Intraspecific differences in thermal tolerance of the diamondback watersnake (Nerodia rhombifer): effects of ontogeny, latitude, and sex.

Ontogenetic shifts in microhabitat use are widespread among taxa and can result in drastic shifts in thermal habitat among age classes. Likewise, geographic variation in climate along latitudinal gradients can cause differences in thermal environments among populations of a species. Using a common garden design, we examined four populations of a single species of semi-aquatic snake, Nerodia rhombifer, to determine whether ontogenetic shifts in habitat use (and/or body size) and latitudinal differences in ambient temperature have resulted in evolutionary changes in thermal tolerance. We found ontogenetic differences in thermal tolerance for all populations, with neonates tolerating temperatures 2 degrees C higher than adults, a pattern that is consistent with ontogenetic shifts in body size and microhabitat use in this species. There were differences in thermal tolerance among latitudes in neonates, suggesting genetic differences among populations, but adults showed no latitudinal differences. In combination, the increased thermal tolerance of neonates and the age-specific response to latitude suggest individuals may be most sensitive to selection on thermal tolerance as neonates. Although latitudinal differences exist in neonates, their tolerances were not ranked according to latitude, suggesting the effects of some other local factor (e.g., microclimate) may be important. Lastly, among neonates, females tolerate higher temperatures than males.