Streams in this region of High Plains are in most instances unsuitable habitats because they are in eroded channels, have a variable and uncertain water supply, and have poorly developed riparian communities. The marsh and wet meadow habitat preferred by sirtalis in most parts of its range is almost absent. T. radix and T. marcianus, well adapted to conditions in this region, perhaps provide competition that is limiting to T. sirtalis. However, several well-isolated populations of sirtalis have survived as relicts in the southern Great Plains, presumably from a time several thousand years ago when mesic conditions were more prevalent, perhaps in an early postglacial stage.
Fig. 1. Map of a part of the United States in the region of the Great Plains and Rocky Mountains, and adjacent northwestern Mexico showing supposed range (shaded) and localities of authenticated occurrence (dots) of Thamnophis sirtalis. 1. T. s. fitchi, 2. T. s. parietalis, 3. T. s. annectens, 4. T. s. ornata. Records from Idaho and Wyoming are based on specimens in the University of Kansas Museum of Natural History collection. Other records are based on [Woodbury] (1931) for Utah, [Hudson] (1942) for Nebraska, [Maslin] (1959) for Colorado, [Smith] (1956) for Kansas, R. G. Webb (MS) for Oklahoma, [Brown] (1950) and [Fouquette and Lindsay] (1955) for Texas, Cope (1900), [Van Denburgh] (1924), [Little and Keller] (1937) for New Mexico, and [Smith and Taylor] (1945) for Mexico.
[Smith] (1956:292) recorded parietalis from three outlying stations in the western quarter of Kansas, from Wallace, Hamilton and Meade counties in the drainages of the Smoky Hill River, Arkansas River, and Cimarron River, respectively. Permanent springs in Meade County State Park perhaps account for the survival of an isolated colony there. Several specimens from that locality seen by Fitch in August, 1960, when recently collected by a University of Michigan field party, seemed to be of the Texas subspecies annectens, as their dorsal stripes were reddish orange, and markings on the dorsolateral area were pale yellow rather than red. Specimens from the Texas Panhandle, from Hemphill County ([Brown], 1950:207) and nine miles east of Stinnet, Hutchison County ([Fouquette and Lindsay], 1955:417) likewise are most nearly like annectens judging from the authors' descriptions. The specimens from nine miles east of Stinnet averaged large; the two largest would have attained or slightly exceeded four feet in length if they had had complete tails. No sirtalis so long as four feet has been recorded elsewhere.
Records are lacking from the drainages of the Republican, North Canadian, Brazos and Colorado River drainages in the High Plains, but possibly isolated populations occur in some of these also. The only record from the Pecos River drainage is that of [Bundy] (1951:314) from Wade's Swamp near Artesia, Eddy County, New Mexico. This locality is separated by some 140 miles from any other known station of occurrence.
From extreme southern Colorado south across New Mexico to the Mexican border T. sirtalis occurs in continuous or nearly continuous populations in the Río Grande Valley, and has been recorded from many localities. It has been recorded from relatively few localities of tributary streams (Los Pinos, Abiqui, Santa Fe) all near the main valley. There is one record from the Ocate River, a headwaters tributary of the Canadian River, in the Sangre de Cristo Mountains near other localities in the Río Grande drainage. The southwestern-most known locality of occurrence is Casas Grandes in the Mexican state of Chihuahua some 130 miles southwest of El Paso, Texas, and near the Continental Divide. The Río Casas Grandes must have once been a tributary of the Río Grande, but now its desert drainage basin is isolated.
Re-description of a Subspecies from New Mexico
Most specimens of a population of sirtalis occurring in New Mexico are recognizably different from most specimens of other populations. This New Mexican population is therefore here recognized as a distinct subspecies: