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NATURAL HISTORY

MEXICAN GARTER SNAKE } Thamnophis eques megalops
FAMILY: Colubridae

DESCRIPTION: Adults of the species are considered medium-sized garter snakes, reaching about 18 to 40 inches long with a maximum length of one meter. They range in color from olive or olive brown to olive gray, with three yellow stripes running the length of the body and darkening toward the tail. A light-colored crescent extends behind the corners of the mouth.

HABITAT: In general, Mexican garter snakes are found in Southwest riparian areas with permanent water, in desert-grassland cienegas, in or along streams in valley floors, and occasionally in desert and lower oak woodland habitats. Populations are most abundant on the margins of intermediate-sized streams at high elevations in association with cottonwood, willow, seep willow, mesquite, and a variety of grasses. Populations also may be found in the shallow waters, banks, and riparian vegetation of large river riparian woodlands and forests.

RANGE: The snake ranges from central and southeastern Arizona to Oaxaca, Mexico. The range in Arizona is from the southeast corner of the San Rafael and Sonoita grasslands to Arivaca. It can also be found along the Agua Fria, Oak Creek, and Verde rivers, and along some parts of the Salt and Black rivers.

MIGRATION: This species is nonmigratory.

BREEDING: Snakes breed in fall and early spring, and females store the sperm until ovulation in late March or early April. Like all garter snakes, the Mexican garter snake gives birth to live young instead of laying eggs. On average, about half the females in a population will give birth each year, and they will birth from 10 to 20 young. Young are born between early June and early July.

LIFE CYCLE: Males mature in two years, while females reach maturity in two to three years.

FEEDING: The Mexican garter snake is classified as a terrestrial-aquatic generalist because it feeds on both aquatic and terrestrial prey. It has a varied diet consisting mainly of frogs, tadpoles, and fish, supplemented by lizards and mice. Native prey species play a large role in the ecology of Mexican garter snakes, and in areas where such prey is scarce, there are few garter snakes. Native prey such as the Chiricahua leopard frog and Gila topminnow are becoming increasingly rare.

THREATS: The Mexican garter snake is threatened by the destruction, modification, and deterioration of habitat, disappearance of native prey, and spread of nonnative predator species.

POPULATION TREND: Population trends clearly demonstrate that the Mexican garter snake is declining in the United States — many populations show negative trends, low densities, and the possibility of extirpation. There may be only a few hundred snakes in the United States, and present trends for the Mexican garter snake in the country can be expected to continue. Although little is known about the snake in Mexico, population decline can be expected there as well, since similar threats are present.

Photo © Erik Enderson