Occurrence of the Garter Snake, Thamnophis sirtalis, in the Great Plains and Rocky Mountains

Transcriber's Note: Original spelling and punctuation have been retained. In particular, both Eutainia and Eutaenia are used in the original, as are both pickeringi and pickeringii.

The common garter snake ( Thamnophis sirtalis ) has by far the most extensive geographic range of any North American reptile, covering most of the continental United States from the Atlantic to the Pacific and from south of the Mexican boundary far north into Canada and southeastern Alaska. Of the several recognized subspecies, the eastern T. s. sirtalis has the most extensive range, but that of T. s. parietalis in the region between the Mississippi River and the Rocky Mountains is almost as large. The more western T. s. fitchi occurring from the Oregon and California coasts east through the northern Great Basin, has the third largest range, while the far western subspecies pickeringi , concinnus , infernalis and tetrataenia , and the Texan T. s. annectens all have relatively small ranges.
Wherever it occurs at all, the common garter snake is usually abundant. Because of its diurnal habits and the concentration of its populations along watercourses, it is not likely to be overlooked. There are few, if any, remaining large areas in the United States where herpetologists have not carried on field work. It may be anticipated that certain rare and secretive species will still be found far from any known stations of occurrence, and seeming gaps in the ranges of these species will eventually be filled. But for the common garter snake the negative evidence provided by the lack of records from extensive areas should be taken into account in mapping the range.
Convincing records of sirtalis are lacking from all of Colorado—except for those in the drainage basins of the South Platte, and the Río Grande east of the Continental Divide—from the eastern half of Utah (east of the Wasatch Range), from New Mexico except for the Río Grande drainage (with one record each for the Canadian and Pecos river drainages), from southwestern Wyoming (at least that part in the Colorado River drainage basin), from the western half of Oklahoma, and from Texas, except the eastern and extreme western and northern parts. The species occurs in Nevada only near that state's western and northern boundaries. The range is therefore much different than it has been depicted heretofore, with the populations living east of the Continental Divide widely separated from those to the west for the entire length of the Rocky Mountains south of the Yellowstone National Park region. The populations of northern Utah, southern Idaho, and Nevada, which have been considered parietalis are thus far removed from the main population of that subspecies to the east and are isolated from them by the barrier of the Continental Divide and arid regions farther west.