Taxonomic History         Species Description         Geographic Distribution




Thamnophis sirtalis sirtalis (Linnaeus, 1758) - Eastern Garter Snake

The Eastern Garter Snake has had a number of different scientific names over the past 250 years, because different scientists have described the species during that time period. For example, the first person to describe the (sub)species was Linnaeus in 1758, but the name he gave it is no longer the one used to identify it. Eventually, all of the animals described below were recognized to be the same thing, but it is uncertain when the genus name Thamnophis was first applied to this subspecies.

Eastern Garter Snake Coluber sirtalis Linnaeus, 1758, sensu Harlan 1827

Coluber ordinatus Linnaeus, 1766

Coluber taenia Schoepf, 1788

Coluber ibibe Daudin, 1803

Tropidonotus bipunctatus Schlegel, 1837

Tropidonotus jauresi Duméril, Bibron, and Duméril, 1854

Eutaenia sirtalis obscura Cope in Yarrow, 1875

Eutaenia sirtalis melanota Higley, 1889

Eutaenia sirtalis graminea Cope, 1889



Species Description

Garter snakes belong to the natricine subfamily of the Colubridae, the most speciose family of snakes. The Eastern garter snake Thamnophis sirtalis sirtalis is a subspecies of the common garter snake, T. sirtalis. Many of the descriptive characteristics listed here apply to more than one subspecies.
from Rossman et al. (1996), p.32 The primary basis for recognizing different subspecies is the dorsal coloration pattern, which varies geographically. In T. s. sirtalis, the dorsal color is usually brown, gray, or black. The medial or vertebral stripe is fairly distinct and ranges from white to yellow to tan, with much individual variation. This stripe often includes just under half of each paravertebral row of scales (in addition to the scale rows directly aligned with the vertebral column). There are 19 dorsal scale rows in all, and this number may reduce to 17 near the anus. The lateral stripe covers rows 2 and 3 and is also yellowish. Between the medial and lateral stripes may be a double row of black spots, which may fuse together either horizontally or vertically (this feature is also highly variable).
There is a brown ventrolateral stripe that extends only slightly onto the ventrals, if at all. The venter is green or yellow tinted. The anal plate is undivided. Males have a simple hemipenis with an unbranched sulcus spermaticus, five large spines at the base, and many tiny recurved spines in oblique rows on the shaft (left, c).

In lateral view, the head has a scale pattern as follows: 1 loreal, 1 preocular, 3 postoculars, 2-3 temporals, 7 supralabials, and 10 infralabials. There are 20 to 26 maxillary teeth, the most posterior of which is slightly longer than the others. Females are larger than males, with the males averaging approximately 83 % of the average female length, and about 55 % of female weight. Males have a greater number of subcaudals, however, and therefore have a longer tail length relative to the body. Finally, the species has a karyotype of 36 chromosomes.
               

Some color variations in the Eastern garter snake. Left and center pictures from Rossman et al. (1996), Plate 12.
Right picture from Ernst and Barbour (1989), color plates not numbered.



Geographic Distribution


from Rossman et al. (1996), p.263

The common garter snake is found throughout North America (see map above). The Eastern garter snake ranges from the Atlantic coast to the Mississippi River, and from Southern Ontario to the Gulf of Mexico. Its range, which includes the entire state of Connecticut overlaps with three other subspecies of T. sirtalis: T. s. pallidus in northern New England, T. s. parietalis along the Mississippi River, and T. s. semifasciatus southwest of Lake Michigan.



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