A test of the thermal coadaptation hypothesis with black rat snakes (Elaphe obsoleta) and northern water snakes (Nerodia sipedon)

1. The thermal coadaptation hypothesis predicts that (1) ectotherms experiencing a narrow range of body temperatures in the wild will evolve to perform well over a narrow range of body temperatures and that (2) the optimal temperature for performance will be equal to the preferred body temperature of the species. 2. We tested the predictions of the thermal coadaptation hypothesis with black rat snakes (Elaphe obsoleta) and northern water snakes (Nerodia sipedon) because black rat snakes experience lower and more variable body temperatures than northern water snakes at our study site. 3. We measured swimming speed, tongue-flicking speed, and striking speed in black rat snakes, and swimming speed and tongue-flicking speed in northern water snakes. 4. Adult water snakes generally had narrower performance breadths and higher optimum performance temperatures than adult black rat snakes. 5. Performance breadths were the same for swimming, tongue flicking, and striking within adult black rat snakes, but performance optima for these behaviours differed significantly. Performance breadths differed and performance optima were the same for swimming and tongue flicking within adult northern water snakes. 6. The relative swimming performance of neonates of the two species was similar in breadth to that of adults, but the thermal optimum for neonate black rat snakes was higher than that of adults. 7. Overall, our results provided support for the thermal coadaptation hypothesis.