals, with offspring available each year. Our breeders are descended from Valle de los Fantasmas animals, and include both normal and black phase material.