Latitudinal variation in thermal ecology of North American ratsnakes and its implications for the effect of climate warming on snakes

Behavioral thermoregulation is expected to be critical in determining the capacity of reptiles to respond to climate warming and how that response will vary with latitude. We used radio-telemetry to compare behavioral thermoregulation among ratsnake (Elaphe obsoleta) populations in Texas, Illinois, and Ontario, a latitudinal distance of >1500 km. Despite numerous specific differences among populations, overall the thermal ecology was surprisingly similar during the months that snakes in all three populations were active. Preferred temperatures varied only slightly across the snakes’ range, the extent of thermoregulation was similar, and by varying when during the day and season they thermoregulated, snakes in all three populations realized body temperatures within their preferred temperature range 15–20% of the time. The ability to use fine-scale behavioral thermoregulation (i.e., selective use of habitats and microclimates) to a similar extent and achieve similar outcomes across such a wide latitudinal and climatic gradient is made possible by large-scale differences in timing of activity (ratsnakes in Texas switch to nocturnal activity during summer, whereas in Illinois and Ontario activity is exclusively diurnal and hibernation lasts 5–7 months). Modeling indicated that a 3 °C increase in ambient temperature will generally improve thermal conditions for all three populations. Our empirical analyses suggest that the snakes’ ability to respond to climate warming will be determined more by their capacity to adjust when they are active than by changes in the extent of fine-scale behavioral thermoregulation. The ability to adjust timing of activity appears to make many snakes fundamentally different from lizards. As such, the consequences of climate warming may be very different for these two groups of reptiles.