Summary
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.
Methodology
Study sites and species
Research was conducted in southern Illinois from 2002–2004 at the Cache River State Natural Area (37° 23′ N, 88° 54′ W) and in central Texas from 2004–2007 at Fort Hood (30° 10′ N, 97° 45′ W). Research in eastern Ontario was conducted from 1997–1999 at the Queen's University Biological Station (44° 34′ N, 76° 19′ W). Although results from Ontario have already been published (Blouin-Demers and Weatherhead, 2001a), we present some of those results here to facilitate comparison among populations. In other cases explained below (2.4), we re-analyzed the Ontario data so the results we present here differ from those published previously.
At all three sites the habitat consisted of forest interspersed with more open habitats. Although patch sizes varied among sites, all habitat types at a site were accessible to all snakes at that site using their normal range of movement. Forest was principally eastern deciduous in Ontario and Illinois (with some differences in component species), and oak-juniper in Texas. An important difference between sites was that in both Texas and Ontario, open habitats were often completely exposed (e.g. bare rock or ground), whereas in Illinois open habitats were fields in various stages of succession, with little bare ground.
The span of approximately 14° of latitude between the Texas and Ontario study sites (a N–S distance of >1500 km) encompasses almost the full latitudinal range of ratsnakes. Based on data from weather stations near each study site, mean annual temperatures for Texas, Illinois, and Ontario are 19.5, 14.4, and 6.6 °C, respectively, and there are 240, 190, and 142 frost-free days, respectively. The snakes’ active season extends from May through September in Ontario, April through October in Illinois, and in Texas the snakes do not hibernate and can be active in any month of the year if the weather is warm (Sperry et al., 2010). From May to September when snakes in all three populations are active, mean monthly high and low temperatures in Texas are, respectively, approximately 5 and 10 °C warmer than in Illinois and 10 and 20 °C warmer than in Ontario.
Although Elaphe obsoleta was historically considered a single species, mitochondrial DNA analyses (Burbrink et al., 2000) revealed three distinct clades that Burbrink (2001) proposed to be considered as separate species. However, Gibbs et al. (2006) found that the Ontario population studied here was a hybrid of the eastern and central clades. Regardless of how the taxonomy is resolved, what is important for our study is that our “populations” are closely related and ecologically similar based on both diet (Weatherhead et al., 2003, Carfagno et al., 2006, Sperry and Weatherhead, 2009) and habitat use (Blouin-Demers and Weatherhead, 2001b, Carfagno and Weatherhead, 2006, Sperry et al., 2009).
Radio-telemetry
To ensure that results were comparable between studies, we followed the methods of Blouin-Demers and Weatherhead (2001a) to the extent possible. In Illinois we captured ratsnakes as they emerged from hibernacula each spring and opportunistically through the season. Because ratsnakes in Texas do not hibernate, all captures were opportunistic. Snakes for which transmitters weighed <3% of their body mass had temperature-sensitive transmitters (Model SI-2T, Holohil Systems Ltd., Ontario) implanted surgically (Weatherhead and Blouin-Demers, 2004a). We relocated snakes approximately every 48 h using hand-held telemetry and recorded their body temperatures (Tb) using transmitter pulse rates, which accurately predicted transmitter temperatures (all R2>0.99). We also used four–six automated radio-telemetry data loggers at each site (SRX 400, Lotek Wireless, Ontario) to record Tb every 10 min around the clock through the active season. By regularly repositioning data loggers we maximized the number of snakes within transmission range, although no snake produced continuous records.