Diagnostic Characters and Color Convergence of the Garter Snakes Thamnophis elegans terrestris and Thamnophis couchii atratus along the Central California Coast

Abstract

Along the central California coast, the closely related garter snakes Thamnophis elegans terrestris and T. couchii atratus have very similar color patterns. Both are characterized by a dark dorsum with three yellow stripes, no red markings, and yellowish throats. Plasma samples from 38 live specimens were subjected to electrophoresis and autoradiography to establish identity on the basis of transferrin mobility. Fourteen morphological characters were studied in the same specimens and correlated with the transferrin types by a stepwise discriminant analysis program. Discriminant functions derived from these specimens were tested on an additional 101 live and preserved specimens. The results indicate that the most important diagnostic characters are the amount of internasal-rostral contact, internasal width versus length, number of dorsal scale rows, and vertebral stripe width. Counts of posterior scale rows and ventral scales, and estimation of relative pupil size, may aid in identification. Further observations have shown that coastal populations of T. elegans terrestris and T. couchii atratus can be distinguished reliably by ventral coloration. However, geographic variation in color patterns is evident, and we suggest that dorsal color patterns of these snakes are strongly convergent in San Luis Obispo and Monterey counties primarily in response to predator selection pressure.