We report on the remarkable variation and frequency of color morphs within and among eight populations of the Common Gartersnake (Thamnophis sirtalis) in central Manitoba, Canada and one on Isle Royale, Michigan, USA. Five color morph categories are identified, one melanistic and four scored on a qualitative scale based on expression of red pigment. In the most northern population (Jenpeg, 54.464°N, 98.115°W) and two island populations (George Island, Lake Winnipeg, 52.819°N, 97.620°W; Isle Royale, 48.102°N, 88.601°W), all five color morphs were expressed. The northernmost population also exhibited sexual color dimorphism, with female snakes expressing significantly more red than males. In contrast, two central and western populations showed very little variation, with only two of the most similar color morphs present. We provide the first report of erythristic snakes in Manitoba, found in three separate locations around Lake Winnipeg. Melanistic snakes are also reported from three new localities in the province, all widely disjunct from previously identified sites around Lake Winnipegosis. Manitoban and Isle Royale populations are compared with color frequency data from sites in mainland Michigan and in Kansas. Color patterns among populations do not follow traditionally recognized zones of intergradation between T. s. parietalis and T. s. sirtalis. The extreme intrapopulational variation suggests that subspecies of T. sirtalis based on color are of questionable validity.

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