The Significance of Knobbed Anal Keels in the Garter Snake, Thamnophis sirtalis sirtalis (Linnaeus)

Abstract

RECENTLY attention has been called by F. N. Blanchard' to certain peculiarities of the dorsal scales of the anal region in various species of snakes. These peculiarities are manifested as keel-like structures on smooth-scaled snakes, and as excrescences or knobs on the keels of keeled snakes. To determine the significance of these excrescences and their relation to sex and age in a species of garter snake, study was made at the suggestion of Dr. Blanchard of a series of 400 specimens of Thamnophis sirtalis sirtalis. All the specimens were loaned by the Museum of Zoology of the University of Michigan through the courtesy of Mrs. Helen T. Gaige, Curator of Amphibians. In this species of garter snake, the well developed knobbed keels usually occur in the immediate vicinity of the anus, extending dorsally as high as the third or fourth row of scales. Keels with poorly developed excrescences may occur several millimeters farther anteriorly and posteriorly, and dorsally to the median line. Ordinarily, however, even poorly developed knobs do not appear higher up than the sixth or seventh dorsal scale rows.