An isolated population of Butler’s gartersnake, Thamnophis butleri (Colubridae), in southeastern Wisconsin has recently been listed as Threatened by the Wisconsin Department of Natural Resources. One of the possible reasons for the decline of T. butleri in Wisconsin is pressure from a closely related species, the plains gartersnake, T. radix. The possibility of hybridization between T. butleri and T. radix has received recent attention because T. butleri is threatened in the area where the hybridization may be occurring. This study addresses the issue of hybridization by studying the behavior of neonatal T. butleri born to females originating from areas where the ranges of the species overlap (southeastern Wisconsin), as well as from areas outside the range of T. radix (Michigan and northern Ohio). Neonatal T. radix from northern Illinois were used as a T. radix “control”. Pregnant females were collected from 4 counties in southeastern Wisconsin, with the southernmost county closest to the range of T. radix. I examined antipredator behavior, prey chemosensory responses, prey preference, and morphological size traits. Several hypotheses have been put forth as to the interactions that occur when two species are sympatric, including character displacement, the importance of local ecology, and gene flow. Morphological and genetic evidence support the hypothesis of gene flow between T. butleri and T. radix, but this hypothesis has yet to be tested with only behavioral data. I had several predictions concerning the hypothesis of hybridization in these species: (1) the behaviors of T. butleri in Wisconsin would differ from the behaviors of Michigan and Ohio T. butleri in the direction of being more like T. radix; (2) the behaviors of T. butleri from the different counties and populations in Wisconsin would differ from one another; (3) T. butleri from the northern part of the Wisconsin range, further removed from the range of T. radix, would show behaviors more like Michigan T. butleri and less like T. radix than would T. butleri from the southern part of their range in Wisconsin.
Michigan and Ohio T. butleri were heavier and longer than Wisconsin T. butleri, thus making the Wisconsin snakes less like T. radix. Populations within southeastern Wisconsin differed, and snakes from the population closest to T. radix were greater in length and mass than snakes from the population furthest from the range of T. radix. Snakes from Wisconsin were found to be more similar in body condition to T. radix than to Michigan and Ohio T. butleri.
Differences were found in antipredator behaviors across the populations of T. butleri from southeastern Wisconsin, with snakes from the population closest to the range of T. radix striking more frequently and therefore more similar to the T. radix studied than to Michigan and Ohio T. butleri. Thamnophis butleri from Michigan and Ohio were more likely to flee than T. butleri from southeastern Wisconsin. Within Wisconsin populations, snakes further removed from T. radix were more likely to flee, and were therefore more similar to Michigan and Ohio T. butleri.
Chemosensory preferences of the snakes also differed among the populations of T. butleri from southeastern Wisconsin. Snakes from the southernmost population showed a chemosensory preference for fish over worms, whereas snakes from the northern part of the range showed a chemosensory preference for worms over fish. Hence, the behaviors of T. butleri from the southern part of their range in Wisconsin, closer to the range of T. radix, are more similar to T. radix than are the behaviors of T. butleri elsewhere in their range. The implications of these findings for the conservation and genetic study of Wisconsin T. butleri are discussed
